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Information - The FFC Bulletin - 2014 V2 March

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Hedging Bets:  Legume Mixtures in Pasture

By Molly Shaw

White and red clovers are choice pasture species for good reason—they’re highly productive and persist well in grazed situations.  But no one species is best at everything, and there is a persuasive argument for considering other species for the qualities they can bring to a pasture mix.  

If a farmer was designing the perfect legume pasture species, the wish list might look something like this:  fast to establish; tolerant of heavy soils, acidic soils, and droughty soils; weed suppressing; delicious to stock; high in nitrogen but resistant to nitrogen leaching; good regrowth after grazing; maybe even providing food for pollinators and other beneficial insects.  Oh, and highly productive too—did we mention that?  And cheap seed?

Well now, we can’t be the best at everything, and it turns out that that’s true with pasture species as well.  Starting in 2008, researchers funded by the UK Department for Environment, Food and Rural Affairs as well as industry partners conducted an extensive study that ran for three years on five research farms and dozens of farm fields across the UK.  They chose 12 species of legumes and 4 pasture grasses to grow alone and in mixes, analyzing their traits and providing almost 200 pages of Project Results [1].  Twenty-two legumes were reviewed and the 12 most promising were chosen for the study.  If a species could not take two of three factors (cold, mowing, or autumn sowing), it was excluded.  Where species were very similar in habit, only one was included to avoid redundancy.   The four grasses in the study were perennial ryegrass, Italian ryegrass, meadow fescue, and timothy.  Highlights from their research are reported in the following summary.  

Twelve legumes were grown separately and in a mixture of all the species together (the “all-species mix”).  Dozens of measurements were taken on the plantings and species were ranked in their performance on 7 criteria.  Table 1 summarizes the conclusions.  

Table 1:  The 12 legumes were scored according to 7 criteria, then ranked.  In this chart a higher number means better performance.  Colours have been assigned to number ranges to make the conclusions easier to see.  Numbers 1-4 are red (generally poor), numbers 5-8 are orange (mid-range performance), and numbers 9-12 are green (best performance).  For the average ranks, 7.9 and above is green, since the top five species scored quite similarly.

Criteria explained:  

  • The Early Development rank was obtained from the average of four variables: speed of emergence, proportion of sown seed emerging, seedling biomass at 60 days, and seedling relative growth rate.  All species were sown in the spring.  Species that emerge slowly tended to grow their bulk later on.  Large-seeded species grew fast early on, but because they tended to be tall and their growing points high up, they re-grew more slowly after grazing.  
  • Variables going into the Productivity rank were plant height in spring, plant regrowth after mowing (low cut to simulate grazing), crop cover (ground cover), crop biomass measured multiple times, and persistence in the All-Species-Mix after three years.  Crop cover, measured in the early spring, reflects how well the species grew over winter.  Grasses were strong in this measure, as were red and white clover, followed by black medic and Lucerne.  The biomass measurement showed the same strong legume species, but among the grasses the perennial ryegrass and Italian ryegrass pulled ahead of meadow fescue and timothy.  The All-Species-Mix was at least as good as any top species alone, both in crop cover (ground cover) and in biomass.  Regrowth after a low cut (simulated grazing) was better in plants with low regrowth points.  White clover is the prime example, since it grows creeping along the ground with horizontal stems, and had the best regrowth after grazing.  Regrowth was also better in plants that were larger before the cut, which would have had more root resources to mobilize for regrowth—red clover is an example of this trait, as well as the All-Species-Mixes.  Lucerne and black medic had relatively lower grazing tolerance than one would expect based on their biomass.
  • Not surprisingly, highly productive species (good ground cover, high biomass) had fewer weeds in the ley.  Weed suppression was scored by assessing weed cover at the end of summer, in the spring, and the change in weediness over two years.  
  • Flowering was an attempt to quantify value to pollinators.  Plants were scored by how early they flowered and how long they continued flowering.   
  • The Pre-crop Value, Resistance to decomposition, and Performance of the following crop were assessed in light of legumes being used by farms to supply fertility (mainly nitrogen) for the next crop, and to condition the soil.  A ley with lots of biomass just before incorporation, especially biomass high in nitrogen, is good for the performance of the following crop (in this case a grain).  When a green manure is ploughed down there is a flush of readily-leachable nitrogen released, often before the following crop has enough roots to utilize it.  Resistance to decomposition is important to reduce leaching of nitrogen, an issue that is becoming more and more critical in New Zealand.  Slower N release (plants higher in lignin and polyphenols, also measured as higher C:N ratio) puts the available N more in line with later crop use, and so reduces leaching….but also makes the plants less digestible to grazers.  Birdsfoot trefoil stood out as being high in N-rich biomass yet slower to decompose, and therefore good for the next crop and for the environment.  
  • The data was hard to generalize beyond that—there was more variation in N accumulation and release between sites than between trial species.  Theoretically, grain yields in the following crop should be highest in legumes and mixtures that (1) grow a lot of biomass, and (2) have enough C (lignin, polyphenols) that their N release is delayed, matching the needs of the following crop better, a situation that was observed at one of the trial locations but not all of them.  

There were five trial sites in the study.  At all sites grass tended to become more dominant over the years, more so in farms that were drier and not grazed.  In grazed situations there were more legumes remaining in the mix, although the “balance of power” wasn’t the same everywhere.  At one site, Alsike clover, white clover and red clover dominated the legumes; at another it was birdsfoot trefoil and black medic; Lucerne, red clover and white clover at a third site.  Among the legumes, black medic did better in grazed situations than without the grazing, while Lucerne was the opposite.  This means it’s important to trial these things on your own farm to see what does best under your management scheme and climate.  

Trade-offs:  Ecologists talk about a concept called “functional complementarity,” which means that one species can’t be best at everything.  Plants have to make trade-offs.  

For instance, the ones that were best at early development and productivity decomposed really fast after plow-down, leading to more potential for N to leach.  Lignin is woody tissue and polyphenols are tannins, and apparently when a plant makes those it is at the expense of fast growth.  

Another pattern the researchers noted was that species that emerged and grew slowly (those with small seeds) tended to increase their biomass over the next two years, whereas those that emerged quickly (large seeds) tended to be smaller than their counterparts over the time, exemplified particularly well with crimson clover (an annual species).  Fast starters dominated early, then fast re-growers took over after mowing/grazing.  Lucerne was an interesting exception, emerging quickly and increasing in biomass over the two years, but for the most part quick starters petered off over time.  Small seeded species also tended to cope with grazing better.  

The researchers used the hundreds of measurements they took on their trial sites with grass and legume species (grown separately and in one giant mix) and used those numbers to plug into an ecological model.  The purpose of this exercise was to guess how mixes of two, three or four species would perform, and find the ones that complemented each other optimally.  Not surprisingly, the best “designer mixes” were mostly the combinations of the highest-performing species, with secondary performers thrown in at times to fulfil a lacking ecological niche.  “Stronger as a team” could be the motto.  

Why try mixtures? 

They are resilient.  In comparison with the monocultures, the all-species mix showed increased ground cover and above-ground biomass, and consequently reduced weed biomass.  The biomass advantage of the all-species mix was more pronounced on poorer soils.  When under stress, a mix with plants of different strengths and weaknesses (heights, root structures) outperformed the more simple leys of species that are high producers under optimal conditions.  

Species profiles:  Farm managers will differ in what qualities they consider the most important for their individual farms.  The following notes will highlight where each of the species shines, and fuller profiles are available at the end of the full report document.

Alsike clover:  Neither the highest nor the lowest performer in any of the criteria, but alsike clover is more tolerant of acidic soils and heavy soils than the others.  Note that it can be toxic to horses!  And its bitter taste makes it suitable as a mix component but not on its own.  

Photo source http://www.anpc.ab.ca/wiki/index.php/Trifolium_hybridum

Black medic:  In contrast to the general view that black medic has low yields, in this study it shown in terms of biomass production, better even than white clover.  It self-seeds and can germinate at any time from immature seeds in the field, and it has a long flowering period (good for beneficial insects).  It is bitter so livestock don’t prefer to graze it.  

Photo source http://www.fao.org/ag/agp/AGPC/doc/gallery/pictures/mediclup.htm

Birdsfoot trefoil: The long tap root on this species gives it drought tolerance, even more so than Lucerne.  It is also more tolerant of poor drainage and flooding than Lucerne, white clover, or red clover.  It has moderate yields and is relatively weak at establishing, but provides good forage value as a non-bloating species (and livestock like it), as well as slower N-release after plow-down.

Photo source http://www.missouriplants.com/Yellowalt/Lotus_corniculatus_page.html

Crimson clover: This winter annual species can be used as a summer annual in cooler climates.  It is suitable for mixing with forage brassicas and is adaptable to a wide range of soil pH.  It has big seeds, giving it the strongest early development score in this study.  It has good first year biomass production and should be grazed before flowering to avoid the hairy flowers that cause livestock digestion problems, though it peters out in subsequent years.  

Photo source: https://www.cotswoldseeds.com/product/crimson-clover-trifolium-incarnatum-organic

Large Birdsfoot trefoil:  This species showed slow early growth and low productivity, and consequently it was ranked near the bottom in this study.  It is reported to have good tolerance of acidic soils.  

Photo source: https://www.kuleuven-kulak.be/kulakbiocampus/lage%20planten/Lotus%20pedunculatus%20-%20Moerasrolklaver/moerasrolklaver.htm

Lucerne:  This species showed high productivity, both at early development and at later stages, and therefore was good at suppressing weeds.  It is drought tolerant due to a long tap root, and needs a soil pH above 6.0.  It doesn’t tolerate grazing well.  

Meadow pea:  This species was one of the poorest performers in the study, and its profile doesn’t offer much to recommend it.

 

Photo source: http://www.naturespot.org.uk/species/meadow-vetchling

Red clover:  This is a well appreciated and highly productive legume species suitable for a wide range of environmental conditions.  Its sizeable tap root gives it drought tolerance.  It outperforms white clover in first year growth, comparable to Lucerne.  It doesn’t persist as long as white clover and doesn’t take low grazing as well, being used more for two-year leys than perennial swards.  Grazing red clover carries a risk of bloat, and can cause problems with fertility in ewes due to a chemical compound with an estrogenic effect.  

Photo source: http://es.wikipedia.org/wiki/Trifolium_pratense

White sweet clover:  This is a biennial species with moderate productivity, good drought tolerance because of a large tap root, and tolerance of alkaline soils.  It can be grazed, but there are several caveats: stock have to get used to its bitter taste, it carries a bloat risk, and coumarin in the leaves can cause “bleeding disease” in stock (hence the development of low coumarin cultivars).

 

Photo source: http://luirig.altervista.org/cpm/albums/15c/008866-melilotus-alba.jpg

Sainfoin:  This is a perennial legume that establishes slowly, but can produce good biomass later on (in years 2 and 3) and is drought-tolerant due to a strong tap root.  It is not super competitive, so is difficult to include in mixtures with grasses, and may be why it performed relatively poorly in this study.  It is highly palatable to stock (they eat more of it than red clover or Lucerne) and non-bloating.  In addition, the aphenolic compounds it contains reduces parasitic worms in lambs.  

Photo source: http://huelsenfruechtler.de/saat-esparsette-onobrychis-viciifolia/

White clover:  This classic legume is popular for good reasons.  It was hard to beat in terms of biomass production and grazing tolerance in ideal (moist, cool) conditions with high soil pH and good soil drainage, but black medic did manage higher overall biomass production in this trial.  Its large N contribution is highly leachable because it has a low C:N ratio.  Because it spreads across the ground by stolons it has outstanding tolerance to grazing.  

Photo source: http://commons.wikimedia.org/wiki/File:Trifolium_repens_blangy-tronville_80_07062007_3.jpg

Winter vetch:  Vicia sativa is an annual species, also called common vetch, spring vetch, garden vetch and tare, but distinct from hairy vetch.  It didn’t perform well in this study.  It has exceptionally low grazing tolerance, so much so that when grown on organic vegetable farms as a green manure, it is commonly killed by a low mowing at flowering.

References

1.    Döring, T.F., Baddeley, J.A., Brown, R., Collins, R., Crowley, O., Cuttle, S., Howlett, S.A., Jones, H.E., McCalman, H., Measures, M., Pearce, B.D., Pearce, H., Roderick, S., Stobart, R., Storkey, J., Tilston, E.L., Topp, K., Watson, C., Winkler, L.R., and Wolfe, M.S., Using legume-based mixtures to enhance the nitrogen use efficiency and economic viability of cropping systems. Project Report No. 513. 2013, Agriculture and Horticulture Development Board, HGCA Division. http://archive.hgca.com/cms_publications.output/2/2/Publications/Final%20project%20reports/Using%20legume-based%20mixtures%20to%20enhance%20the%20nitrogen%20use%20efficiency%20and%20economic%20viability%20of%20cropping%20systems.mspx?fn=show&pubcon=9373

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Information - The FFC Bulletin - 2014 V2 March

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'Can healthy soil feed the world?' Seven scientists give their opinion

By Charles Merfield

The FFC considers the soil to be the most valuable asset humanity owns and has thus made soil management (or husbandry to use the old term) a core focus.  This was highlighted in the presentation made at the FFC’s launch. I was therefore very pleased when recently the ABC (Australian Broadcasting Corporation) Rural asked seven Australian and international soil scientists (see below) for their perspective on the question 'Can healthy soil feed the world?', on the eve of the Australian 'Soil Change Matters' symposium in Bendigo, Victoria.  I think there is considerable commonality between what these scientists said and what the FFC is trying to achieve.

Go to the ABC page with the scientists answers.  

Direct links to the scientists answers are below.

Why is soil missing from the 'big five' environmental questions of our time? Prof. Johan Bouma, Netherlands.

Could a global grab for fertile soil could bring civil unrest. Dr Luca Montanarella, Italy.

Soil security on the political agenda. Prof. Alex McBratney and Andrea Koch, Australia.

Long-term soil experiments 'profoundly undervalued'. Prof. Daniel Richter, United States.

Spending on Earth: an alternative to studying soils on Mars.  Prof. Iain Young, Australia.

Finding optimism in the challenge to double food production.  Dr Damien Field and Prof. Alex McBratney, Australia.

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Information - The FFC Bulletin - 2014 V2 March

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Fire-resistant shelter belts

By Molly Shaw

Last September many shelter belts in Canterbury were blown down or damaged, and land owners now have an opportunity to re-assess the reasons for having shelter in their fields. Sure, there has been a trend toward cutting down shelters in irrigated pastures the past few years. But as climate change research predicts that Canterbury will be a hotter, drier place in the next few decades, the economic picture may not always look like it does now.

Farmers expect shelter belts to benefit them in some or all of the following ways:

  • Wind breaks to reduce drought stress on pasture (even farmers with access to irrigation can’t run it for free)
  • Shelters for livestock during southerlies
  • Habitat for beneficial species that subsequently decrease pest pressure in crops
  • Some degree of nutrient scavenging to reduce leaching into water sources, particularly for shelter belts lining streams or ditches

For the ecologically minded, you can add the less tangible benefits of preserving native species, aesthetic appeal, etc. Landowners prioritize one or more of the above benefits, take into account establishment cost and plant growth rate, and then choose shelterbelt species accordingly. In the past, species like pines, macrocarpa, gorse and poplar have come out on top.

Now we have one more characteristic to take into consideration: flammability. The tendency for vegetation to facilitate fire spread will become more and more important in Canterbury’s hotter, drier future.

Wild fires are most likely to break out and spread at a devastating rate when we’ve had a hot, dry summer. It’s easy to think back to this past summer, cool and wet as it was, and wonder what we’re worrying about. But think back one more year to the heat and drought of the 2012-2013 summer—that’s the type of summer predicted to become more common in the upcoming few decades.

Flammability isn’t rocket science. Remember your basic scout fire building skills? Fires catch best in super dry, lightly packed, twiggy material, and these qualities vary between plant species. Fire spreads quickest in plants that:

  1. Hold more dead material (dead plant matter is drier than live and requires less energy to ignite)
  2. Hold fuel, especially small twigs, evenly spaced along the branches, facilitating fire spread
  3. Contain oils and resins—sparks flying ahead of the fire can ignite these substances easily

For example, young gorse isn’t rated as highly flammable but older plants are highly flammable because they can be made up of 65% dead material arranged neatly along the branches. And manuka and kanuka burn with high intensity probably due to the essential oils they contain and their small leaves which are continuously spaced along their branches.

In wet conditions or if you irrigate your shelter belts, then species choice probably isn’t so important when it comes to flammability—regardless of what it is, if it’s wet then it generally won’t burn. But if you don’t have irrigation, or it doesn’t reach to the shelter belts, then read on.

In 2001, the NZ Fire Service Commission harnessed the experience of about 60 fire managers via surveys, asking them about their experience with fires in different native NZ species. The information gathered is somewhat subjective, but trends did emerge, see figure 1.

Figure 1: Flammability guide for 42 NZ native trees and shrubs [1].

Botanical Name

Maori/European Name

Flammability class

Kunzea ericoides

kanuka

High

Leptospermum

scoparium manuka

High

Cyathea and Dicksonia spp.

tree ferns

Moderate/High

Cyathodes fasciculata

mingimingi

Moderate/High

Dodonea viscosa

ake ake

Moderate/High

Phormium cookianum and P. tenax

flax/harakeke

Moderate/High

Podocarpus totara

totara

Moderate/High

Agathis australis

kauri

Moderate

Beilschmiedia

tawa tawa

Moderate

Dacrydium cupressinum

rimu

Moderate

Metrosideros umbellata

southern rata

Moderate

Pittosporum tenuifolium

kohuhu

Moderate

Podocarpus dacrydioides

kahikatea/white pine

Moderate

Weinmannia silvicola

tawhero/towhai

Moderate

Aristotelia serrata

makomako/wineberry

Low/Moderate

Cordyline australis

ti kouka/cabbage tree

Low/Moderate

Coriaria arborea

tutu

Low/Moderate

Hebe salicifolia and H. stricta

koromiko

Low/Moderate

Hoheria spp.

houhere/hoheria/lacebark

Low/Moderate

Knightia excelsa

rewarewa

Low/Moderate

Melicytus lanceolatus

mahoe wao

Low/Moderate

Melicytus ramiflorus

mahoe/whiteywood

Low/Moderate

Myoporum laetum

ngaio

Low/Moderate

Nothofagus menziesii

tawhai/silver beech

Low/Moderate

Phyllocladus glaucus

toatoa

Low/Moderate

Pittosporum crassifolium

karo

Low/Moderate

Pittosporum eugenioides

tarata/lemonwood

Low/Moderate

Plagianthus regius

manatu/ribbonwood

Low/Moderate

Weinmannia racemosa

kamahi

Low/Moderate

Carpodetus serratus

putaputaweta

Low

Coprosma grandifolia

raurekau, kanono

Low

Coprosma repens

taupata

Low

Coprosma robusta

karamu

Low

Corynocarpus laevigatus

karaka

Low

Fuchsia excorticata

kotukutuku/fuchsia

Low

Geniostoma ligustrifolium

hangehange

Low

Griselinia littoralis

papauma/broadleaf

Low

Griselinia lucida

puka

Low

Macropiper excelsum

kawakawa/pepper tree

Low

Psuedopanax arboreum

five finger

Low

Pseudopanax crassifolius

horoeke/lancewood

Low

Solanum aviculare

poroporo

Low

Tim Curran, ecologist and lecturer at Lincoln University, has been researching flammability of plants grown in NZ and was able to add comments about non-natives used for shelter belt species. He classifies some of the most common shelter belt species as highly flammable, such as:

  • Eucalyptus, e.g. manna gum E. viminalis (high in natural oils)
  • Pines, such as radiata pine, especially when it retains dead material
  • gorse (especially old gorse hedges with high levels of dead material)

Deciduous species such as willow and poplar tend to have leaves with higher moisture content and therefore are quite a bit less flammable.

Dr. Curran’s recommended low-flammability species for shelter belts are:

  • Broadleaf (Griselinia spp.)
  • Coprosma species
  • Pseudopanex (five-finger, lancewood)
  • Pittosporum eugenioides (lemonwood)

Admittedly, these species are slower growing than the standard pines and macrocarpa often used for shelter belts. A compromise might be to plant the slow-growing natives interspersed with fast-growing non-natives, then remove the big trees as the native ones mature. It is important to realize that if the weather is hot and dry enough, even the low flammability species are likely to burn.

Dr Curran’s Plant combustion BBQ measuring maximum burning temperature

Dr. Curran’s research team is currently conducting flammability tests of NZ shelterbelt species in order to make objective recommendations for “green fire break” plantings, lines of low-flammability species that could serve as fire breaks in the greater Canterbury landscape.

References and further reading

Fogarty, L.G., A flammability guide for some common New Zealand native tree and shrub species, New Zealand Fire Service Commission Research Report. 2001. http://www.fire.org.nz/Research/Published-Reports/Documents/89fa12a030b48531cf396dcdba52c6e2.pdf

Berry, Z.C., Wevill, K., and Curran, T.J., The invasive weed Lantana camara increases fire risk in dry rainforest by altering fuel beds. Weed Research, 2011. 51(5): p. 525-533. http://dx.doi.org/10.1111/j.1365-3180.2011.00869.x.  Please email This email address is being protected from spambots. You need JavaScript enabled to view it. if you would like an ePrint of this paper

Lantana fuels rainforest fires ABC Science (web page)

Shelter and nature conservation in Canterbury – a practical guide ECan (PDF file)

Botanical Name

Maori/European Name

Flammability class

Kunzea ericoides

kanuka

High

Leptospermum

scoparium manuka

High

Cyathea and Dicksonia spp.

tree ferns

Moderate/High

Cyathodes fasciculata

mingimingi

Moderate/High

Dodonea viscosa

ake ake

Moderate/High

Phormium cookianum and P. tenax

flax/harakeke

Moderate/High

Podocarpus totara

totara

Moderate/High

Agathis australis

kauri

Moderate

Beilschmiedia

tawa tawa

Moderate

Dacrydium cupressinum

rimu

Moderate

Metrosideros umbellata

southern rata

Moderate

Pittosporum tenuifolium

kohuhu

Moderate

Podocarpus dacrydioides

kahikatea/white pine

Moderate

Weinmannia silvicola

tawhero/towhai

Moderate

Aristotelia serrata

makomako/wineberry

Low/Moderate

Cordyline australis

ti kouka/cabbage tree

Low/Moderate

Coriaria arborea

tutu

Low/Moderate

Hebe salicifolia and H. stricta

koromiko

Low/Moderate

Hoheria spp.

houhere/hoheria/lacebark

Low/Moderate

Knightia excelsa

rewarewa

Low/Moderate

Melicytus lanceolatus

mahoe wao

Low/Moderate

Melicytus ramiflorus

mahoe/whiteywood

Low/Moderate

Myoporum laetum

ngaio

Low/Moderate

Nothofagus menziesii

tawhai/silver beech

Low/Moderate

Phyllocladus glaucus

toatoa

Low/Moderate

Pittosporum crassifolium

karo

Low/Moderate

Pittosporum eugenioides

tarata/lemonwood

Low/Moderate

Plagianthus regius

manatu/ribbonwood

Low/Moderate

Weinmannia racemosa

kamahi

Low/Moderate

Carpodetus serratus

putaputaweta

Low

Coprosma grandifolia

raurekau, kanono

Low

Coprosma repens

taupata

Low

Coprosma robusta

karamu

Low

Corynocarpus laevigatus

karaka

Low

Fuchsia excorticata

kotukutuku/fuchsia

Low

Geniostoma ligustrifolium

hangehange

Low

Griselinia littoralis

papauma/broadleaf

Low

Griselinia lucida

puka

Low

Macropiper excelsum

kawakawa/pepper tree

Low

Psuedopanax arboreum

five finger

Low

Pseudopanax crassifolius

horoeke/lancewood

Low

Solanum aviculare

poroporo

Low

Normal 0 false false false MicrosoftInternetExplorer4

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Crushing Californian thistles to death and other on-farm, non-chemical control techniques

By Charles Merfield

Download this article as a PDF report.

1. Introduction

Following the appeal for sightings of white tip disease (Phoma macrostoma) on Californian thistle (Cirsium arvense) in previous editions of the FFC Bulletin (2013-V2), a number of farmers and growers contacted the FFC asking about other non-chemical control methods for Californian thistle. This article explains some less well known means of non-chemically controlling Californian thistle that farmers and growers can implement themselves.

Globally, most non-chemical control techniques are based around defoliation of the aerial parts of the thistle during the growing (summer) season, using mowing and/or surface / shallow tillage. While these techniques can be effective against Californian thistle, they are not a silver bullet, as it typically takes three years to achieve a significant, e.g., 90% reduction of thistle biomass. As complete elimination of plants can take several more years, it is clear that defoliation is a long row to hoe, albeit a reliable one. However, there are other techniques that can be added to or substituted for a mechanical defoliation regime to enhance Californian thistle control, one of which does approach silver bullet status, at least for flat to undulating land where tractors can operate effectively. First however, to know how to effectively deal with any weed or pest in agriculture you need to understand and think like the enemy.

2. Thinking like the enemy: the biology of Californian thistle

Californian thistle is unlike all the other thistles in New Zealand which are annuals or biennials, as it is a perennial that can live for decades and theoretically it is just about immortal as it is a creeping weed that spreads via its root system, so it is constantly regenerating. A patch of Californian thistle in a paddock is in fact one individual cloned plant, connected underground by the spreading roots that grow horizontally through the soil, and which also act as the food storage system that allows the plant to remain dormant through the winter. Figure 1 shows patches of Californian thistle in a recently cultivated field. Individual clones can achieve considerable size with diameters of 10 to 20 meters possible.

Figure 1. Californian thistle patches, each of which are individual cloned plants.

 

Figure 2. Left, Californian thistle spreading root and a vertical stem. Right, four month old plant originating from seed that has already produced the first horizontal spreading root along with a vertical shoot.

Another way to think of the structure of Californian thistle is that of a tree, with the individual thistle stems that grow out of the ground akin to leaves on a tree and the underground spreading roots as the trunk that connects all the branches that hold the leaves. This analogy illustrates the fact that it is the underground spreading roots that are the ‘heart’ of the Californian thistle plant, and that the above ground shoots are more ephemeral and expendable, like the leaves on a tree.

The spreading roots grow horizontally, between about 15 and 30 cm deep (Figure 2), depending on soil type, soil structure, tillage approach (e.g. ploughing vs. min-till or no-till) and production system (e.g. pasture vs. cropping), though in very loose soil they can be deeper still. The plants also produce ‘normal’ roots, i.e. that grow downwards and are not thickened, which have the normal root functions of taking up water and nutrients. These roots can penetrate to a considerable depth, e.g. a meter or more, the same as many other weeds and crop plants, however, they don't have the same regenerative ability of the spreading roots. The University of Minnesota extension service has some impressive photos of the root system.

As shown in Figure 2, the Californian thistle stems arise from the spreading roots. A side root from the main spreading root starts to grow upwards and then transforms from a true root to a true shoot, with the boundary between root and shoot delimited by the end of the small feeder roots emerging from the true root.

2.1. Californian thistle’s ‘life strategy’

The annual lifecycle of Californian thistle, is also akin to a deciduous tree: in spring, like new leaves on a tree, new stems emerge from the soil, in summer the plant flowers and in the autumn produces seeds before the above ground parts die back for winter, often following a good frost, like the leaves falling from the trees. The plant then remains dormant over winter, existing only as the underground root system, until spring when the cycle repeats.

In comparison, reproduction by seed is generally considered to be a much lower level strategy for Californian thistle. This is not to say that Californian thistle’s cannot produce a large amount of seed, and that seeds can’t survive in the soil for a number of years, even decades, however, and especially compared with annual weeds such as the infamous fat hen (Chenopodium album), it is very uncommon to find Californian thistle seedlings, even where there are large patches of thistles nearby. In my 20 years of farming and agricultural research experience in the UK, Ireland and New Zealand, I have only positively identified a couple of handfuls of Californian thistle seedlings on working farms, even those that have Californian thistle problems. In comparison, I must of seen billions of fat hen seedlings over that same time, with some fields producing thousands of seedlings per square meter.

However, this is not to say that Californian thistle’s reproduction by seed should not be ignored entirely: I know of one farm that took a home-saved crop of barley seed from a field with Californian thistle that had gone to seed, and through inadequate seed cleaning, planted a load of thistle seed heads with the barley, and turned a clean field into one full of Californian thistle. Interestingly, over 10 ha, there were only a few tens of thistle patches i.e. individual clones (although very large ones) indicating that the number of planted seeds that resulted in mature patches was again very small. This is in part explained by the slow early growth and poor competitive ability of Californian thistle seedlings.

The ‘life strategy’ of Californian thistle is therefore all about growing the spreading root system. The rate of growth of the roots and therefore individual plants can be quite profound: under ideal conditions of good soil, warm weather and most importantly, the absence of other plants, i.e. crops or pasture, individual seedlings can produce plants 10 meters across in a couple of years and the University of Minnesota Extension Service (MSE) photos (noted above) also show massive root growth. Also as noted in the MSE webpage, and many scientific papers, even very small lengths of root, such as 3 cm long, are able to regenerate and create new plants, again emphasising that the roots are the heart of Californian thistle.

A final point are the soil conditions that appear to favour Californian thistle. It is clear that Californian thistle favours some soil conditions over others, for example, the lack of Californian thistle across wide areas of farmland where the climate is amenable for the plants, while in similar climatic areas Californian thistle is the number one weed. Also within individual farms with a range of soil conditions, Californian thistle thrives in some areas but not others. Figure 3 shows a strip of Californian thistle that has been growing in an old stream bed, on the Canterbury plains, for over two decades, but that has never grown up and out of the stream bed into the rest of the field.

Figure 3. Californian thistle that has been growing in an old stream bed between Springston and Leeston on the Canterbury plains for two decades but that has not invaded the rest of the field over all that time.

Unfortunately, little research has been undertaken to accurately and reliably determine what soil conditions favour Californian thistle. The relationship between soil conditions and weed flora is also an area of much pseudoscience so should be treated with considerable caution. However, observations indicate that Californian thistle prefers moister soils, i.e. heavier soils that contain moderate and higher proportions of clay, and also some chalk and limestone derived soils that also tend to be heavy with a high water holding capacity. Dry soils, i.e. with moderate to high levels of sand, do not appear to allow Californian thistle to thrive. If this is correct, it is also likely that soil damage, such as compaction and cultivation / tillage pans that increase soil moisture, may well also favour Californian thistle. However, due to the lack of detailed research these observations should be taken as an indication not proof.

2.2. It’s the roots, stupid! (apologies to Bill Clinton)

So with Californian thistle’s life strategy focused on expanding the spreading root system and having a minor (but not zero) focus on reproduction and spread via seed, not only should the root system be the focus to kill existing plants, killing existing plants should be the dominant focus of a whole-of-farm Californian thistle management strategy. This because unlike the annual and biennial weeds where it is the seeds that are the fundamental life stage and the means of perennating from year to year and decade to decade, for Californian thistle its spreading root system is the means of reproduction, spread and perennation. That is why ‘it’s the roots, stupid’. If all Californian thistle plants are eliminated from a farm, and the few new plants that arise from seed, are rapidly dispatched, then Californian thistle will no longer be a problem. This is also why I consider Californian thistle to be a zero tolerance weed, because once eliminated, management is relatively easy and cheap, while just trying to keep a lid on the problem is a Sisyphean task.

3. Killing Californian thistle plants

However, while its easy to say that the best means of controlling Californian thistle is to kill all the plants, the reason Californian thistle is such a problem on so many farms, is the things are B. difficult to kill! Clearly some tools to break this circular problem are needed.

3.1. Crushing Californian thistles to death

Of all the non-chemical management tools for Californian thistle one stands head and shoulders above the rest in terms of potentially achieving an instant kill: subsoiling / deep ripping. This is one of those tools that is known of and being used by a small number of farmers, but one that has apparently completely bypassed the scientific establishment. Figure 4 Shows a field which was wall-to-wall with Californian thistle the previous year. In the autumn the field was subsoiled to kill the thistle, however, the driver did not match the bouts up correctly so there were strips of ground that were not subsoiled between the subsoiled bouts. This accidently made for a great experiment, in that the parts that were subsoiled were almost completely clear of Californian thistle, while the strips of field that were missed, show up as long thin strips of nearly solid thistle, which is now going to seed among the linseed crop.

Figure 4. Narrow strips of Californian thistle (gone to seed) between wider thistle free strips following subsoiling with insufficient bout overlap in a linseed crop (with fathen).

While there is positive farmer and grower experience of the effectiveness of subsoiling for killing Californian thistle, the lack of scientific research is a concern because the exact mechanism by which subsoiling works has not been determined. The working hypothesis, is that the pressure created by subsoiling crushes the spreading root system, effectively to a pulp, thereby instantly killing the plant. What is not considered to be the cause is the subsoiling 'merely' breaking up the spreading root system into small fragments, while leaving the individual root section intact, because there is extensive scientific and practical evidence that shows fragmented but still living roots have remarkable regenerative capacity, and, if that were all that was occurring during subsoiling, then what were individual plants before subsoiling, would end up being a much larger number of smaller plants afterwards, somewhat like Goethe’s sorcerers apprentice who cuts the broom in half with an axe, only to end up with two brooms doing twice the work!

If this hypothesis is correct, it indicates that the subsoiling must be done in such a way that all the spreading root system is crushed. As the spreading root system is typically 15 to 30 cm deep, best practice subsoiling for compaction remediation may not be sufficient to achieve this. Figure 5 shows the key subsoiling issues relating to killing Californian thistle’s spreading roots.

  1. First the larger size of shatter cone, both at the surface and at depth from a winged subsoiler compared with a standard / traditional chisel point subsoiler. It is now generally regarded that winged subsoilers are superior to chisel points on a wider number of factors (see Figure 6 for examples of chisel and winged subsoilers).
  2. Best practice for the spacing between subsoiler legs is that the shatter cones should meet at the soil surface (therefore the deeper the ripping, the wider the leg spacing).
  3. Best practice leg spacing for compaction remediation, is however, insufficient to completely crush the spreading root system of Californian thistle as it is typically 15-30 cm deep in the soil, so not all of the root system will be within a shatter cone.
  4. Either, closer leg spacing (which will require legs to be offset longitudinally to avoid excessive draft) or better, undertaking a second pass of the subsoiler with the bout position offset by half the distance between legs, so that all of the spreading root system is within a shatter cone and therefore crushed to death.

Figure 5. Diagram of subsoiling issues relating to killing Californian thistle spreading root systems. Section 1: increased width of shatter cone from winged vs. standard chisel point subsoilers. Section 2: Standard recommendation for correct spacing between legs. Section 3: the standard spacing for subsoiling, leaves some of the spreading roots unaffected / un-killed by the shatter cone so they will survive. Section 4: decreasing the spacing between legs, or better using overlapping bouts offset by half the distance between legs, results in all of the spreading roots being crushed within a shatter cone.

Figure 6. Standard chisel point subsoiler (left) winged subsoiler (right).

All of the above also requires that subsoiling / deep ripping is done correctly. Unfortunately, there are many cases where it is done badly with more harm than good being done. The key failure is subsoiling at incorrect soil moistures. Soils must be sufficiently dry, to the full depth of subsoiling, that they are no longer plastic, because if they are plastic they will deform not shatter. Equally, soils must not be so dry, that they no longer shatter into small crumbs, but break into large blocks. The only way to confirm if conditions are correct, and a good shatter to depth is achieved is by digging a soil pit after the subsoiler has passed. It cannot be over-emphasises the importance of doing the job right, and the key reference for this is Davies, Eagle and Finney’s classic text “Soil Management” now rebranded for the 6th edition as “Resource Management: Soil”[1].

If done correctly, farmer and grower experience is that Californian thistle can be completely killed in one operation. If some thistles regrow after treatment, it is important to re-subsoil them as soon as soil conditions are correct, and/or use other means to combat the regrowth.

In addition, if the common belief that Californian thistle like moist soil conditions is correct, subsoiling has the additional advantage of breaking up compaction and plough pans that can increase soil moisture, especially at depth where the spreading roots grow, thereby drying out the soil at depth and making the conditions less hospitable for Californian thistle’s spreading root system.

3.2. Goats

Sheep and cattle generally avoid grazing Californian thistle unless pushed hard or when the plants young and tender. Goats however will graze Californian thistle move avidly than sheep and cattle so they can be used as part of a Californian thistle management strategy, and they clearly have the edge over mowing in steeper terrain. However, goats, like other means of defoliation are no silver bullet, and they need the correct management to get the best weed control effect. For example, if there are other more palatable fodder species available, goats will graze those in preference. Goats won't clear Californian thistle overnight, as the control mechanism is the same as mowing, i.e. exhausting the underground reserves, so it will take several years to eliminate Californian thistle under intensive grazing pressure, and longer under more infrequent / lower pressure.

Goats will also graze other thistles and are quite partial to the flower heads, but, it is important to manage grazing so that thistles don't go to seed as the seed will pass right through animals guts and therefore be spread in the manure. This especially important for annual and biennial thistles as for these species, the seed, not the root, is the fundamental stage of the lifecycle and only means of reproduction and spread.

Milking goats are not suitable, due to disease and parasite problems, and only meat goats are up to the task. However, the final plus for goats, is unlike mowing and subsoiling, if well managed, they will produce an income, rather than just burning diesel. One useful resource in NZ for information on meat goats is http://caprinex.com/

3.3. Mowing in the rain

Another example of where science has been behind practical farmer and grower experience is mowing Californian thistles in the rain (without any assistance from Gene Kelly). This originated from farmer and grower observations that when mown in the rain, compared with dry conditions, the thistles were set back more. This was then tested, using on-farm trials, by scientists at AgResearch and found to be correct, with up to 30% reduction in thistle biomass compared with mowing in the dry. The likely cause is that wet conditions allow for greater transfer and infection of a range of fungal diseases between the cut stems, when then go on to harm, and even kill the plants. Both Beef+Lamb who help fund the research and AgResearch have more information on the technique. Beef+Lamb information. AgResearch information.

3.4. Non-mowing, mowers: Combcut

While mowing Californian thistles is a well proven technique, the downside, especially in an era of rising fuel costs and climate change, is the energy required by toppers and especially flail mowers. For farms with major Californian and other thistle problems, more energy efficient means of cutting down thistles could be beneficial. One machine, reported on in the last issue of the Bulletin (2014-V1) that could well fit that need is the Combcut. While currently designed for three point linkage mounting, there is no reason, and with a bit of kiwi ingenuity, that towed versions could not be constructed with small on-board engines to power the brushes, for use behind quad bikes and farm utes. Considerably faster forward speeds than toppers and mulchers should be possible using Combcut, thereby also reducing field time and total costs.

3.5. Crop and pasture competition

Following the logic of the old adage that a ‘sheep’s worse enemy is another sheep’ a plants worse enemy is another plant because they are competing for soil and light resources. A wide range of research and farmer and grower experience indicates that Californian thistle grows much faster / larger in low or no competition situations and can struggle in high competition. However, when many people think about plant competition, they mostly think only of the above ground aspect, i.e. competition for light. However, root competition, underground and therefore out of sight and therefore often out of mind, can have as big, or even much bigger effect than above ground foliar competition. So, once again, its back to the roots. However, with the spreading roots of Californian thistle being several tens of centimetres down the soil profile, and the feeder roots deeper again, it indicates that shallow rooted species, especially pasture species, such as rye grass (Lolium perenne) and white clover (Trifolium repens) are going to be less competitive with Californian thistle below ground than deeper rooted pasture species as chicory (Cichorium intybus) and lucerne (Medicago sativa).

Chicory appears to have particular value: one trial looking at mixed species pastures found the plots that had chicory, had very low Californian thistle populations compared with neighbouring plots with high thistle populations that did not have chicory even seven years after the trial started [2]; In addition chicory is a host to the sclerotinia fungus that attacks Californian thistle and that has been researched as a bio/mycoherbicide. There are reports of pure chicory fields used for seed production succumbing to sclerotinia which also eradicated the Californian thistle present in the fields. These are clearly interesting observations that need following up with more investigative experiments.

Other farmers report considerable success using highly competitive green manures / cover crops, such as a mixture of triticale (× Triticosecale) or rye (Secale cereale) with a climbing legume such as vetch (Vicia sativa) during the time Californian thistle actively growing, i.e. over spring and summer. Some report dramatic reductions in Californian thistle numbers following such crops. Such vigorous cover crops may well be achieving such an effect through both root and shoot competition, and rye and triticale are known to be allelopathic which may possibly be adding to the suppressant effect. Again this is an area farmers are leading the research, and more experiments are needed to optimise such approaches.

Another key benefit of using plants to fight Californian thistle, is that doing so can yield a double dividend, in that such techniques, even in the absence of thistle, can increase profit, and, they don't require fuel intensive tractor operations such as subsoiling and mowing, so can be much more financially attractive than getting the mower or cultivator out.

And, relating back to the potential for goats to control Californian thistle compared with sheep and cattle, where stock are avoiding thistles, but grazing the rest of the pasture species, this is giving Californian thistle a (significant) competitive advantage over the pasture, especially where there is tight grazing, than where the pasture is ungrazed. Research from the UK looking at Californian thistle management in highland and other biologically sensitive / traditional upland grazing areas, found that (somewhat counterintuitively) reducing grazing pressure in autumn, and thus allowing the pasture to grow up and compete more with the thistles produced a large reduction in thistle biomass the following year [3].

However, despite some initial positive research and farmer and grower experience in this area a lot more research is required to really maximise the potential of using pasture species, cover crops and other types of plant competition and to also dispel potential myths out there.

4. Conclusions

Californian thistle is a major problem weed for a wide range of producers, from growers, through cropping farmers, to stock farmers across a wide range of climates and land topographies in NZ and globally. With a growing realisation that chemical / herbicide control of weeds faces an increasingly uncertain future, the need for effective non-chemical management techniques continues to grow (see 'Challenges for pest management' in New Zealand in this issue). While there are some well developed techniques, principally mowing and shallow tillage, that when used at the right times can achieve good management over several years, other less well know techniques, such as subsoiling, integrating goats into stocking systems, especially for country too steep for easy machinery access, mowing in the rain and fighting fire with fire by using ‘good’ plants to defeat Californian thistle, also have considerable potential, but many of these also need research to underpin them and refine them, to get the best bang for your buck (or doe!).

5. References

1.         Davies, B., Finney, B., and Eagle, D., Resource Management: Soil. 2001, Tonbridge: Farming Press Books. ISBN 0-85236-559-4

2.         Musgrave, D.J. and Daly, M.J. Assessment of the performance of non-ryegrass pasture mixtures. in Proceedings of the New Zealand Grassland Association. 2004. http://www.grassland.org.nz/publications/nzgrassland_publication_449.pdf

3.         Pywell, R.F., Hayes, M.J., Tallowin, J.B., Walker, K.J., Meek, W.R., Carvell, C., Warman, L.A., and Bullock, J.M., Minimizing environmental impacts of grassland weed management: can Cirsium arvense be controlled without herbicides? Grass and Forage Science, 2010. 65(2): p. 159-174. http://dx.doi.org/10.1111/j.1365-2494.2010.00735.x

The BHU Future Farming Centre

Information - The FFC Bulletin - 2014 V4 October

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The final frontier: Non-chemical, intrarow, weed control for annual crops with a focus on mini-ridgers

By Charles Merfield

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Summary

  • Controlling weeds non-chemically in the interrow is now a straightforward task due to modular interrow hoes and automatic tractor and implement steering systems.
  • The intrarow, and especially close-to-crop-plant weeds, are now the final frontier for non-chemical weeding in annual row crops.
  • There are two main approaches to intrarow weeding:
    • ‘discriminatory’ (high tech) weeders driven by computer vision or sensors that weed around crop plants:
    • ‘non-discriminatory’ (low tech) weeders where the crop is resistant / tolerant to the weeding technique while the weeds are susceptible.
  • There are pros and cons to both techniques, as they suit different situations, but a key and inherent limitation for discriminatory machines is they cannot kill close-to-crop weeds, while non-discriminatory weeders can.
  • Deciding among non-discriminatory weeders requires an understanding of how they kill weeds, viz: uprooting, severing / breaking and burial.
  • Different tools kill weeds by these three modes in different proportions and with varying aggressiveness. Non-discriminatory tools also handle soil and weather conditions (texture, compaction, stones, wet vs. dry) very differently.  It is therefore essential to correctly match the tool to the situation.
  • However, of all the non-discriminatory weeders described, including finger weeders, torsion weeders, vertical spring tine weeders, and thermal weeders, the mini-ridger stands out as having a great combination of simple engineering, working in a wide range of crops and weeds, handling diverse soil conditions and maintaining a high weed mortality in wet conditions, and ability to reliably achieve 100% weed kill, including close-to-crop weeds. 
  • The key issue for mini-ridgers is it is impossible to tell in the field if the burial depth will kill the weeds, unlike the instantly obvious effects of other weeders.  The FFC has therefore undertaken preliminary research to determine a rule-of-thumb for minimum burial depths. 
  • Initial indications using potting mix are considered to require burial depths considerably in excess of field experience, however, they do cast doubt on the idea of timing weeding operations at the white thread stage, particularly where the majority of weed death is caused by burial.  The FFC will be undertaking field trials in the 2014-15 season to build on these results. 

Introduction: The interrow is under control

Non-chemical weeding machinery has made huge advances over the last couple of decades, particularly compared with the machinery in use before the widespread uptake of herbicides in the 1950s.  Out of all the great diversity of machines and techniques that have been tried, two approaches stand out as the foundation of modern interrow hoeing in annual crops:

  • First, the development of modular parallelogram tool frames (Figure 1), that have allowed hoes to achieve excellent depth control across a wide range of widths, including individual toolbars in excess of ten meters, and multi unit machines with total working widths in the 25 meter range. 
  • Second, GPS and computer vision steering systems for both tractors and implements, which have transformed interrow hoeing from a highly specialised and mentally demanding job, often requiring mid-mounting on tool-carrier tractors and top-end drivers, to ‘just’ another standard tractor job using three-point mounted equipment. 

Figure 1.  Standard design of modern interrow hoe using modular parallelogram toolframes (red) being steered by a computer vision guidance system (green). 

These two technologies have revolutionised interrow hoeing from being a task only undertaken when there is absolutely no other alternative, to one that can fit alongside herbicides in the toolbox as an equally effective and even cheaper weed control option.  Controlling interrow weeds without herbicides is now a straightforward and reliable task thanks to autosteer and modern interrow hoes.  The action has therefore moved to the intrarow (within the crop row) and especially the ‘close-to-crop-plant’ weeds that exert the highest competitive effect against the crop (due to their proximity to individual crop plants).  However, while GPS, computer vision and modular hoes have made interrow hoeing straightforward, intrarow weeding is a much tougher challenge. 

The final frontier: the intrarow and close-to-crop weeds

There are two main approaches to dealing with intrarow weeds: 

  • ‘Discriminatory’ weeders (high tech);
  • ‘Non-discriminatory’ weeders (low tech).

Discriminatory weeders (high tech) are powered by some kind of ‘intelligence’ that allow them to identify what is a crop plant and what is a weed and therefore ‘discriminate’ between them.  They then apply an aggressive weeding technique, e.g. hoe blade or flame to kill the weeds, but, if they get it wrong and misidentify a crop plant as a weed, then the weeding technique will kill the crop plant. 

Non-discriminatory weeders (low-tech) don't have the ‘intelligent’ identification system of the high tech approaches, instead, they rely on the crop plants having a (much) greater resistance / tolerance to the weeding technique than the weeds; they then apply the weeding technique equally to both crop and weeds at the same time, with the result that weeds are killed but the crop survives.

Discriminatory weeders

Discriminatory weeders use two means of discriminating between crop and weed plants:  (1) computer vision systems (2) sensors such as wands and light beams. 

Computer vision

Computer vision systems are truly NASA grade technology - trying to differentiate between lots of similar, small, green, crop and weed plants against a highly variable background of soil, over a wide range of lighting conditions, in an agricultural setting has only recently become possible. The fact that only a handful of people / groups have actually developed machines that are effective in real-world farming, while many more have tried and failed, indicates how difficult it is.  For example videos of these machines see http://www.garford.com/products_robocropinrow.html, https://www.youtube.com/watch?v=bhdeCk5PJGU, https://www.youtube.com/watch?v=qeYyWiLfiYw, https://www.youtube.com/channel/UC01nc4j8eSKXv_jIqIqXPnA (This is neither an endorsement of these companies nor a non-endorsement of other companies with similar products). 

Sensors

The sensor based systems using light beams, wire wands or nudge bars, and are therefore technically simpler than the computer vision systems (no computers or cameras), but the crop plants needs to be significantly bigger than the weeds so the sensor can detect them but detect the weeds.  Once a plant is detected, then the weeding technique / mechanisms is typically no different from the weeding tools used on computer vision systems - indeed the same weeding mechanism could be attached to either a computer vision or sensor crop plant detection system.  The weeding mechanisms typically consist of small horizontal blade hoes that are moved in and out of the intrarow, though there are a number of other, often quite innovative, techniques such as Garford’s kidney shaped rotating hoe and VisionWeeding’s micro-flame banks. 

Sensor based systems are also used in perennial crop (e.g. pip fruit, vines, bushes, etc.,) intrarow (under plant) weeders.  For an example of a sensor based system for annual crops see http://www.plantdetectionsystems.com/ and perennial crops in Figure 2. 

Figure 2. Perennial crop intrarow weeder (power harrow type) with sensor wand to the guide weeder around plants and posts. 

Both computer vision and sensor based weeders achieve the same overall weeding job, as the weeding tools themselves are the same.  The key difference is that computer vision systems can work with smaller crop plants and other situations where physical sensors cannot differentiate weeds from the crop (e.g. the weeds are as large as the crop).  The trade off for the greater abilities of the vision systems is a higher (sometimes much higher) capital cost and complexity (e.g. repairs require a computer technician, not a hammer).  Overall, one approach cannot therefore be said to be better than the other, rather, it is a case of horses for courses and matching the technology to the individual weeding situation, production system and its economics. 

Machine vs. operator intelligence

A key point about discriminatory weeders, like herbicides, is that the smarts that make them work are provided by their creators (engineers for machines, biochemists for herbicides).  The amount of smarts the end user therefore has to supply is pretty limited, some initial setting up, and then ‘just’ driving the machine up and down the field.  This means that operator skill has a lower impact on the outcome, but that is paid for in higher capital costs.  In contrast many non-discriminatory weeders require considerable skill in setting up by the operator and constant monitoring and adjustment to ensure optimum results.  Skilled operators are therefore essential, but the capital cost of the machinery is lower. 

Discriminatory weeds cannot control close-to-crop weeds

However, and this is the big however, due to the nature of the weeding technique, the really critical close-to-crop weeds cannot be killed by discriminatory weeders, because the weeding tools will kill the crop if it gets to close to them, so there has to be a ‘no-weeding zone’ directly around the crop plant.  This is where low tech / non-discriminatory tools, somewhat surprisingly, have an advantage.

Non-discriminatory weeders

Compared with the mostly convergent designs of discriminatory weeders, non-discriminatory weeder designs are highly divergent because there are a multitude of ways of creating a machine that will kill weeds but not crop plants.  It is not therefore possible to cover all the many different designs in this article, but, the major / widely applicable approaches are outlined below.  Importantly, to be able to compare the pros and cons of different designs it is vital to understand how different machines / weeding techniques kill weeds.

How non-discriminatory intrarow weeders kill weeds

There are three fundamental techniques whereby all mechanical weeding machinery kills weeds.  These are:

  • Uprooting;
  • Severing / breaking;
  • Burial.

Uprooting leaves weeds mostly intact, i.e. the foliage and roots are still joined by an intact stem, though some root loss and leaf / upper stem damage / loss may occur.  Weed death then occurs due to the plant no longer being able to absorb water through the exposed root system, and it therefore dies through desiccation.  To be effective, uprooting therefore requires at least dryish soil and in particular, dry weather conditions, otherwise the weeds may be able to re-root before they are desiccated, and therefore regrow.

Severing / breaking is where the weed is cut or broken at, or close to, the hypocotyl (in dicots) or mesocotyl (in monocots) (the region of stem below the cotyledon leaves and above the roots).  The hypo/mesocotyl, is akin to the neck of an animal in that severing, or breaking it, separates the water and nutrient gathering system of the roots, from the photosynthesising leaves, which means the plant is no longer able to grow, and therefore dies.  If the stem is broken rather than severed, the damage needs to be sufficient to stop the phloem and/or xylem (vascular system) working. 

Severing or breaking the weed close to, e.g. less than 1 cm, from the hypo / mesocotyl also generally results in plant death, as there are insufficient roots or stem left to allow the weed to survive. However, if the weed is larger and does has sufficient reserves, then it has the potential to regrow. 

Severing / breaking of the hypo/mesocotyl is instantaneously lethal for small weeds so it is effective regardless of environmental conditions.  However, cutting and breaking the roots or stem close to the hypo/mesocotyl is an aggressive form of uprooting, so it therefore results in higher weed mortality in dry than wet conditions.  However, in reality , it is very difficult to consistently sever or break the hypo/mesocotyl of all weeds present in a field.  So even with weeding techniques (e.g. horizontal knife blades) whose principle aim is to sever weeds at the hypo/mesocotyl, most end up being severed close to, but not through, the hypo/mesocotyl and are therefore technically uprooted, so dry soil and weather are required to achieve maximum weed death. 

Some weeders with a very aggressive weeding technique, e.g. interrow brush hoes, kill by severing and breaking, but in an extreme form where the weeds may be cut into multiple parts and broken throughout their roots and leaves.  This generally results in much higher mortality than simple cutting tools such as knife blades, especially for larger weeds. 

Burial is where the intact weed is covered by a soil layer thus blocking sunlight from reaching the leaves and therefore killing the plant.  To be effective the soil layer / burial depth needs to be sufficient to prevent the plant growing up through the soil to regain access to light.  Burial is therefore mostly independent of soil and weather conditions, i.e. it is as effective in wet conditions as dry, as the plant is intact, the exception is where there is sufficient rain to wash the soil layer off the plants allowing them to restart photosynthesis. 

Matching weeding technique to the situation

Most non-discriminatory intrarow mechanical weeders kill weeds through a combination of uprooting, severing / breaking and burial.  Different weeders use different proportions, and levels of aggressiveness, of the three techniques, which, in turn, determines how effective they are in any given situation.  This means no one weeder is ‘better’ than others overall, as some will excel in some situations but give poor results in others.  The weeding techniques therefore need to be matched to a number of variables including:

  • Weed size;
  • Crop size;
  • Weather conditions (wet vs. dry);
  • Soil conditions:
    • Temporary soil conditions:
      • Soil moisture;
      • Tilth.
    • Permanant soil conditions:
      • Texture;
      • Stoniness.

The permanant soil conditions of texture and stoniness will completely rule some weeders in or out as they cannot deal with some situations, e.g. stones.  The rest of the variables will all need to be considered on a case by case basis.  However, it is unlikely to be economically viable to purchase every possible weeding tool in case it may be the best, so, the usual compromises have to be made to choose an optimum set of weeders or just one weeder, that will achieve the best result overall. 

Thermal weeding

And, to complicated matters, thermal weeding, i.e. flame and steam weeders, add a fourth dimension to this mix as they kill weeds through heat rather than mechanical action. 

Thermal weeders control weeds by killing all of the aerial meristems / buds (i.e. above the hypo/mesocotyl) and/or by killing the phloem in a complete ring around the hypo/mesocotyle (akin to ring-barking a tree).  Thermal weeding is therefore akin to severing / breaking the hypo/mesocotyl in mechanical weeding (above), but if not all of the buds are killed, then it is akin to cutting the stem close to the hypo/mesocotyle, and so, if the weed has sufficient reserves (especially bigger weeds) then it can regrow. 

If the thermal treatment kills all the aerial buds of most species of weeds when they are small, then the plant will die regardless of environmental conditions, but, if not all buds are killed, the plant is more likely to die in hot dry conditions and survive in moister conditions.  The exception to this are plants that can regenerate true stems from true roots, but even among the weed species that can do this, most cannot do it at the seedling stage. 

There is also a widely held misunderstanding (including in research and extension publications) about the ‘thumb print test’ where, after thermal treatment, a leaf is pressed between thumb and finger, and if a fingerprint remains, then the thermal weeding has worked.  This is incorrect as it is the buds that have to be killed by the heat not the leaves, and buds require a longer treatment time to kill than leaves as they are protected in leaf axils.  This means, at best, the failure of a leaf to take a fingerprint, means that there was also insufficient heat to kill the buds, but just because a leaf takes a print, does not mean the buds are dead.  The only sure-fire way to determine the treatment speed, is to undertake a speed test a day or two before treatment is required, whereby a decent length of crop, e.g. 10 meters is treated at one speed, then another 10 meters 1 kph faster, over say, five speeds, and then checked for weed death the following day.  To provide a margin of error, a slower speed (e.g. 1 kph) than the speed that achieved 100% kill is then used. 

The most common, non-discriminatory, intrarow, weeders

Finger weeders

Finger weeders (Figure 3) work by breaking up the soil in the intrarow thereby uprooting, breaking, and burying weedlings.  There are a very large range of options on the basic design, including different diameters / sizes, a wide range of materials used for the weeding fingers, from steel, through a range of plastics, fabric reinforced rubber and even brushes.  This means that the weeding action can be varied from very aggressive (amplified by higher speed and down pressure) to exceptionally gentle.  As the weeders are ground driven, only require a simple pivot depth control (not parallelograms), they have a reasonable latitude in adjustment accuracy (i.e. if they are not perfectly set, they will still achieve a good result), and they can work in a wide range of conditions (especially across the many different designs), they are one of the most popular intrarow weeders. 

However, they are most effective against cotyledon stage weeds, and work best in dryer soil conditions and drying weather to help kill uprooted but undamaged weeds.  They also have reduced effectiveness in stony soils.  In optimum conditions weed kill can be about 85% but that can decrease significantly in sub-optimal conditions (wet soil and weather). 

 

Figure 3.  Finger weeders. 

Torsion weeders

Torsion weeders (Figure 4) also work by breaking up the soil in the intrarow, but with more of a shattering effect than the mixing / churning effect of finger weeders.  Also like finger weeders they are most effective against cotyledon stage weeds, and rapidly loose their efficacy as weeds grow beyond two true leaves.  They also require loose level soil and only work in dry soil as the shattering effect does not work when soil is plastic.  They are also unable to penetrate hard soil or be effective where there are stones.  While they can be quite effective, they require precise setup, as only small differences in both vertical and horizontal placement can result in very large variation in the aggressiveness, from no effect at all to killing the crop.  Very good depth control and steering are therefore required.  In compensation, the tools are very simple - just shaped spring steel rod, so are cheaper than finger weeders (though more expensive parallelogram depth control systems are required).  The summation of this means that maximum weed mortality is around 75% and it is often much lower. 

 

Figure 4.  Torsion weeders

Vertical spring tine weeders

The most common / standard vertical spring tine (spring steel wire) weeders are spring tine harrows (Figure 5) however, these are normally too aggressive for row-crops (i.e. cause crop damage) while at the same time they do not have a sufficiently intensive weeding action within the crop row.  They are therefore generally of limited use for focused intrarow weeding. 

Successful intrarow weeding with vertical spring tines therefore mostly requires dedicated equipment. This includes designs such as the rotary spring tine weeder (Figure 5), the vertical spring tine oscillating weeder (Figure 6) horizontal axis rotating spring tine weeder (Figure 6) plus many more designs on this general theme of vertical spring tines. These machines are best suited to thin upright crops such as maize/sweetcorn, leeks, and cereals, while they can cause significant damage to leafy spreading crops, e.g. lettuce, spinach. 

 

Figure 5.  Spring tine harrow (left) rotary spring tine weeder (right). 

  

Figure 6.  Vertical spring tine oscillating weeder (left) horizontal axis rotating spring tine weeder (right). 

These machines kill weeds mainly by uprooting, severing and breaking, and a small amount via burial.  The technique generally requires dry soil and weather, so the tines cause soil shattering thereby maximising uprooting and other weed damage, and weeds that are not directly killed by the weeding action are more likely to die if the weather is hot.  While stones are very unlikely to damage these machines, they will reduce their effectiveness, especially when stones reach a size where they resist being moved by the tines.  The machines also perform better in loose friable soil, and they loose effectiveness in hard-packed soil. 

Depending on the design the weeding action can be quite aggressive, so high levels of weed mortality can be achieved, even reaching 100%.  However, they rapidly loose effectiveness as weed size and rooting depth increases as the force and depth of cultivation needed to uproot the weeds increases exponentially and if achieved crop damage and death would also result. 

Thermal weeder

Some crop plants when they have been growing for a while (e.g. more than a month or two) have stems that are relatively tolerant / resistant to thermal treatment such as flame and steam weeders.  For example the monocots such as sweetcorn/maize and onions have their growing point at the center of their stem so it is well protected from heat, while some dicots have thick stems that can tolerate a moderate amount of heat, e.g., cabbages .  This means that a thermal weeder is able to kill small weeds growing in the intrarow of such crops (Figure 7).  This technique is the dominant thermal weeding technique in North America, but is less know outside that region. 

Figure 7.  Non-discriminatory intrarow thermal weeder operating in sweetcorn. 

However, by the time most crop plants are sufficiently large to be resistant, many of the weeds will also be larger (as most emerged at crop establishment) and so are also more resistant to thermal weeding.  In this situation, thermal weeding can still be of benefit by defoliating the weeds (i.e. reducing their biomass) rather than killing them outright, thus setting them back and giving the crop a competitive advantage.  Non-discriminatory, intrarow, thermal weeding is therefore mostly restricted to a small range of crops that have good thermal resistance and situations where there are thermally susceptible weeds that are abundant. 

Intrarow mini-ridgers

The last non-discriminatory intrarow weeder discussed here is the ‘mini-ridger’.  Despite none of the intrarow weeders being ‘the best’ as all have their strengths and weaknesses, the mini-ridger stands out from the crowd on a number of points:

  • It is definitely the simplest from an engineering perspective being made of only mild steel flat bar;
  • It can work with a wide range of crops and crop growth stages;
  • It can handle many different soil types, structures and stoniness;
  • It is efficacy is mostly independent weather conditions and soil moisture;
  • It can reliably achieve 100% weed kill.

Mini-ridging has been independently invented by many different growers and machinery designers, despite this, it remains little known or understood. 

The technique works by creating a small, e.g. two to six centimetre ridge of soil within the intrarow, thus burying small weedlings but leaving the larger crop plants above the soil mound (Figure 8). 

Figure 8.  Mini-ridger in transplanted cabbages. 

It is therefore akin to potato ridging, but on a much, much smaller scale.  The key to the system is getting the ridger design correct, which rather counterintuitively, the simplest design works best: it being just a flat metal bar with the long edge horizontal to the ground, angled at about 45° to the direction of travel / crop row, with the short edge set vertically (not tilted) and placed in the interrow, such that it funnels a small wave of soil into the intrarow (Figure 9).

 

 

Figure 9.  Various mini-ridger designs: Basic, vertical leg, V design, with two different ridger heights (top left), single blade, rotatable design on a rotary hoe / rotovator with wide crop gaps for field tomato production (top right), V design on a sloping sprung loaded leg (bottom left) and vertical leg V design with adjustable wings mounted behind a V blade hoe (bottom right). 

Typically the design sets a pair of the flat bars in a V shape with a leg attached at the centre of the V with the ridger placed in the center of the interrow, but there are also single blade designs  (Figure 9, top right) which are used where there are very wide interrows, e.g. field tomatoes, squash, maize/sweetcorn, and on outside rows of a bout. 

The critical design criteria is the height of the flat bar, as this determines how much soil is moved laterally, which in turn determines the size of the ridge.  Very simply, a smaller bar height creates a smaller ridge as the bar can only push a wave of soil sideways the same height as the bar, as any excess soil simply flows over the top (Figure 10).  This means that the ridge height can be precisely controlled just by changing the height of the blades.

Figure 10.  Diagram of how the height of the mini-ridger affects ridge height. 

The other main design criteria, are:

  • The angle of the bar to the crop row / direction of travel.  45° (a 90° V shape) is about as shallow as possible otherwise soil will not flow along the front of the blade, while narrower angles e.g. up to 30° (a 60° V) are better suited to higher speeds as they don't throw the soil sideways as much as larger angles.  However, narrower angles require longer blades. 
  • The crop gap i.e. the space between the end of the blade and the crop plants / center of the intrarow.  Generally, lower height blades require a smaller crop gap, so that the blades funnel soil to the center of the intrarow, and conversely, larger blades require a larger crop gap, so that there is a sufficiently wide base to support a larger ridge. 
  • Achieving reasonable depth control:  as excess soil flows over the blades, this means they have a reasonable tolerance to variations in depth, but, if they are too deep, they will no longer create a ridge and will rather start tilling the soil, and if they are too high, they wont pick up sufficient soil to create a ridge.  Some form of depth control is therefore required, typically a parallelogram or telescope system using a depth wheel, mounted on another tool, e.g. a basket or brush weeder frame, or a pivoting system such as the bottom left image in Figure 9. 

The pros and cons of mini-ridging for intrarow weed control

Mini ridging kill weeds entirely by burial, which makes it qualitatively different from the other mechanical approaches such as finger, torsion and vertical spring tine weeders that kill by a mix of uprooting, severing / breaking and burial.  Mini-ridgers also typically bury weeds much deeper (>3 cm) and more consistently than these weeders, where burial depth may be only a few millimetres.  Ridging therefore has more in common with thermal weeding where all of the foliage is destroyed. 

As weed death is caused by depriving the plants of light, it means that it is less affected by soil and weather conditions, except where rain is sufficiently heavy to flatten the ridges or the soil crumbs are so coarse, or stones so numerous and large, they allow light through to the buried weeds.  This makes it an excellent tool for wet early season conditions, and it can cope with harder soils and stones, though large numbers of stones will reduce weed kill, and sticky soils, e.g. those with a high clay and silt content wont flow when wet / plastic. 

Also unlike the other mechanical techniques, where 100% week kill is the exception, mini-ridging has the ability to consistently kill 100% of the intrarow weeds, including close-to-crop weeds, as the ridge affects the whole of the intrarow, compared with other weeding tools where they have a small point of contact (end of the wire or finger) with the soil, so they don't always weed every last square millimetre of the intrarow. 

As mini-ridging is generally gentle on the crop plants i.e. pushing soil up around them, crop plants that would be killed, uprooted or damaged by other techniques, can be weeded with mini-ridges.  The main limitation is that there needs to be a sufficient size difference between the crop and the weeds, such that the weeds can be buried to a lethal depth while the crop remains above the ridge.  This typically means it is restricted to transplanted and large seeded crops.  However, if the requirement for 100% weed death is dropped, and the aim is lowered to achieving only partial weed mortality but also setting some of the weeds back (while they grow up through the ridge) thus giving the crop a competitive advantage, then crops with a much smaller size differential with weedlings can be treated, e.g. carrots at four true leaves.  However, there is no reason to stop at one ridging, so where crops are ridged when small and thus don't achieve 100% weed kill, they can then be ridged again later when they are bigger, with a larger ridge, either again setting weeds back, or achieving a high kill rate. 

Mini-ridging is also a great tool to use in combination with finger, torsion and vertical spring tine weeders as the former puts a soil mound up, and the latter, especially the vertical spring tine weeders do a great job of pulling the mound down again, thus creating the classic potato ridge weeding technique of alternately pulling ridges up and down but on a much smaller scale. 

Optimum burial depth and the white thread stage

One of the ‘problems’ with burying weeds to kill them, compared with uprooting, severing / breaking, is that it is not possible to tell just by looking at the results in the field if it has been effective.  For uprooting and severing / breaking, it is easy to inspect the weeding result and identify if sufficient weeds have been affected and if intensity needs to increase or decrease.  However, for burial, and especially mini-ridging, the weeds are still intact and as death will take days even weeks to occur, so there is no in-field visual indication of success.  What is therefore required is a priori information, aka a rule-of-thumb, of the lethal burial depth for common weeds at a range of growth stages.  Unfortunately, while there has been a lot of research on how deep seeds have to be buried so their seedlings cannot emerge, there has been no research on lethal weedling burial depth.  The Head of the Future Farming Centre, Dr Charles Merfield, along with Drs Simon Hodge and Dean O’Connell of Lincoln University has therefore undertaken preliminary research to study this. 

Five plants (mustard, alyssum, buckwheat, fescue, onion, poppy) chosen to be representative of the size and shape of a range of crop and weed plants were grown in pots in a glasshouse and then buried under five depths of potting mix (0, 2, 3, 4, 6, 7, 10 cm) at four growth stages (seed, cotyledon, two and four true leaves) to determine the minimum burial depth required to ensure 100% plant mortality (Figure 11). 

Figure 11.  The minimum lethal burial depth (i.e. zero survival) of five plant species at four growth stages and seven burial depths. 

The key points from the results in Figure 11 are:

  • There is considerable variation in the lethal burial depth among species at the same growth stages;
  • Generally, the bigger the seedling the greater the burial depth required to kill the plants. 
  • That seeds can survive burial at greater depths than seedlings, in most cases. 

Is weeding at the white thread stage really a good idea?

The latter point, that seeds can typically emerge from greater depths than already emerged seedlings, is of critical importance. In some quarters, much is made of the importance of controlling weeds as early as possible when using physical techniques, because as weeds get bigger they get much harder to kill.  This advice often extends to the ‘white thread stage’ i.e. when a seed has germinated and put out a root and shoot, but before the shoot breaks the surface to complete emergence.  However, the white thread stage is part of the seed classification in Figure 11 so if weeds are buried at the white thread stage, then they will have the same soil penetrating ability as seeds, i.e. burying white thread stage weeds will kill far fewer than if they are allowed to emerge and are then buried.  This also indicates that the advice of weeding at the white thread stage for other mechanical approaches that achieve a significant proportion of weedling death through burial, may be misguided.  There is also a paucity of research comparing the efficacy of weeding at white thread vs. cotyledon stage, so although it appears to be common sense, it may be an example of where common sense may be wrong.  More research is clearly required which the FFC will be undertaking in the 2014-15 season.

Limitations of this initial research

Two issues with this research are: (1) that it is pot based, which is not a perfect simulation of field conditions, and (2) potting mix was used as the burial medium not soil.  The results suggest much deeper burial is required than field experience has established, and it is hypothesised that the plants could grow up through the lighter and more friable potting mix than they could through heavier soil.  The FFC therefore will be undertaking more research, to compare different soil textures with potting mix.  In the interim, it is suggested that 3 cm is the minimum lethal burial depth for small stage cotyledon stage weeds, but it is best to experiment with your particular crops, weeds and soils to determine what works in your individual situation.  As ridgers are so simple to make, the good news is that making a range of sizes is not expensive. 

High tech vs. low tech, to discriminate or not?

Just as neither computer vision or sensors is ‘best’, nor that any one non-discriminatory weeder is perfect, the overall situation for intrarow weeders is one of ‘horses for courses’. 

  • Discriminatory weeders are simpler to operate, but cost more; they can kill big weeds, but they can’t touch the most critical close-to-crop weeds;
  • Non-discriminatory weeders often require a skilled operator, but they are less expensive, even cheap, they typically can kill only small weeds, but they can kill close-to-crop weeds. 

There is therefore a lot of benefit from being able to mix and match tools, within economic constraints.  Discriminatory weeders, have a ‘get out of jail’ ability due to being able to kills larger weeds that escaped non-discriminatory weeders, especially if conditions were wet at the optimum weeding time.  Non-discriminatory weeders can make a good impact on close-to-crop weeds and can work faster for less cost than discriminatory machines.  Table 1 therefore summarises the key differences between discriminatory and non-discriminatory weeders.

Table 1.  Comparison of the key differences between discriminatory and non-discriminatory weeders. 

Discriminatory

Non-discriminatory

Mechanical complexity

High

Low

Price

High

Low

User skill

Low

High

Crop plant size and shape

Vision - small, sensor -large

Variable

Close-to-crop weeds

No

Yes

Kills small weeds

Yes

Yes

Kills big weeds

Yes

Unlikely

Dependence on dry conditions

Moderate

Variable - low to high

Cope with wide range of soil conditions inc. stones

Good

Variable - low to high

Conclusions

No so long ago the only means of controlling intrarow weeds without herbicides was hand weeding.  With the technology (both low and high tech) that is now available, and outlined in this report, it is now possible to get very effective control of intrarow weeds, even in sub-optimal conditions.  In addition other techniques, such as intrarow soil thermal weeding (http://www.bhu.org.nz/future-farming-centre/information/weed-management/istw) currently under development, have the potential to achieve even better intrarow weed control than herbicides.  I think it is therefore fair to say that the non-chemical weeding frontier has been reached, and with future advances, it will well and truly be achieved.