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The FFC Bulletin 2013 V1 – “Sort and treat” parasite control for lambs

Written by Molly Shaw. Research by Robin McAnulty and Andy Greer, Lincoln University

How would you like to cut the lamb-drench bill in thirds, while at the same time slowing the development of resistant parasites? A new method to quickly weigh and sort lambs for treatment makes this feasible.

Intestinal parasites in lambs cause the animals to loose appetite, eat less, and fail to grow at a satisfactory rate. The lack of weight gain is because of eating less, rather than a result of energy loss caused directly by parasite feeding. It turns out that some lambs can tolerate a belly full of worms and still eat fine, gaining weight satisfactorily.

Some farmers simply drench the flock on a pre-determined schedule, whether or not lambs are suffering from parasites. It’s a tempting approach. Drenches are cheap and, for the moment at least, effective. The problem with this approach is that sooner or later, every parasite evolves resistance to the chemical used to kill it. The more frequently drenches are used, the faster we select for the resistant parasites in the population, and the faster our chemicals become useless. Just a couple years ago the sheep industry faced the bleak possibility of having no remaining effective chemical wormers, until two new products came out and saved the day. The more pessimistic among us realize that it’s only a matter of time before these also cease to work.


Robin McAnulty at the BHU trial site. Photo credit Molly Shaw.

Other farmers attempt to drench only when needed based on faecal egg counts. That’s an improvement over the calendar approach, but recently we have found that faecal egg counts are actually a pretty poor indicator of when lambs need treating. First, by the time you can count eggs in faeces, the worms have been in the lambs for three weeks, causing appetite loss for at least the last two weeks. Second, there are some lambs that can tolerate surprisingly high parasite levels and still have good weight gain. The reasons for this haven’t been fully worked out, but it appears that it’s the animal’s own immune response to the worms that make it go off its food and fail to gain weight, rather than a direct energy loss from parasite feeding. Those robust lambs don’t benefit from drenching, while lambs that fail to gain weight do.

The real concern is making sure all lambs are putting on weight quickly. So naturally, using a scale to measure that gain directly is the most accurate method of determining which lambs need help.

Several years ago a method for measuring this individual weight gain was tested and verified in Scotland, and researchers found that they could cut the overall use of chemical drenchers by 50% and still maintain excellent weight gain. The method, called Targeted Selected Treatment (TST), involves tagging animals with unique electronic identifiers, running them through a set of scales, then separating them (automatically) into a group that needs treatment and a group that is doing fine on its own. Lambs are weighed every two weeks and each animal’s weight gain is measured, correcting for the natural variation in lamb size due to sex, age, and genetics. Successful trials in dairy cattle have been run in NZ for the past three years and some farmers are beginning to adopt the practice.

A trial to test this “sort and treat” method, suitable for organic producers who cannot use drenches, was begun in 2012 at Lincoln University’s Biological Husbandry Unit and run by Robin McAnulty and Andy Greer of Lincoln University. Instead of using drenches a “hospital paddock” was used that contained a range of ‘bioactive’ forages such as red clover, chicory and plaintain, which the lambs were put on to graze until their weight returned to the desired levels. The forages were chosen based on the literature for their ability to reduce internal parasites and also what local organic farmers use. While these forages are considered to be bioactive, i.e., to contain chemicals that directly reduce worm burdens, the science is not yet clear as to the relative contribution from the plant chemicals inhibiting parasites directly or from other factors such as a higher quality nutrition and reduced parasite loadings on such plants. However, from a farmers perspective that is somewhat academic as it is the end result that really counts.

McAnulty started with land that hadn’t seen sheep for many years, so the trial area was split in two and one half was “sown” with black scour (Trichostrongylus colubriformis) and other half with brown stomach worm (Teladorsagia circumcinct, also known as Ostertagia circumcincta), via pasturing inoculated ewes. Pastures had spectacularly high levels of parasites, a strong challenge to the trial lambs. Each half of the trial had its own hospital paddocks of chicory, plantain, red clover, and ryegrass.

Sixty male Coopworth lambs were conventionally raised at a Lincoln research farm and drenched before arrival with abamectin (1g/l), albendazol (25g/l) and levamisole hydrochloride (40g/l), so they started out the trial parasite-free. They were then rotationally grazed on ryegrass and white clover. Every two weeks lambs were weighed, and those that didn’t make their individually-calculated target weights (about 60% on the first weighing) were shunted off to graze the herbage lay for the next two weeks, after which they were weighed and sorted again.

2012 turned out to be a stellar growth season for ryegrass. Although ryegrass seed was added to the drilled herbage lay seed mixture at only 10%, it ended up dominating the hospital paddocks at roughly 90% of leaf matter. Consequently the 2012 trial wasn’t able to test whether chicory, plantain and red clover consumption benefited the parasitized animals. However, the trial results do show strong promise for the “sort and treat” method of parasite control.

Even without significant chicory, plantain and red clover in the hospital paddocks, just the two weeks grazing on parasite-free pasture gave half the initially “hospitalized” lambs the respite they needed to recover from the effects of the parasites. After the six week weigh-in, only six lambs remained on the hospital paddocks, the others having recovered.

By the end of the trial (12 weeks in), 10% of the black scour-infested lambs and 43% of the brown stomach worm-infested lambs had needed a chemical drench, a huge decrease from the 3-5 drenches a lamb might receive in a conventional calendar-based parasite management system. Remember, this was an extremely high parasite load situation initially (faecal egg counts above 2000 eggs/gram), so Robin expects this year’s trial with more normal parasite loads to look even better.

The trial is being continued in 2013 trial, with the herbage lays sown in strips instead of intermixed, allowing individual species of herbage to flourish, and white clover was added as a nutritional boost rather than ryegrass. The stands look strong and Robin is looking forward to even better results.

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