Manage Lambs Before Weaning for Future Parasite Control

By Sally Colby

Photo courtesy of angeladd/

When it comes to weaning, shepherds usually concentrate on the lamb's birth date and its size and/or weight. Ideally, the lamb ready for weaning weighs about 45 to 50 pounds, and by that time milk production in the ewe has started to decline. In preparation for weaning, lambs should be eating a forage-based diet that includes a concentrate (usually fed as creep) and a mineral supplement.

Pasture-based operations that aim for April and May lambs usually leave lambs with the ewes for the entire summer. Although ewes grazing on new growth can usually support twins for an extended period of time, parasite burdens from spring pasture are a significant stressor to ewes, and milk production may decline more rapidly. Effective management of the parasite burden in the ewe flock gives lambs an easier transition to life without their mothers.

Lambs that are born on spring pasture don't usually acquire high levels of parasites until early summer. If the ewe flock is already being monitored through the FAMACHA diagnostic tool, check lambs at the same time ewes are checked, and be prepared with a plan to manage parasites before lambs have such a heavy burden that they become debilitated. Remember that FAMACHA is only useful for monitoring Haemonchus contortus, or barber's pole worm, and that FAMACHA is based on estimating the level of anemia caused by this parasite.

"When ewes give birth to lambs, they are somewhat immunologically depressed," said Dr. Bill Shulaw, extension veterinarian for The Ohio State University. "Pregnancy and lactation are stressful. The sheep's immune system is not as functional during late pregnancy and early lactation, so they tend to have a much higher egg count when lambs are first born."

Shulaw says that worm eggs can come from two sources: adults from the sheep's intestine that are carried over from the previous fall, and from the periparturient rise. "This is an important source of worm larvae and eggs," said Shulaw, citing a Sustainable Agriculture Research and Education (SARE) study. "Starting before lambing time, egg counts were roughly 400 EPG [eggs per gram of feces]. By about two weeks post-lambing, egg counts started to rise, and by four to five weeks post-lambing, egg counts reached an average of 1,800 EPG. Then the counts started to decline. This is the classic periparturient egg rise, and the decline tends to happen after the ewes start waning in milk production."

Pasture contamination in spring is directly related to the parasite load on that pasture the previous fall and winter. "There can be a lot of larvae left in February, March and April," said Shulaw. "If you turn out in March, animals will consume overwintered larvae. Moisture throughout the winter, even snow cover, affords a level of protection to larvae. Larvae are more likely to be killed by dry weather."

However, the parasite issue for lambs comes early - far before weaning time - and is directly related to the physiology of the immune system. Shulaw says that pastured lambs don't consume a lot of grass until they're several weeks old, and parasite eggs don't show up in lambs' feces until they are about 6 weeks old. Even then, counts are usually low. "Lambs are immunologically naive when they're born, and they remain that way with respect to worms until they're 6 to 8 weeks of age," said Shulaw. "Potential contamination for lambs from July through September can be very high, so while the ewes have started it [pasture contamination], the lambs really magnify it."

Understanding refugia can help the shepherd plan an easier transition for weaned lambs while helping protect their immune system from future parasite burdens.

Refugia is the concept of allowing a portion of the parasite population susceptible to treatment with anthelmintic (deworming) products to survive in order to slow down the resistance to available anthelmintics. Shulaw cites Australian research that shows that if 10 to 15 percent of lambs that are carrying the heaviest parasite burden are untreated and moved to a clean pasture, those nontreated lambs provide sufficient contamination of pasture with unselected worms to help create a change in drug resistance.

"Another method we've explored through the SARE project is to treat lambs as they're moved with their mothers to a clean pasture, but not treat the ewes," said Shulaw. "If that's combined with grazing strategies, like strip grazing and a back fence to prevent them from going back over ground they've already been on, they go across that clean pasture and stay clean, and the ewes provide refugia and reduce selection pressure." Another strategy to preserve refugia is to treat only thin ewes, or those rearing twins or triplets. This maintains a reasonable number of ewes that will still carry unselected worms.

There is evidence that supplemental feeding of a lamb in the form of creep ration helps the lamb's immune system develop, which will help it withstand parasites as an adult. "It's clear that supplemental energy and protein at critical times can make a huge difference in the animal's ability to withstand a parasite load," said Shulaw, citing a study in which pastured lambs were fed corn and soybean meal. "Lambs had counts as high as 5,000 EPG, but we didn't see significant parasitism and had good daily gains."

Shulaw says that if lambs that are intended for market are only on pasture with ewes until weaning, then go to a dry lot, the parasite problem is essentially solved because lambs aren't exposed to parasites. "Adult ewes can handle a contaminated pasture much better than lambs can," he said. "Early weaning of lambs offers the opportunity to use your forage for more ewes, so it may be possible on the same acreage to have a few more sheep."

The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania. Comment or question? Visit and join in the discussions.