Inborn Worm Resistance In Lambs: How To Breed For It

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Inborn Worm Resistance In Lambs: How To Breed For It

Originally published in the July/August 2016 issue of sheep! magazine.

By Tim King – “Parasite resistance in Katahdin hair sheep can be improved by selective breeding. That improved resistance in Katahdins may be able to be passed on to wool sheep breeds through cross breeding.”

Those are the general findings of a decade of on-farm research led by Kathy Bielik of Misty Oaks Farm near Wooster Ohio. Bielek’s research, funded by the USDA’s Sustainable Agriculture Research and Education (SARE) program, involved other farmers across America’s Midwest and Southeast, as well as a number of researchers at various academic institutions.

Kathy and Jeff Bielek have been raising Katahdins for fifteen years and currently have a flock of between thirty-two to thirty-six ewes. Parasite resistance (PR) has always been of concern to the Bieleks.

“Selecting for animals resistant to parasites allows us to graze more animals on our limited acreage,” Kathy Bielek says. “Parasite resistance is one of the signature traits of the Katahdin breed, yet we’ve found that not all Katahdins are equally resistant. And we’ve found that the trait can be lost if it isn’t being actively selected for.”

The Bieleks raise and sell breeding stock. They also use Estimated Breeding Values (EBVs) to select for growth, prolificacy and mature size.

“Following the advice of Dr. David Notter and other researchers we respect, we use a matrix that weighs each of these traits,” Bielek said. “In selecting a ram, we try to improve at least one of these three areas without going backwards on the other traits. It can be a challenge. But EBVs are essential to this process.”

Bielek says that today she’s using many of the techniques developed during the ten years of on-farm research to continue building resistance in the Misty Oaks Katahdin flock.

Identifying Rams With High Resistance to Worms

In 2005 Bielek, in cooperation with nine other farmers in Ohio, Pennsylvania, and Missouri, received a $17,950 grant from the North Central Region of SARE. One of the farmers chose to use a Katahdin and a Hampshire ram on Dorset ewes. All the other farmers used Katahdins.

The purpose of the two year research project was primarily to identify Katahdin rams with the ability to transmit resistance to gastrointestinal parasites to their offspring. The farmers worked with Dr. William P. Shulaw, the State Extension Veterinarian in beef and sheep at Ohio State University and Dr. Charles Parker, professor emeritus, Department of Animal Science, also at Ohio State.

Dr. Shulaw and Dr. Parker helped develop the projects monitoring plan and provided input in analyzing the results. Dr. David Notter—now semi-retired but then Professor of Animal Science at Virginia Tech—performed a genetic analysis on the results. Dr. Notter also directed the National Sheep Improvement Program Genetic Evaluation Center at Virginia Tech.

The farmer researchers analyzed results from four hundred and fifty-six lambs and thirty-one rams using FAMACHA eye-socket red-to-pallid color evaluation, body condition and vigor scoring, and worming history.

Fecal egg counts (FEC) were done on a randomly selected subset of fifteen lambs per sire.

Based on the data—collected by the farmers and analyzed by Drs. Shulaw, Parker, and Notter—Bielek concluded in her final report that:

FECs taken at average lamb ages of thirteen weeks or greater showed clear sire differences. The estimate of heritability at these ages was 0.52, which was consistent with the estimate of 0.54 obtained from NSIP Katahdin flocks and considerably larger than the estimates obtained from various wool breeds around the world. Thus selection to reduce FEC in Katahdin lambs at ages of thirteen weeks or more should clearly be successful.

At all ages, there was a strong positive association between FEC in full-sibling littermate lambs. This result suggests that lambs born to the same ewe have similar FEC, both early (8 weeks) and later (after 13 weeks) in life, Bielek wrote in her project report.

Selection favoring low FEC will be effective in increasing parasite resistance (PR) in Katahdin flocks.

Roxanne’s Ram
Roxanne Newton’s best 2015 ram at 90 days, at home in Hahira, Georgia. His EBVs for FEC were minus 96 at weaning and minus 100 after. (Minus 100—As good as it gets.)

Worm Resistance In  Katahdin Ewes & Lambs

The first SARE research project made clear that lambs from certain Katahdin sires have greater heritability for PR than lambs from other sires.

The research also posed the question of heritability via the mother.

So, in 2007 Bielek applied for a second SARE grant to do on-farm research. The USDA awarded her, and her group, $14,215 over a two year period.

For the new project, three farmers involved in the previous SARE project went on to study rams and ewes that had been selected as replacements, based in part on their low FECs as lambs.

The idea was to assess their adult parasite resistance and also the resistance of their offspring.

They also investigated methods of identifying ewes with reduced FEC during the periparturient rise (PPR) which is also referred to as first challenge, refers to the increase in parasite eggs produced by ewes during a period of four to eight weeks after lambing. The PPR is the major source of pasture contamination in spring.

By the time the 2007 grant proposal was made, Bielek and her group had concluded that taking FECs was the most effective way of selecting for resistance.

“Quantifiable FECs, using the McMaster technique, are the most accurate way to identify potentially resistant animals but they are labor intensive and can be expensive,” Bielek said. “I collect and perform my own FECs. Samples are collected directly from the animal and kept chilled. Using a modified McMaster technique I weigh and then mix the fecal material with a flotation solution. Then, using a special chambered slide, I calculate the results under a microscope.”

The results of the 2007 to 2009 research showed that a low first challenge FEC in a sire or dam lamb is related to their offspring’s FEC at first challenge.

There was also a positive correlation between the FEC at first challenge as a lamb and the subsequent FEC of that ewe during the PPR at lambing.

Additionally, the study showed that a ewe’s average PPR fecal egg count was related to her dam’s average PPR FEC. That is to say, if the dam had a low FEC the daughter was likely to have a low FEC as well.

“We found this project to be more challenging than our previous work on identifying PR sires,” Bielek said. “The issues appear to be complex and are affected by farm-level management factors including grazing management, time of lambing, weather, level of parasite challenge from the pastures, and especially genetics and prior selection for resistance at the farm level.”

Katahdin Ewes
A few of Roxanne Newton’s Katahdin ewes, bred for worm resistance. Pond in background is off limits to sheep: It harbors alligators and water moccasins. But she says, “I don’t manage for parasites: I want my lambs to be exposed.”

Different Types of Resistance &  “Need For Parasite Infection”

Bielek said the project participants learned a number of general things.

“One of the most important things we learned is that there are differences in the parasite resistance of ewes during the periparturient rise,” Bielek said. “Identifying and selecting ewes with more resistance and lower FEC during lactation results in fewer parasite eggs shed on the pastures and so less contamination for lambs.”

“Another important point is that in order to identify differences in ewes or lambs, there has to be a parasite challenge. An average FEC of at least 500 eggs per gram (epg) is necessary. It can be tricky on a small acreage like we have to achieve enough pasture contamination to see these differences without compromising growth and performance of lambs later in the season.”

Roxanne Newton, a Katahdin breeder from Hound River Farm near Hahira, Georgia agrees that sheep must be challenged if you’re going to select for PR. She says she manages sheep and not parasites.

“I don’t really manage for parasites since I want my lambs to be exposed,” Newton says. “I feel like if I’m going to sell sheep that are parasite resistant they better not die from worms, they should never need treatment for worms. And ideally, they’ll pass this trait on to their offspring. If I were managing for parasites, the pastures used for lambing should be rested for several months. However, I intentionally put my weaned lambs back on the pasture the ewes lambed on because I know it’s highly contaminated and I want to determine which lambs are resistant at an early age. I would not recommend this method to anyone that isn’t monitoring the lambs closely and collecting frequent FECs.”

Newton, like Bielek, collects her own FECs. She uses the same collection method as Bielek.

“I collect the first FEC in mid to late April and the second one four to six weeks later,” Newton said. “The exact time varies depending on conditions. Generally, once I start to see FAMACHA scores trending downward on the lambs (getting less pink; more pale), going from ones-and-twos to twos-and-threes with a few fours, I collect.

Sheep Flock
Some of Kathy Bielek’s flock: “In order to identify differences in ewes or lambs, there has to be a parasite challenge.” Grassy camps under trees make friendly habitat for worms.

“FAMACHA is great for determining resilience. But you can have lambs with a FAMACHA score of 1 and still have a FEC of 5,000. Often it’s a single born lamb that’s been getting good nutrition and is able to fight off the worms. But that lamb is depositing eggs on the pasture that will infest less resilient lambs.”

Newton says FAMACHA scoring has another value as a management tool.

“For me, FAMACHA is not as useful for resistance, since I do so many FEC collections,” she says. “Its strength is in a shepherd’s ability to treat only the lambs that have become anemic from the worm load, based on FAMACHA scores. That saves money on worm remedies. More importantly it preserves their
usefulness. Worms that survive treatment can become resistant to the few such products available. As long as we’re only dosing a small percentage of lambs, those worms that survive will mate with the susceptible worms and keep the next generation susceptible. If all lambs are treated every four to six weeks (whether they need it or not) the only worms left on the pasture are the ones that survived or have become resistant.”

Both Roxanne Newton and Kathy Bielek are part of a broad effort to improve the parasite resistance in Katahdin sheep. Bielek continues her work at
Misty Oaks Farm and she writes and speaks about what she’s learned from her SARE research. Newton, in addition to her work at Hound River Farm is part of a consortium of Katahdin breeders that share their highest performing rams with each other.

Both women are also working to move the resistance they’ve been building in Katahdins into wool breeds. Newton is selecting for PR in Katahdin x Texel crosses made at Hound River Farm. Bielek conducted a SARE on-farm research project from 2010 to 2012 to investigate the heritability of parasite resistance in Katahdin x Polypay crosses.

More about Hound River Farm can be found on the internet at and Misty Oaks Farm is at

Originally published in the July/August 2016 issue of sheep!

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