Breeding Strategies for Improving Resistance to Gastrointestinal Nematodes in Wool Breeds of Sheep

Project Overview

Project Type: Farmer/Rancher
Funds awarded in 2010: $17,640.00
Projected End Date: 12/31/2012
Region: North Central
State: Ohio
Project Coordinator:
Kathy Bielek
Misty Oaks Farm

Annual Reports


  • Animals: sheep


  • Animal Production: parasite control, livestock breeding
  • Education and Training: on-farm/ranch research
  • Pest Management: genetic resistance

    Proposal summary:

    Sheep can be a profitable livestock species for diversified farming operations. However, for those interested in foraged-based production, GIN that have become resistant to available dewormers are a serious and growing problem. In common with most sheep producers in the North Central region, we have all identified dewormer-resistant parasites in our flocks. Our sustainability and that of the sheep industry depends on utilizing methods of parasite control other than chemical dewormers.

    The Anderson and Rickard farms have been looking for a solution to the parasite problem for years. Management strategies we have tried include severely reducing the number of grazing sheep, rotating anthelmintics, weaning early, moving lambs off grass to stored feeds, rotating cattle, sheep and hay-making to try to produce parasite larvae-free grass, and growing annual crops. Successful control of GIN remains difficult and stilI requires selective use of dewormers. Dewormer-resistant parasites severely compromise our efforts of control and may force further reduction of sheep numbers or the elimination of sheep altogether from our farms.

    Published research and our work on previous SARE grants has shown that selecting sheep with genetic resistance to parasites is possible and has the potential to reduce parasite challenge to a flock. Sheep considered genetically resistant to GIN have the ability to suppress or resist the establishment of GIN infection. Parasite resistance is heritable, and the degree of genetic variability among individuals within some breeds can be high. This provides producers an opportunity to improve overall parasite resistance within flock by selecting individual sires with above average resistance. Hair sheep, including Katahdin, have shown greater parasite resistance than wooled sheep, with innate and early-acquired immune responses.

    In this two year project we plan to adapt methods identified in Katahdin sheep in previous SARE grants to attempt to reduce the parasite burden in our wooled flocks. Our specific objectives are to:
    1. Demonstrate that progeny of Katahdin rams selected for their genetic resistance to GIN, are likely to have lower FECs than their wooled contemporaries and can be effectively used in a commercial wooled flock to increase overall genetic resistance to GIN in the flock;
    2. Demonstrate the same methods that were successful in identifying resistant sires in Katahdin sheep can be used to identify more resistant sires in a purebred Polypay flock;
    3. Develop a blueprint for other sheep farmers (purebred or commercial, hair or wool) to reduce the parasite burden in their flock by the use of parasite resistant sheep.

    To accomplish our objectives, animals had to be bred in the fall of201O. On the Rickard farm, four unrelated Katahdin rams that had been selected for genetic resistance to GIN were purchased. For year one, 21 mature ewes were bred to each of three Katahdin ram Iambs, and 60 mature ewes were bred to one mature Katahdin ram for lambing in March 2011. Fifteen test lambs will be identified per Katahdin sire plus two IS-Iamb control groups from the wooled sires for a total of 90 lambs.

    Composite FECs on the lambs will be done on each sire group at 8-10 weeks of age, then monthly through August, the time of heaviest challenge, for a total of three sample dates. By sampling statistically significant groups of lambs with the same sire under the same management, regardless of age at weaning, published research and our previous work have shown it is possible to estimate the sire's ability to transmit parasite resistance to his lambs. For year two, each Katahdin ram will be bred to both unrelated 1 st reneration Katahdin cross ewe lambs produced in year 1 and commercial ewes as in year 1. The 2nd year progeny of two Katahdin rams - two groups of 15 lambs per sire (one 1st and one 2nd generation) - will be compared to two control groups from the wooled sires for a total of 90 lambs.

    For year one on the Anderson farm, five rams were bred to 8-10 ewes each for lambing in May 2011. Using the timing described above for a total of two sample dates, fecal samples will be collected on 12-15 lambs per sire for a total of 60 lambs. It is important to establish the variation in FECs in this purebred flock and to identify individual progeny with lower GIN infestation, thus FEC will be done on each lamb. Two ram lambs with lower FECs in year one will be used to sire part of the second year lamb crop and compared to some of the original rams using the same protocol.

    All the test lambs will be ear-tagged, identified by sire, and managed the same on common pasture on each farm. At each fecal sample collection date for lambs, the following information will be collected and recorded: FAMACHA score (a visual way of classifying sheep based on their level of parasite-induced anemia), body condition score, and weight. All lambs in the sire groups that require deworming at any time will be recorded. Whenever possible, dam ID, birth date, age of dam, and type of birth and rearing, and weights will also be recorded. The data will be sent to the Bieleks after each collection date to be recorded and maintained in an electronic database. Periodic reports will be provided to all participants and cooperators.

    Based on past experience on theses two farms, it is possible for the pasture contamination to be so severe that it would overwhelm even naturally resistant lambs. On both farms, levels of pasture contamination will be estimated by monitoring worm egg output by ewes using composite FECs on randomly selected, fresh fecal samples at lambing and at intervals through the summer. Based on these data and previous information collected on these farms, we will use grazing management strategies and selected deworming to provide overall flock parasite control while still providing sufficient worm challenge to measure differences in lamb groups.

    Fecal egg counts will be performed by East Holmes Veterinary Clinic using the McMaster technique. Dr. Shulaw has developed a composite fecal egg counting technique to estimate a group average that will reduce the time and cost of FEC determination. Using this technique to sample all the ewes and the Rickard lambs will allow us to sample more animals and demonstrate it to be a more practical and affordable tool that Dr. Shaver could then offer to other shepherds. Dr. Shaver will hire a veterinary or pre-vet student to assist one of his licensed technicians in performing the FECs and to assist with on-farm sample collection. Hiring a veterinary student will help increase the awareness of parasite resistance problems in a new generation of veterinarians.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.