The Impact of Estimated Breeding Values on Parasite Resistance and Reduced Parasitism in Sheep

Project Overview

Project Type: On-Farm Research
Funds awarded in 2019: $15,000.00
Projected End Date: 03/31/2022
Grant Recipient: USDA-ARS
Region: Southern
State: Arkansas
Principal Investigator:
Dr. Joan Burke
USDA, Agricultural Research Service

Information Products


  • Animals: sheep


  • Animal Production: animal protection and health, genetics, livestock breeding

    Proposal abstract:

    Infection with gastrointestinal nematodes threatens economic viability of small ruminant production in the South and is the major health concern due to widespread GIN resistance to dewormers. Parasitic worms are a year-round problem in the South affecting well over 1 million sheep on more than 230,000 small to mid-size farms that raise small ruminants in the U.S. Gastrointestinal worms can cause reduced weight gains, anemia and death of infected animals. Because of the high prevalence of dewormer resistance (worms that are resistant to all available dewormers), alternative tools that aid in control of worms are extremely important. Some of these are the use of copper oxide wire particles, improved forage programs, use of condensed tanin rich plants, optimal nutrition, and perhaps, most importantly, parasite resistant genetics.

    The National Sheep Improvement Program provides predictable, economically important genetic evaluation information to the American sheep industry by converting performance records into relevant decision-making tools. By using sheep with predictable genetics or performance attributes, including growth, reproductive or maternal traits, and parasite resistance, a superior flock can be built for both breeding stock and commercial flocks (which focus on lamb production). The genetic predictability uses estimated breeding values, which are science-based, industry-tested measures of an individual animal. A study was conducted on the Mickel Brothers Sheep Ranch in Utah comparing lamb body weights born from either NSIP sires with good growth traits or non-NSIP sires selected by the rancher ( Lambs from the NSIP sires weighed over 3 pounds more than non-NSIP lambs at weaning as predicted by EBVs.

    No similar study has been conducted to examine the impact of parasite resistant EBVs. Most of the data on parasite resistance EBVs came from the Katahdin breed, but any breed and goats can submit data. There is a large variability in the marker for parasite infection, fecal egg counts, within a breed. Within the Katahdin breed, FEC EBVs rang from -100 to 769 (-100 is the best EBV for parasite resistance), indicating great potential for genetic selection. A sheep with -50 FEC EBV would be expected to have a FEC 25 percent lower (a 50 percent improvement) than progeny from an individual with 0 percent. If you were to mate a ram with -100 percent to a group of ewes, then you would expect FEC of his lambs to be 50 percent lower than FEC of lambs sired by a ram with 0 percent FEC EBV. Flocks that have selected for parasite resistance have markedly reduced FEC in their flocks and have flock averages of -60 to -70 percent. The need exists to disseminate parasite resistant genetics and better understand their contribution to sustainable sheep production.

    Significant progress has been made in identifying sheep that are superior for parasite resistance in the last few years. This greatly helps with raising sheep sustainably, organically, and in forage systems on pasture. Coupling genetic resistance with better management will allow more producers to be economically successful. Our aim is to increase the number of flocks with parasite resistance and show producers the value of sheep with high resistance. Our research group has been providing guidance to encourage seedstock producers to identify resistant animals. Recent expo sales of a relatively small number of sheep with very high parasite resistance indicates a willingness by several buyers to pay a premium.

    Parasite resistance is the ability of the host or animal to initiate and maintain an immune response to suppress establishment or eliminate the parasite. Traditional breeding programs and selection for traits such as GIN resistance have been used successfully to establish flocks of sheep with high levels of resistance to GIN in Australia and New Zealand. NSIP has partnered with SheepGenetics in Australia, which provides analyses of U.S. flock data to generate EBVs. Based on these EBVs, several flocks in the U.S. also possess parasite resistance.

    Small ruminant production is the Southeastern U.S. has become a viable farming entity for small and mid-sized farms. Profitable farms include hair sheep such as the Katahdin in their operation and serve ethnic markets, conventional American markets, or grass-fed and organic markets. A growing market is providing parasite resistance genetics to large commercial operations who often do not have the infrastructure to collect data, but have a strong need for parasite resistance in their flock. The wetter summers in Texas, a major sheep production state, during the last two years led to parasite problems and a realization that dewormers do not work and genetic resistance is highly desirable. A more resistant flock will be less likely to succumb to losses due to parasites and remain sustainable. The practice of genetic selection of superior animals or those that possess parasite resistance will increase the sustainability of existing flock management and will have particular relevance to organic and pasture-based production.


    Project objectives from proposal:

    In order to meet our objective, farmers will use EBVs to select for parasite resistance in their flock. There are five farms that will be used in this proposed project. Three of these flocks have been involved in previous SARE funded research (OS09-045) and continue to work closely together sharing genetics and have served as role models for other farms. Parasite resistance in these flocks is high, measured by NSIP genetic trends of FEC EBVs, between -60 and -70 percent (-100 percent is the best and represents the top one percentile in the breed). Using NSIP data on the current five flocks which produce 40 to 200 lambs per farm, and sales records of prices paid for replacement stock, a value will be estimated for parasite resistance. Determining savings in reduced need for dewormer, increase weight gains, and reduced death losses are other important savings realized from improved parasite resistance in flocks, and will be estimated using data from the ARS flock.

    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.