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

Final report for OS19-124

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
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Project Information


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, including 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 (or goats) 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. No data exists on the impact of parasite resistant EBVs in sheep flocks. There is a large variability in the marker for parasite infection, fecal egg counts, within a breed. Within the Katahdin breed, FEC EBVs range 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 to seedstock and commercial farms and better understand their contribution to sustainable sheep production. 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. We collected data from NSIP flocks to understand the economic benefit of parasite resistant genetics along with superior growth or maternal traits captured by NSIP EBVs and reported their value in terms of farm sales of breeding rams.

Project Objectives:

The objective is to show producers the value of sheep with high resistance by examining farm gate sales of sheep with EBVs generated by NSIP.


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Materials and methods:

Four farms enrolled in NSIP were used. All animal procedures at ARS were approved by the ARS Animal Care and Use Committee, and producers requested to view humane animal handling procedures available from the American Sheep Industry at Katahdin flocks used were the USDA, ARS (AR; >200 lambs/yr), Roxanne and Milledge Newton (GA; 80 lambs/yr), Kathy Bielek (OH; 50 lambs/yr), and Lisa Weeks (VA; 80 lambs/yr). All Katahdin flocks used for this project were enrolled in NSIP, use a minimum of two sires during the breeding season, with a minimum of 10 offspring per sire. Lambs were managed (along with ewes preweaning) as a single group on pasture.  Farmers collected fecal samples for fecal egg count (FEC) analyses from lambs around the first threat of worm infection (usually 60 to 90 days of age), and body weights and FAMACHA scores are determined. A second set of samples/data was collected 30 to 60 days later. Data were entered into NSIP by producers.  Fecal samples are collected directly from the rectum (at least 5 g or large pellets) using lubrication in a plastic bag removing all air, labeled and shipped (can be refrigerated for up to one week) wrapped with newspaper with an ice pack to the Louisiana State University parasitology lab. Feces were analyzed for FEC using a modified McMaster's technique, and, when possible, cultured to determine nematode genera to verify whether Haemonchus contortus (barber pole worm) is the predominant infection.

Producers, whose data will not be disclosed to the public, sent an Excel spreadsheet of FEC, FAMACHA scores, body weights, and EBVs from lambs, along with details of markets that lambs entered into (retained as replacements, sold as market lamb, or sold as replacement) and prices to PI. Data were analyzed using correlations among all variables, regression between sire FEC EBV and offspring FEC and worm infection markers to determine value of FEC EBVs (SAS; version 9.3, Cary, NC). The ARS flock has data recorded from 2004 to present on multiple FEC, deworming treatment based on high FAMACHA score, pedigree data, genomic EBVs that were used to determine relationships between parasite resistance EBVs and need for deworming, production, and other pertinent data to develop economic models. Economists from the University of Arkansas completed an analyses on sale value of breeding rams among flocks.

Expected outcomes: Expanding producer knowledge on organic and conventional worm control, and additional producer publications of exceptional quality (see links below in Outreach). Adoption of parasite resistance genetics in flocks will reduce the number of farms exiting the industry, and increase the number transitioning to organic production.

Research results and discussion:

Genetic selection of sheep for resistance to gastrointestinal nematodes (GIN) has become a priority for pasture-based production of lambs to minimize the need for deworming. The objective of this experiment was to determine the impact of sire weaning or postweaning fecal egg count (FEC) estimated breeding value (EBV) or WFEC, PFEC, respectively, determined by the National Sheep Improvement Program (NSIP) on GIN infection of lambs born in fall or winter.  Katahdin lambs were born in fall (Oct – Nov; n = 459) or winter (Jan – Feb; n = 378) of 2018 – 2021.  Blood samples and feces were collected from the lambs to determine packed cell volume (PCV), FAMACHA scores, and FEC at 60, 90, 120, and 150 days of age.  Lambs were selectively dewormed if anemic. Data were analyzed using mixed models with year, sex, day of sampling as repeated measure, and interactions, and linear regression to look at relationships between GIN traits of offspring and sire EBV. A second objective examined sale of breeding quality ram lambs to estimate whether sale price was impacted by EBV for WFEC and the Ewe Productivity Trait (EPT) of lamb prior to sale and included the ARS and 3 private flocks. The FEC of offspring was positively related to their sire WFEC and PFEC (P < 0.001) in that there was a reduction of up to 8.2 eggs/g in offspring for every 1-unit reduction in FEC EBV. Similarly, sire PFEC was negatively correlated with offspring PCV in that for every 10-unit reduction in sire PFEC, there was a 0.5% increase in PCV (P < 0.001). Sale price effects were dominated by sale type with premiums relative to direct sales from the farm for those animals with premium (NSIP or Katahdin Hair Sheep International Expo or KHSI) sales designations at $1,086/head and producers valuing animals retained for their own breeding herd at a premium of $107/head. The next most impactful variable was EPT where a one SD increase of 2.3 in EPT yielded an extra $39/head whereas a similar one SD decrease in EPT was estimated to result in a $32/head discount. Lambs born in fall (Oct – Nov) were discounted by $6/head relative to those born in winter months (Jan – Mar). Not deworming lambs led to a $29/head premium over lambs that were dewormed likely as a signal that lambs were resistant. A 1 SD changes in WFEC more or less was estimated to be meaningless in the sense that sale value changed by less than $0.01/head. This latter finding of WFEC may be confounded by whether or not a producer dewormed lambs prior to sale. Thus, while the more parasite resistant a sire was, the lower the GIN infection in his offspring. However, apparent sale value of breeding rams may have been associated more with the individual buyer’s desire to meet their flock goals through other or balanced traits rather than solely considering the parasite resistance status of the ram.

Participation Summary
5 Farmers participating in research

Educational & Outreach Activities

100 Consultations
3 Curricula, factsheets or educational tools
1 Online trainings
5 Published press articles, newsletters
1 Webinars / talks / presentations
3 Other educational activities: Presentation given to producers in Romance, AR on April 8, 2022 on current research on sheep and goats including use of genetic selection to increase value to lambs and kids. Abstract presentations were given on use of genetic selection to increase value value of small ruminants during virtual meetings of the American Society of Animal Science (small ruminant extension specialists and educators in attendance), SCC-81 (Sustainable Small Ruminant Production in the Southeastern U.S.), and NC214 (Increased Efficiency of Sheep Production).

Participation Summary:

100 Farmers
10 Ag professionals participated
Education/outreach description:

Numerous producers were educated by PI and farmer cooperators through phone calls, emails, and even texts. Due to the pandemic, travel restrictions remained in place until March 2022. Thus, opportunities were created during online and auction sales of ARS sheep by posting information in ARS newsletters and website (Dale Bumpers Small Farms Research Center : USDA ARS), sending in depth emails to producers, and spending sometimes long phone conversations with producers interested in genetic selection for parasite resistance. A fact sheet has been written and pending review and formatting for the ACSRPC website ( once the technical publication is accepted (In Review). A fact sheet on the peri-parturient rise (increased parasitism around the time of lambing/kidding) was published on the website which outlined the importance of genetic selection of ewes/does to minimize pasture contamination and worm infection in lambs. The first live presentation was made to 30 producers in Romance, Arkansas in early April on genetic selection and increasing the value of offspring. 

Learning Outcomes

50 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation

Project Outcomes

20 Farmers changed or adopted a practice
2 Grants received that built upon this project
1 New working collaboration
Project outcomes:

Infection with gastrointestinal nematodes (GIN) threaten economic viability of small ruminants 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 and 1.4 million goats (NASS, 2021). Multi-drug dewormer resistance of small ruminants is highly prevalent. GIN can cause reduced weight gains, anemia, and death, particularly due to Haemonchus contortus or barberpole worm which is a blood sucking nematode. Genetic selection for resistance and/or resilience to GIN is the most promising means to minimize GIN infection, and with good pasture management and nutrition, can nearly eliminate the need for deworming. Results of the project (increased awareness of the value of parasite resistance and increased use of resistant stock on commercial and seedstock farms) contribute to cleaner manure (fewer chemicals, fewer worm parasites), improvements in regenerative agriculture, and improved resilience of the farm system.


An analyses was conducted on ARS lambs that considered fate of lamb after 150 days of age based on selection goals to improve worm resistance (WFEC and PFEC) and for dams to produce adequate milk in this challenging southern climate with oftentimes poor quality forages. Balanced traits or those in absence of below average EBVs for any particular trait was also considered. Using these criteria, it was not surprising that market or cull lambs had higher FEC and FAMACHA scores and lower PCV, body weight and ADG compared with lambs considered as breeding stock (retained or sold in premium sales). Considerably more of these market lambs required deworming. It is also clear that lambs retained in the ARS breeding program were more resilient to worms (less anemia, higher body weight and average daily gain) compared with premium sale lambs. This demonstrates the value of using NSIP EBVs to select the future generation of sheep within a flock.


Sale type (retained for breeding, sold direct from farm, or sold in a premium sale) of breeding rams had the largest influence on sale price.  Premiums for NSIP ($296/head) and KHSI (Katahdin Hair Sheep International Expo) sales ($637) showed the largest deviation in comparison to farm sales or sales at local auctions. At the same time, animals retained for breeding on the farm were valued higher ($126/head) than those sold. Second in relative impact was the EPT EBV (a Katahdin ewe productivity index that encompasses offspring body weight and reproductive performance) suggesting buyers attribute more importance to this EBV than WFEC.  Also, premia and discounts for ±1 SD changes in EPT resulted in $18/head and -$17/head, respectively.  By comparison a similar change in WFEC led to sale price projections that varied by on $0.02/head and in opposite sign of expectations (a more negative WFEC should be more valuable). Whether the animal was dewormed prior to sale was tertiary in relative impact and resulted in an average $55/head premium for animals that were not dewormed, perhaps as proof of resistance concept to buyers. At the same time, deworming may have been performed as a result of high FEC and as a result, the impact evaluation may have been confounded with WFEC effects. Finally, fall born lambs (only in the ARS flock) were discounted as their parasitic exposure was least and thereby resistance effects may have been masked resulting in lower estimated sale prices or an average discount of $24/head for animals born in fall months from September to December.


It is often said that the sire or ram is half the flock due to his large contribution of genetics to offspring and future replacements. Using NSIP EBVs, particularly with high GIN resistance led to fewer problems in managing GIN in often vulnerable weaned lambs. Resistant FEC EBVs of ram lambs sold had little impact on sale price, whereas the EPT or estimate of dam production was a greater consideration.



Future research could measure the effect of improved parasite resistance in the animal on pasture larval contamination and impact to soil health. Animals with lower parasite infections may express different grazing behaviors which could be measured using smart technologies (GPS, accelerometers), and may be less prone to stressors such as heat stress. Minimal inputs to the animal including chemical dewormers will lead to a more wholesome protein source benefitting producer’s income in marketing a superior product and society for better protein. These could be measured in the future using marketing objectives.

Information Products

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.