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November 2017

Impact of Selection and Breed on Resistance

Dr. Ken Andries, Ph.D.

Kentucky State University


With the results we have to date, it would support continued selection based on
eye color score to reduce parasite loads in kids as well as the breeding herd.

Most producers of sheep and goats are experiencing resistant parasites to the common products utilized to control them.  This continues to be a growing issue resulting in recommendations for change in management and selection practices.  Selection for resistance can improve overall parasite status of a herd and reduce the need for treatment.  Finding the individual within the breed that is more resistant is the issue when using selection.

For years, producers have been told they need to utilize selective treatment and cull animals that require greater numbers of treatments. Using the eye color score system (FAMACHA©), we are able to select for resilience, but there is little evidence of the impact on actual resistance using this method.  There is also limited information on the impact of different parasites loads on performance of kids.

Kentucky State University started with a grade Boer herd in 2005 and added Spanish (2010) and Savanna (2011) breeds to evaluate breed differences under Kentucky’s environment.  The herds were selected for production and parasite resilience/resistance by culling any doe that did not bring a kid to weaning and number of dewormings a doe received during a 12 month period. 


We moved the number of dewormings down over the years from 5 dewormings in 2008, to our current standard of culling any doe that is dewormed 3 times, or more, in a year.  We utilize purebred bucks on the grade Boer and Savanna does and have a Spanish herd that is a combination of 4 different heritage Spanish lines. We started evaluating parasite loads in the kids at weaning and 60 days post weaning in 2016.

The average actual egg counts were 677.2 epg at weaning and 1,458.7 epg 60 days post weaning.  We used log transformed egg counts to normalize the data for analysis. At weaning, sex of the kid was significant (P = 0.017), breed was not significant (P = 0.336), nor was year or any interaction.  At 60 days post weaning however, breed and year by breed interactions were significant (P = 0.035 for both) and sex was still significant (P=0.004). The Spanish and Savanna breeds were similar while the Boers had the higher egg count post-weaning. 

To get an idea if parasite load was impacting performance of the kids, we looked at the correlation between log transformed egg count and weight per day of age when the sample was collected.  We found a significant correlation of -0.198 (P=0.026) at weaning, but at 60 days post weaning the correlation was not signification (P=0.363).

The preliminary results from this project do show that there are differences between breeds even when they have been selected for resistance, though the level of infection was relative low overall.  We believe that the reason we are not seeing greater differences is due to the fact that the herds have been selected for parasite resistance/resilience over the years resulting in a herd that is not as susceptible to parasite infection. 


The lack of a significant correlation between weight per day of age and egg counts is an indication that the selection for resilience has resulted in desired performance with the higher egg counts.  More data will be needed to make a final recommendation.  However, with the results we have to date, it would support continued selection based on eye color score to reduce parasite loads in kids as well as the breeding herd.

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