PARASITES AND GENETIC SELECTION
Genetics play a major role in how goats are affected by parasites. However, it is not necessarily easy to breed for reduced parasitism. Some of the major concerns are explained here.
Except for herdsires, Lookout Point does not test goats for parasite "resistance," which is an improved immune response that decreases the number of worm eggs produced. While on the surface, "resistance" may sound appealing, numerous studies have shown that breeding for this trait may actually lead to decreased productivity. This is likely due to the high physiological "cost" of an aggressive immune response. Worse, though, is the fact that unless resistant animals are housed only with other resistant animals, so that the overall pasture egg burden is low, they will likely not perform well. Since it is unlikely that commercial herds will ever be uniformly populated with resistant animals, we have chosen not to rely on this route. (We do, however, assess parasite resistance as a secondary selection criteria for herdsires.)
Instead, we breed primarily for parasite "resilience," which is an improved ability to maintain health and productivity despite parasite challenge. We think this is a more realistic approach, since success does not depend on how many eggs other goats are producing. A resilient animal is prepared to face a pasture full of eggs, and continue to produce.
Research has found that there is no genetic correlation between resistance and resilience. Instead, it is a fork in the road that producers must choose: resistance, resilience, or both? If breeding for both traits is the plan, then those must be selected for separately. Next, producers must decide how to measure genetic potential for the selected trait. Five tools we have tried are:
1) Fecal Egg Counts (FEC). We use the McMaster technique. We used to do extensive FEC testing, but never found a correlation between low FEC and improved performance. We now understand why, since FEC can only measure "resistance," which can be negatively correlated with performance. We now run FEC's only on herdsire candidates, and favor those that produce fewer eggs, so long as they meet all other selection criteria.
2) FAMACHA. This is a method of gauging anemia caused by the blood-sucking Haemonchus contortus. It is ineffective when other worms, such as Ostertagia circumcincta and Trichostrongylus, are dominant. This is usually the case on our farm. Thus, for most of the year, FAMACHA has little value for us.
3) Dag scores. This is a measure of scouring, usually caused by parasites. While we do have a few cases of serious scours every year, the differences between the remaining goats are not significant enough to inform culling decisions.
4) Weight gain under challenge. This is our primary selection tool for yearling animals. In a recent buck test, parasites caused large weight gain differences between un-wormed candidate herdsires over a 100-day winter test period. Read more about our buck selection here.
5) Reproductive efficiency under challenge. Our production does are never wormed, even when parasite immunity is lowered at kidding time (except to prevent death when a goat has failed). Only does that are "resilient" can succeed under these conditions.
We do not think that a goat's genetic value for improved performance can be determined using FEC or FAMACHA scores. The 2011 Western Maryland Pasture-Based Meat Goat Performance Test ranked bucklings by average daily gain (ADG), fecal egg counts (FEC) and anemia score (FAMACHA). They apparently didn't correlate those results, but we did. This recap directly compares the rankings each goat received. It reveals essentially no correlation between ADG, FEC and FAMACHA. For example:
Goat #34 was the best for ADG. But he was 25th for FEC, and 45th for FAMACHA.
Goat #62 was the worst for ADG. But he ranked 20th for FAMACHA.
Goat #74 was 62nd for ADG, but 9th for FEC.
Goat #58 was 5th for FEC, but 69th for FAMACHA.
Based on the results of this study, we conclude that trying to breed for FEC or FAMACHA and hoping that will translate into increased production is probably a fool's errand. We will continue to breed for weight gain and reproductive efficiency under challenge.