Tuesday, September 2, 2014

Parasite Resistance and Resilience

With regards to internal parasites, resistance and resilience are two different traits, with different heritabilities. Resistance is the ability to prevent infection. It is quantified by fecal egg counts (FECs), which are an estimate of the number of worms in the animal's gut.

Bucks which shed a lot of eggs onto pasture are not desirable, regardless of their growth rates and other qualities, because they will serve as a constant source of pasture contamination. Even if they don't require deworming, other more susceptible animals may. More importantly, bucks which are not resistant to parasites will sire kids that are more likely to be heavy egg shedders. Parasite resistance is more important in bucks than does because a buck will produce many more offspring than a doe.

Resilience is the ability to tolerate parasitic infection. It is quantified by FAMACHA© scores, which are an estimate of packed cell volume (PVC), a measure of red blood cells. The additional criteria in the Five Point Check© are also measures of parasite resilience:  bottle jaw, body condition, coat condition, and scours, as is weight loss/gain.

It is obvious why you want a buck that is resilience to internal parasites. He will not require deworming or he will require fewer treatments than other goats. Many of his offspring will also be less likely to require frequent treatment. Parasite resistance and resilience are genetically correlated. Historically, the correlation has been weak to moderate among the bucks in the Western Maryland Pasture-Based Meat Goat Performance Test.

To qualify for the sale, a buck must excel in both parasite resistance and resilience, in addition to growth. This year, fecal egg counts were especially high, making it difficult for many bucks to meet the Gold, Silver, or Bronze Standards for parasite resistance.  However, eleven bucks met at least the Bronze standard of performance for parasite resistance:  no fecal egg count above 2000 epg and an average FEC of less than 1000 epg. However, many of these bucks did not meet the performance standards for growth (ADG).

Many more bucks met the standards for parasite resilience, which do not allow more than one anthelmintic treatment nor FAMACHA©  scores of 4 or 5. Bucks with  FAMACHA©  scores of 3 were sometimes dewormed if they displayed additional risk factors, e.g. weight loss, poor body condition, and scours. A buck that was dewormed once, with a FAMACHA©  score of 3, can still qualify as a Bronze buck, if he meets all other standards of performance.

Final egg counts soar
Fecal egg counts on August 28 ranged from 75 to 16550 epg and averaged 3710 ± 3047 epg. The large standard deviation indicates there was a large variation in egg counts, among the 71 bucks that finished the test. The median egg count was 2825 pg. Hopefully, the administration of copper oxide wire particles (0.5 g) will reduce egg counts in the bucks, as they leave the test.

Parasite Resistance (FEC) Summary
Parasite Resistance (FEC) Rankings

Parasite Resilience (FAMACHA©) Summary
Parasite Resilience (FAMACHA©) Rankings