PSV-7 - Sire Fecal Egg Count EBV are Predictive of Offspring Response to an Artificial Gastrointestinal Nematode Infection in Rambouillet Sheep

Gastrointestinal nematodes (GIN) pose a significant risk to the health and productivity of Rambouillet sheep, a noted wool and meat producing breed, in the United States. Capturing phenotypic measurements of Rambouillet while infected with GIN is essential for the calculation of estimated breeding values (EBV) as a selection tool to improve host resistance to GIN. Weaned (age= ~120 d.) Rambouillet lambs (n = 75) were each inoculated with 10,000 Haemonchus contortus L3 larvae and placed in a feedlot. Lambs were offspring of two representative sires, A (n = 26) and B (n = 49), with post-weaning fecal egg count EBV (PFEC) from LAMBPLAN, of -9.94 and +9.20%, respectively. Lamb fecal egg count (FEC), packed-cell volume (PCV) and body weight (BWT) measurements were captured at 7-day intervals for 6 subsequent weeks. Actual FEC were log_e transformed (lnFEC) to meet normality and then backtransformed least squares means (LSM) are reported. lnFEC, PCV and BWT were analyzed using a mixed model with repeated measures including fixed effects of sex, sire and average daily gain (ADG) classification (for lnFEC and PCV), and sex and sire (for BWT). Significant differences (P = 0.0216) in average lnFEC were observed between offspring from sires A [2242 eggs per gram (epg)] and B (2914 epg). Weekly lnFEC differed (P < 0.0001) and a trend identified between sexes (females = 3414 epg vs males = 2712 epg, P = 0.0909). Interestingly, females exhibited a higher average PCV than males, (34.93% v. 31.15%, P < 0.0001), suggesting potential pathological differences by sex in response to GIN infection. Artificial GIN infections offer a more controlled test than under natural conditions and are a viable option to aid in identifying Rambouillet sheep more resistant to GIN. These results indicate EBV calculated with FEC from a natural GIN infection are predictive of the response of Rambouillet lambs to an artificial infection of H. contortus.