Adding Parasite Resistance to a Hair sheep breeding Objective

The U.S. Hair Sheep Index was designed to increase total weight of lamb weaned per ewe lambing (TW) using estimated breeding values (EBV) for direct (DWWT) and maternal (MWWT) weaning weight, and number of lambs born (NLB) and weaned (NLW), as selection criteria. Producers are interested in adding post-weaning fecal egg count EBV (PFEC), an indicator of parasite resistance, to the index to achieve genetic change in both TW and PFEC. First, the effects of adding PFEC alongside TW to the breeding objective of indexes that used either phenotypes or EBV as selection criteria were studied. Since MWWT is not phenotypically recorded, it was combined with DWWT as a selection criterion. The genetic (co)variance matrix among the criteria and goal traits was singular and, thereby, corrected with bending. Indexes were developed using TW as the breeding objective without PFEC as a selection criterion (benchmark), including PFEC as a selection criterion, or adding PFEC as both a selection criterion and goal trait. A scaled economic value (EV) of +3 for TW and -1 for PFEC achieved balanced genetic gains in both traits. Relative to the benchmark, TW retained more than 94% of its genetic response, PFEC favorably decreased by >5%/yr, while ram and ewe selection decisions were similar. To mimic the current United States Hair sheep index, indexes were then developed with DWWT and MWWT as separate selection criteria. Like before, due to its singularity, the genetic (co)variance matrix was modified (bent). The EV of +3 for TW and -1 for PFEC remained the ‘best’ choice: TW retained 97% of its benchmark response, PFEC was reduced by 8.00%/yr, and 73% and 83% of males and females, respectively, were chosen in common with the benchmark. However, index coefficients differed substantially from those in the United States Hair Sheep Index. The coefficient for NLB went from negative to positive while that for NLW was reduced. Those changes were likely caused by the decreased genetic correlation between NLB and NLW after bending. Validation of the effects of applying the revised index on selection decisions in commercial flocks is needed before its implementation by hair sheep breeders.