Adding gastrointestinal parasite resistance to the breeding objective in hair sheep: Initial steps

Authors: FORBES, ROBERT - University Of Nebraska; Murphy, Thomas - Tom; Burke, Joan; NOTTER, DAVID - Virginia Tech; SPANGLER, MATTHEW - University Of Nebraska; MACNEIL, MICHAEL - Delta G; LEWIS, RONALD - University Of Nebraska

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Gastrointestinal parasite resistance is a heritable trait important to breeders of hair sheep. However, the U.S. Hair Sheep Index, provided by the National Sheep Improvement Program, omits parasite resistance. Selection indexes were designed to continue improving total weight of lamb weaned per ewe lambing (TW), the current goal, and reduce post-weaning fecal egg count (PFEC), an indicator of parasite resistance. These indexes placed varying emphasis on PFEC relative to TW. Annual genetic gains in TW and PFEC were predicted. Measurements for the traits used to construct the indexes were simulated to compare selection decisions—the number of rams and ewes chosen in common—between the current and the alternative indexes. Placing 79% of the selection emphasis on TW, and thereby 21% of the emphasis on PFEC, appeared preferable. With this balance, genetic gains in TW reduced by only 2% relative to the current index, while PFEC was reduced favorably by as much as 8%. Additionally, over 60% of the males and 80% of the females were chosen in common between the current index and the proposed index including PFEC. Parasite resistance, therefore, can be incorporated as another selection criterion, with positive impact on the productivity and fitness of hair sheep.

Technical Abstract: The U.S. Hair Sheep Index was designed by the National Sheep Improvement Program (NSIP) to increase total weight of lamb weaned per ewe lambing (TW). Producers are interested in adding gastrointestinal nematode (GIN) resistance to this breeding objective since many hair sheep enterprises incur substantial economic losses to GIN infection. The NSIP provides producers with estimated breeding values (EBV) for post-weaning fecal egg count (PFEC), an indicator of GIN resistance. Our objective was to assess the effects of including PFEC as another selection criterion and goal trait. Selection index theory was used to construct 11 indexes. First was a benchmark index with TW as the objective. Weaning weight, and number of lambs born and weaned, were selection criteria. An index was then designed with PFEC added as a selection criterion. In 9 more indices, PFEC was also included as a goal trait with a scaled economic value (EV) of -1; the EV for TW was increased from +1 to +5 at +0.5 increments. Selection criteria were modelled as EBV or phenotypes. Annual genetic responses in goal traits were predicted. The top 3% of males and 26% of females were selected. Responses in PFEC were expressed as a percentage of the NSIP mean PFEC of 2,089 eggs/g as in Katahdin sheep. Breeding values and phenotypes were simulated for 200 lambs by Cholesky decomposition and used to generate index scores, with 100 replicates run. Concordances with the animals selected in the benchmark scenario were determined. Using EBV as selection criteria, TW increased by 1.46 kg/yr in the benchmark scenario. However, unfavorably, PFEC increased by 2.24%/yr. When using phenotypic criteria, TW increased by 0.52 kg/yr and PFEC by 0.28%/yr. Once added as a goal trait, PFEC decreased regardless of the EV of TW. However, responses in TW were also reduced, although less so as its EV increased. An EV of +3 for TW appeared reasonable over other choices with 79% of the emphasis placed on TW in the breeding objective. With EBV as criteria, PFEC declined by 7.96%/yr and 98% of the response in TW retained. Also, on average, 64% of males and 80% of females were chosen in common with the benchmark. With phenotypic selection criteria, PFEC declined by 5.13%/yr and 94% of the response in TW was retained; 61% of males and 80% of females were chosen in common with the benchmark. Implementing a more holistic index with EV of +3 for TW and -1 for PFEC would result in balanced responses that increase TW and reduce PFEC.