The U.S. National Sheep Improvement Program: across-flock genetic evaluations and new trait development.

The U.S. National Sheep Improvement Program (NSIP) began in 1987 to provide within-flock genetic evaluations for the American sheep industry. An evaluation of operating procedures and methodology for NSIP began in 1993 and has resulted in across-flock, multiple-trait EPD for three breeds: Targhee, Suffolk, and Polypay. Traits reported in the across-flock analyses included direct and maternal effects on weaning weight in all breeds, postweaning weight at 120 d in Suffolk and Polypay and at 365 d in Targhee, greasy fleece weight in Targhee and Polypay, and wool fiber diameter in Targhee. Number born per ewe lambing was also evaluated in single-trait analyses for all breeds. Genetic parameters were derived separately for each breed. Important genetic antagonisms include an unfavorable genetic correlation of .51 between fleece weight and fiber diameter in Targhee and a genetic correlation of -.55 between direct and maternal effects on weaning weight in Polypay. Estimates of genetic trends between 1987 and 1995 were consistent with perceived breed roles. In Targhee, direct and maternal effects on body weights increased, fiber diameter declined, fleece weight was maintained, and number born declined. In Suffolk, direct effects on body weight increased, but number born and maternal effects on weaning weight did not change. In Polypay, number born and maternal contributions to weaning weight increased, but direct genetic merit for body weight declined. Prospective enhancements to NSIP include methods for genetic evaluation of performance in accelerated lambing and of carcass leanness and development of breeding objectives and selection aids for various production systems.