Effects of genotype and mating weight on ovulation rate, litter size, and uterine efficiency of Coopworth, Polypay, and crossbred ewes.

Six genotypes were produced by mating Coopworth (C), Polypay (P), and Suffolk (S) rams to Coopworth-type and Polypay ewes. In 1989, BW and ovulation rate (OR) were recorded for 188 naturally ovulating 5- and 6-yr-old ewes representing all six genotypes, and litter size (LS) was recorded for 174 ewes (93%) lambing to the ovulation observed by laparoscopy. Overall OR and LS averaged 1.85 and 1.71, respectively. Mean OR and LS did not differ (P greater than .40) among daughters of the various sire breeds; however, daughters of Polypay dams exhibited both higher OR (P less than .05) and LS (P less than .01) than daughters of Coopworth-type ewes. Heterosis estimates for OR and LS were 6 and 7%, respectively. Overall uterine efficiency (UE) of ewes conceiving to twin ovulations was .83. Polypay-derived ewes exhibited higher UE than other genotypes (P less than .05). Premating BW had significant but small effects on both OR and LS. Overall estimates of OR and LS response to a 10% increase in BW were 5 and 3%, respectively. Only Polypay-sired ewes showed a significant relationship between BW and reproductive performance; the estimated responses to a 10% BW increase were 9% for OR and 6% for LS. Variation in BW had no effect on UE of ewes conceiving to twin ovulations (P greater than .50).