Date of this Version
Rams express differences in sexual performance during the breeding season. Breeding rams with high sexual performance scores as measured during a sexual performance test can improve flock fertility. Whether selecting rams for high sexual performance score will indirectly improve ewe reproductive performance is not known. The objective of this study was to estimate the genetic correlation between sexual performance scores of rams and reproduction of ewes. Sexual performance scores of rams and reproductive performance (number of lambs born per ewe exposed and number of lambs weaned per ewe exposed) from four breeds were analyzed with univariate and bivariate animal models using REML. A total of 4685 records for sexual performance scores of rams were obtained from the US Sheep Experiment Station (Columbia, n = 807; Polypay, n = 1668; Rambouillet, n = 1208; and Targhee, n = 1002). Reproductive performance of ewes was based on 35,154 records (Columbia, n = 7693; Polypay, n = 9229; Rambouillet, n = 10,954; and Targhee, n = 7278). Estimates of heritability for sexual performance score of rams were larger for the Columbia (0.31±0.09) and Polypay (0.30±0.08) than that for Rambouillet (0.14±0.07) and Targhee breeds (0.17±0.08). Overall breed heritability estimate was 0.22 ± 0.04. Heritability estimates for number of lambs born were larger (0.05–0.11) than for number of lambs weaned (0.02–0.05). Estimates of genetic correlation between sexual performance score and number of lambs born were near zero (−0.09 to 0.02) except for the Columbia breed (0.24±0.20). Estimates of genetic correlation between sexual performance score of the ram and number of lambs weaned varied by breed (Columbia, 0.28±0.26; Polypay, 0.00±0.25; Rambouillet, −0.17 ± 0.25; and Targhee, 0.32 ± 0.28). Overall breed genetic correlations of sexual performance of rams with number of lambs born and weaned were 0.00 ± 0.10 and 0.00 ± 0.12, respectively. Because of the low heritability of ewe reproductive traits and their apparently nil to low genetic correlation with sexual performance scores of rams, selection and use of rams with high sexual performance scores would not be expected to result in much indirect response for improved reproduction of ewes.