Correlation between production traits and sexual behavior in white-faced yearling rams and Effect of sex of co-twin and breed on ewe flock productivity

In prolific species, such as the sheep, co-twinning with a male fetus changes the uterine environment which may influence life-time flock productivity. Lambing records from the University of Wyoming purebred sheep flocks were analyzed to determine if sex of a co-twin affects number of offspring, flock longevity, or age at first lambing. Breed differences were also evaluated. Nine years of lambing records (ewes born from 1995 -- 2003) for Columbia, Hampshire, Rambouillet and Suffolk ewes (n = 547) were analyzed. Total number of lambs born to each ewe and number of years each ewe remained in the flock was evaluated. As expected number of lambs born (P < 0.001) and years in the flock (P = 0.05) was affected by breed, but there was no breed by co-twin sex interaction (P > 0.4). Suffolk ewes were the most productive with the most number of lambs and longest flock longevity. Flock longevity of Suffolk ewes did not differ ( P = 0.7) from Rambouillet ewes. Columbia ewes had the fewest number of lambs, and shortest flock longevity, but flock longevity did not differ (P = 0.9) from Hampshire ewes. Rambouillet ewes produced the fewest number of offspring but remained in the flock longest. Number of lambs born, but not (P = 0.22) years ewes remained in the flock, tended (P = 0.08) to be affected by twinning. Ewes born as a single (n = 138) had fewer lambs during their productive lifetime than ewes born co-twin with ewes (P = 0.05; n = 193), but did not differ (P = 0.8) from ewes co-twined with rams (n = 216). To determine effect of a male co-twin, ewes born as singletons were removed from the data set. Presence of a male co-twin tended ( P = 0.08) to decrease the number of lambs born but did not affect (P = 0.13) number of years a ewe remained in the flock. Sex of the co-twin did not affect (P = 0.7) age at first lambing. This data suggests that flock productivity would benefit from selection of replacement ewes which are twinned with females.