Efficacy of methods used to test fertility of beef bulls used for multiple-sire breeding under range conditions.

A study was conducted during the 1982, 1983 and 1984 breeding seasons with 277 crossbred bulls, 1 to 3 yr of age, that were evaluated for physical soundness, testicular development, seminal quality, and both sexual and social behavior immediately before exposure to crossbred cow herds ranging in size from 89 to 329 cows. Crossbred cow herds were exposed to 4 to 24 bulls per breeding group (mean of 14) at a mean female: male ratio of 21.2 +/- .6:1 under extensive range conditions for 31 to 62 d (mean 46.6 d). All resulting calves were blood-typed to determine the number of calves sired by each bull as an estimate of his fertility. The mean number of calves sired by 1- (n = 116), 2- (n = 126) and 3-yr-old (n = 35) bulls was 4.7 +/- .1, 8.2 +/- .1 and 10.5 +/- .1, respectively. A regression model for predicting bull fertility under multiple-sire, range breeding conditions was selected that accounted for 29% of the total variance in fertility. Similar models accounted for a greater proportion of variance in fertility of 1-yr-old (37%) than of 2-yr-old bulls (22%). Due to the large amount of unexplained variation, the model could not predict individual bull fertility precisely. However, this study demonstrated that selection of herd sires with large scrotal circumference, low backfat thickness, low levels of primary sperm defects, and a low number of mounts in combination with a moderate number of services during libido testing would be expected to improve fertility of beef bulls used under extensive range conditions.