Abstract
Pregnancy rates and calving dates of beef cows in four multiple-sire breeding herds in 1980 and 1981 were used to evaluate bull fertility. The bulls were from Hereford, Beef Synthetic and Dairy Synthetic breed groups, and were measured for scrotal circumference, scored for libido and evaluated for semen characteristics 1 wk before the commencement of breeding. Three yearling bulls were assigned to approximately 70 cows in each of the four herds (a total of 12 bulls) during a 60-d breeding season. The progeny of each sire were identified by blood typing. There was one bull in each breeding herd which sired at least 50% of the calves. The average age was higher in bulls with the maximum fertility (P < 0.05) than those with the minimum fertility within each herd. The bulls with minimum fertility within each herd tended (P = 0.08) to lose more weight during the period between yearling and breeding man those with higher fertility. Age of bull and percent normal spermatozoa were the only traits which were associated (P < 0.05) with bull fertility when data were pooled over the herds. Scrotal circumference, sperm volume, preweaning average daily gain, weaning weight and daily gain in the period between yearling and breeding showed positive but insignificant correlation with bull fertility. The measures of libido did not show any association with fertility. Key words: Cattle (beef), bull fertility, multiple-sire mating
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