Reproductive Capacity of Beef Bulls. I. Postpuberal Changes in Semen Production at Different Ejaculation Frequencies

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Abstract
Semen characteristics and sperm output were determined for 17 Angus and Hereford bulls: seven ejaculated biweekly from puberty to 1 year and weekly from 1 to 2 years of age (1× group), five ejaculated three times weekly from puberty to 2 years of age (3X group) and five ejaculated at 3× from puberty to 1 year and six times weekly from 1 to 2 years of age (6× group). All semen was collected by artificial vagina and three false mounts preceded each ejaculation. By changing the stimulus conditions as needed, only 58 ejaculates, or 2% of the specified number of 2880, were not obtained during the first 60 weeks after puberty. At puberty mean age for the 17 bulls was 44 ± 1 weeks and mean body weight was 321 ± 12 kg. Mean body weight at 2 years of age was 660 ±11 kg. Highly significant increases in ejaculate volume and weekly total and total motile sperm output and a significant improvement in sperm concentration were found as the bulls aged. Sperm motility showed a highly significant increase during the first 20 weeks after puberty, but changed little thereafter. As ejaculation frequency increased, ejaculate volume was highly significantly smaller, but total sperm and total motile sperm per week were highly significantly greater; differences in sperm concentration were not significant. Total motile sperm output from puberty to 2 years of age for the 6× and 3× groups was 3.0 and 2.1 times, respectively, that for the 1× group. Sperm concentration and weekly total sperm output were significantly higher for Hereford than Angus bulls. Bull variation within breed and frequency groups was significant for all semen traits studied. Semen from three unrelated Herefords in the 3× group decreased in sperm motility and concentration during July, August and September, but ejaculate volume showed no summer decline. A bull × season interaction is proposed as the cause of this decline. Ejaculation frequencies up to six times per week were not harmful to semen characteristics or weekly sperm output of young beef bulls.