Onset of Puberty and Seasonal Fertility in Bison Bulls

Onset of puberty was observed in 12 bison bulls by the collection of semen at monthly intervals for 12 months beginning at 13 months of age. Onset of puberty was defined as the time in development when an ejaculate contained a minimum of 50x10 sperm showing at least 10% progressive motility. At each collection, data were recorded on body weight, semen quality, fecal testosterone concentration, and physical development. Semen was evaluated for gross motility, individual progressive motility, sperm morphology, sperm concentration and volume. From these data bison bulls attained onset of puberty at an average age of 16.5 months and an average body weight of 353 ± 52.8 kg. Age was the greatest determining factor for onset of puberty in this group of bulls. Monthly abattoir collections of epididymal sperm (n=288) and testicular tissue (n=120) were evaluated to determine if bison bulls undergo seasonal changes in sperm production. Although epididymal sperm morphology did not give any indication of seasonal variation, the histological study of testicular tissue showed greater seminiferous tubule diameter (27.0 ± 4.3 μm) during the breeding months (July, August and September) than during any other seasons. Semen collected at 4 different occasions during the year (June, November, January, and April) from live mature breeding bulls (n=21) was used to verify data collected from abattoir samples. Semen from mature bulls showed a significantly greater proportion of normal sperm in June than in November (73.8 ± 9.1%; 44.1 ± 24.3%), respectively. There was little improvement in sperm morphology at the January sampling but in April morphology improved to a level close to that observed in June. Fecal testosterone concentrations were highest in June (128.6 ± 67.4 ng/g) and lowest in April (48.5 ± 33.3 ng/g). Although there was no clear seasonal trend in sperm morphology from bulls sampled at the abattoir, mature bulls showed slight seasonal variations in semen quality.