The relationship of reproductive performance of yearling virgin beef bulls in natural mating systems with semen characteristics, and physical reproductive measurements

Beginning in the year 1979 and continuing for three breeding-calving periods, twelve (12) virgin yearling Angus bulls at the University of Tennessee Plateau Experiment Station (PES) and eight (8) Polled Hereford bulls at the Tobacco Experiment Station (TES) were evaluated. The bulls were first electroejaculated within one week prior to exposure to females the first of April and then again during the first week of July following removal from the breeding pasture.

The semen was evaluated for various qualitative traits which were: volume of ejaculate, color of ejaculate, concentration of sperm within an ejaculate, vigor score, and the percent of sperm cells that were motile.

Sperm cell morphological data were collected. The parameters examined were: the number of normal cells, cells with broken heads, misshapen heads, bent tails, coiled tails, shoehooked tails, and immature sperm. The integrity of the acrosomal cap (galea capitis) was recorded as the number of disrupted caps and the percent intact acrosomes (PIA). Scrotal circumference and testicular tone were also measured.

Reproductive efficiency was calculated for each bull as the mean number of estrous cycles exhibited by the females from the time the bull was placed in the breeding pasture until conception occurred.

Results indicate highly significant correlations between and within some semen quality, sperm cell morphological, and physical reproductive traits.

The conclusions of this study reflect a need for refinement of evaluation techniques for yearling virgin beef bulls to more accurately predict their breeding efficiency by scrutinizing existing parameters believed to be influential.