Abstract
Bull management for reproduction begins at birth. The objectives of this invited review are to provide a brief summary of the impact of nutrition on sexual development and reproductive function in beef bulls, followed by a discussion of management before, during and after the breeding season. Increased calfhood nutrition results in greater testicular development at maturity. Conversely, poor calfhood nutrition results in delayed puberty and reduced testicular development at maturity. Unfortunately, it is not possible to compensate for the effects of poor calfhood nutrition by subsequently providing supplemental feed. Previous research investigating sexual development and reproductive function in bulls 6 to 16 months of age focused on associations of average daily gain (ADG) and body weight with age at puberty, scrotal circumference, sperm production and morphology. Researchers reported no significant correlations between cumulative ADG and any of the factors investigated, and concluded ADG of 1.0 to 1.6 kg per day did not result in excessive scrotal fat accumulation, increased scrotal temperature, or reduction in semen quality. A common recommendation is for yearling bulls to have a body condition score of 5.5 to 6.5 (on a 9-point scale) at the start of the breeding season. Spermatogenesis occurs over a 60-day period; therefore, nutritional effects of inappropriate nutrition (over- or under-feeding) on sperm will have a carryover effect. During the breeding season bulls are usually limited to the same nutritional plane as the cow herd; however, bulls losing an excessive amount of weight should be replaced. Nutritional management of bulls after the breeding season is influenced by age (potential for further growth) and weight loss during the breeding season. Managing nutrition of bulls through early life phases and the first breeding season provides the opportunity for desired sexual development and reproductive function.