Effects of dietary energy on scrotal surface temperature, seminal quality, and sperm production in young beef bulls.

The objective of this study was to determine the effect of dietary energy, breed (British vs Continental x British crosses), and their interactions on scrotal surface temperature (SST), seminal quality, and sperm production in bulls. This experiment, replicated over 2 yr, included 72 Angus, Angus x Simmental, or Hereford x Simmental bulls fed either a moderate- (100% forage) or high-energy (80% grain, 20% forage) diet for 168 d after weaning. At the end of the feeding period, SST was determined by infrared thermography, seminal samples (two ejaculates) were collected by electroejaculation, and reproductive tracts were collected at slaughter. Bulls fed the high-energy diet were heavier (P < .0001; diet x time interaction), had thicker backfat (P < .05; diet x line x time interaction), and had a larger scrotal circumference (P < .05). Testicular tone decreased over time (P < .0001) with a diet x time interaction (P < .05). There was no significant effect of diet on top, bottom, or average SST. However, bulls fed the moderate-energy diet had a larger (P < .02) SST gradient (3.9 vs. 3.4 degrees C). Bulls fed the moderate-energy diet had more (P < .01) morphologically normal spermatozoa (68.8 +/- 2.1 vs 62.5 +/- 2.5%) and a higher proportion (P < .006) of progressively motile spermatozoa (53.4 +/- 2.1 vs 44.5 +/- 2.4%). No effects (P > .05) of dietary energy on epididymal sperm reserves or daily sperm production were detected. Increased dietary energy may affect scrotal or testicular thermoregulation by reducing the amount of heat that can be radiated from the scrotal neck, thereby increasing the temperature of the testes and scrotum.