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 ene rgy, breed (British vs Continental x British crosses), and their inter actions on scrotal surface temperature (SST), seminal quality, and spe rm production in bulls. This experiment, replicated over 2 yr, include d 72 Angus, Angus x Simmental, or Hereford x Simmental bulls fed eithe r a moderate- (100% forage) or high-energy (80% grain, 20% forage) die t for 168 d after weaning. At the end of the feeding period, SST was d etermined by infrared thermography, seminal samples (two ejaculates) w ere collected by electroejaculation, and reproductive tracts were coll ected at slaughter. Bulls fed the high-energy diet were heavier (P < . 0001; diet x time interaction), had thicker backfat (P < .05; diet x l ine x time interaction), and had a larger scrotal circumference (P < . 05). Testicular tone decreased over time (P < .0001) with a diet x tim e interaction (P < .05). There was no significant effect of diet on to p, 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 fe d the moderate-energy diet had more (P < .01) morphologically normal s permatozoa (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 rese rves or daily sperm production were detected. Increased dietary energy may affect scrotal or testicular thermoregulation by reducing the amo unt of heat that can be radiated from the scrotal neck, thereby increa sing the temperature of the testes and scrotum.