1048 1 Financial support was provided by the Canadian Association of Animal Breeders (Project No. 87024N), the Canada/Alberta Livestock Research Trust Inc., and Agriculture and Agri-Food Canada. Appreciation is expressed to T. Entz, K. Stanford, and G. Wallins for technical support. LRC Contribution no. 3879644. Received June 17, 1996. Accepted November 4, 1996. Effects of Dietary Energy on Scrotal Surface Temperature, Seminal Quality, and Sperm Production in Young Beef Bulls G. H. Coulter, R. B. Cook, and J. P. Kastelic Research Centre, Agriculture and Agri-Food Canada, P.O. Box 3000, Lethbridge, Alberta, T1J 4B1, Canada ABSTRACT: The objective of this study was to determine the effect of dietary energy, breed (British vs Continental × British crosses), and their interac- tions on scrotal surface temperature (SST), seminal quality, and sperm production in bulls. This experi- ment, replicated over 2 yr, included 72 Angus, Angus × Simmental, or Hereford × 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 × time interaction), had thicker backfat ( P < .05; diet × line × time interaction), and had a larger scrotal circumference ( P < .05). Testicular tone decreased over time ( P < .0001) with a diet × 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°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. Key Words: Testes, Scrotum, Thermoregulation, Diet, Energy, Bulls J. Anim. Sci. 1997. 75:1048–1052 Introduction Postweaning dietary energy profoundly affects sper- matogenesis. Most bulls fed moderate-energy diets (100% forage) after weaning had substantially greater reproductive potential than those fed high- energy diets (80% grain, 20% forage; Coulter et al., 1987). Feeding Angus and Hereford bulls moderate- vs high-energy diets from weaning to 15 mo of age resulted in greater efficiency of sperm production (13%), daily sperm production (19%), and epididymal sperm reserves (52%; Coulter et al., 1987). The mechanism by which feeding higher- energy diets decreases reproductive capacity is not clear. Perhaps excess fat in the neck of the scrotum and(or) scrotal tissues insulates the testes and increases scrotal/testicular temperature, thereby decreasing sperm production and seminal quality. The objective of the current study was to determine the effect of dietary energy, breed (British vs Con- tinental × British crosses), and their interactions on scrotal surface temperature, seminal quality, and sperm production. Materials and Methods This experiment, replicated over 2 yr included crossbred and straightbred bulls ( n = 72) of four lines: Angus × Angus, ( AM × AM) ; Simmental × Hereford ( SM × H); Simmental × Angus ( SM × AM); and Simmental × Angus ( SM × AH). Bulls in the Simmental × Angus lines resulted from breeding the same Simmental sires to cows from Angus lines selected for five or six generations for feedlot gain on either high- ( AH) or moderate- ( AM) energy diets (Gilbert et al., 1993). Bulls were fed either a moderate-energy (100% forage) or high-energy (80% grain, 20% forage) diet for 168 d after weaning (Coulter and Kozub, 1984; Coulter and Bailey, 1988).