Reproductive performance of bulls divergently selected on the basis of blood serum insulin-like growth factor I concentration

The objectives of this study were to examine differences in scrotal circumf erence, sperm motility, and percentage of normal sperm cells between two li nes of Angus beef cattle divergently selected for blood serum IGF-I concent ration. Data were obtained from an ongoing experiment involving 100 spring- calving (50 high and 50 low line)and 100 fall-calving (50 high and 50 low l ine) purebred Angus cows. Scrotal circumference, percentage of motile sperm cells, and percentage of normal sperm cells did not differ between high an d low IGF-I line yearling bulls (P = .79, .50, and .56, respectively). The IGF-I concentrations measured at d 28, 42, and 56 of the postweaning test a re abbreviated as IGF28, IGF42, and IGF56, respectively. Coefficients for t he quadratic regression of scrotal circumference on IGF28 and IGF42 tended to be negative (P = .07 and .08, respectively), as did the coefficient for the quadratic regression of the percentage of motile sperm cells on IGF42 ( P = .08). The coefficient for the linear regression of percentage of normal sperm cells on IGF28 was positive (P = .02). The coefficient for the quadr atic regression of percentage of normal sperm cells on IGF56 was negative ( P = .04). Coefficients for the quadratic regression of scrotal circumferenc e and percentage of normal sperm cells on mean IGF-I concentrations were ne gative and important (P = .04 and .08, respectively). Thus, scrotal circumf erence and percentage of normal sperm cells are related to blood serum IGF- I concentration in yearling Angus bulls.