SEXUAL DIMORPHISM OF CIRCULATING SOMATOTROPIN, INSULIN-LIKE GROWTH-FACTOR-I AND GROWTH-FACTOR-II, INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEINS, AND INSULIN - RELATIONSHIPS TO GROWTH-RATE AND CARCASS CHARACTERISTICS IN GROWING LAMBS

The effects of sex and age on pattstuffof circulating somatotropin (ST ) concentration and plasma IGF-I, IGF-II, insulin, and IGF binding pro tein-3 (IGFBP-3) were studied in ram, wether, and ewe lambs (n = 7 or 8) sampled at mean ages of 81 (I1) and 158 d (I2). Between 81 and 158 d of age, rams grew more rapidly than wethers (P < .01), and wethers g rew more rapidly than ewes (P < .01). The sex differences in growth we re reflected in empty body weight at slaughter: rams > wethers > ewes (P < .05). Mean plasma ST concentrations, ST pulse amplitude, and inte grated plasma ST concentrations were greater (P < .05) in rams than in ewes at I1 and I2. Characteristics of the ST plasma profile in wether s were generally intermediate between those of rams and ewes. The inte rpulse interval was greater in ewes than in wethers at I2. The IGF-I a nd IGFBP-3 concentrations were greater in rams than in ewes at both sa mpling times. Plasma IGF-II was greater in ewes than in rams at I2. Me an plasma ST was approximately two thirds less at I2 than at I1 regard less of sex. Mean plasma ST and IGF-I at both ages were positively cor related with growth. Mean plasma ST at I2 was negatively correlated wi th fatness at slaughter. Sex and age significantly affected patterns o f circulating ST and concentrations of IGF-I and IGFBP-3 in prepuberta l growing lambs, under conditions for which growth rates and compositi on were also sexually dimorphic.