Effects of winter nutrition and summer pasture or a feedlot diet on plasma Insulin-like Growth Factor I (IGF-I) and the relationship between circulating concentrations of IGF-I and thyroid hormones in steers

Abstract

Effects of two winter nutritional levels (LOW, MOD) and two summer pastures (bahiagrass, BG; perennial peanut, PP) on plasma IGF-I, and the relationship between IGF-I and average daily gain (ADG), thyroid hormones, plasma urea, packed cell volume (PCV) and steer type were determined in 101 steers (217 kg) varying in breed composition, frame size and initial condition. Relationships between body composition or composition of gain and IGF-I were determined in 11 contemporary steers assigned directly to the feedlot. Initial IGF-I (57.9 ± 3.5 ng/ml) was positively correlated (P<.05) to initial condition, estimated percentage of Brahman and plasma T 3, but not related to subsequent ADG. During the 126-day wintering period, ADG was .21 kg for the LOW winter treatment and .47 kg for the MOD winter treatment. Concentration of IGF-I in the wintering period was affected (P<.01) by nutritional level (LOW = 71.8 ng/ml, MOD = 150.6 ng/ml) and was positively related to winter ADG in MOD steers (P<.01) but not in LOW steers. Concentration of IGF-I in winter was also positively related to condition at the end of the winter period (P<.01), T 3 (P<.05) and T 4 (P<.05). There were no effects of winter treatment on IGF-I during the subsequent summer pasture period. During the 145-d summer period, ADG was .53 kg for BG and .68 kg for PP. Concentration of IGF-I during the summer period was affected (P<.05) by pasture treatment (BG = 138.6 ng/ml, PP = 181.9 ng/ml), was positively related (P<.01) to PCV and percentage of Brahman, and was negatively related (P<.05) to estimated percentage of English breeding. In steers assigned directly to the feedlot, IGF-I was correlated with empty body (EB) weight (r=−.59, P<.10), EB water (r=−.59, P<.10) and EB protein (r=−.60, P<.10) at slaughter, and with days on feed (r=−.65, P<.05), but was not correlated with ADG or rate of component gain. These data indicate that IGF-I is related to nutritional status in steers as in other species, that there may be significant breed or cattle type differences in circulating concentrations of IGF-I, and that circulating concentration of IGF-I may be functionally related to plasma concentration of thyroid hormones.