Endocrine and Metabolic Changes during Altered Growth Rates in Beef Cattle

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Abstract
Eight steers from a group of 14 were fed ad libitum from 240 to 510 kg live weight, gaining at 1.4 ± .2 kg/d. The six other steers were diet-restricted and grew at .37 ± .09 kg/d from 240 to 307 kg, prior to ad libitum realimentation on the same diet to a final weight of 510 kg. Blood samples taken during the growth phases from both treatments were analyzed for insulin-like growth factor-I (IGF-I), triiodothyronine (T3), thyroxine (T4), glucose (GLU), nonesterified fatty acids (NEFA), and blood urea nitrogen (BUN) and(or) growth hormone (GH). During restricted growth, mean serum concentrations of GH were elevated (45.6 vs 23.4 ng/ml; P < .05), serum concentrations of IGF-I decreased (108 vs 167 ng/ml; P < .05) compared with control steers with ad libitum access to feed. Levels of T4 and GLU also were lower (P < .05) during restricted than during normal growth. During early realimentation, levels of GLU (P < .05), IGF-I (P < .01), T4 and BUN (P < .01) increased. Levels of T3 remained unchanged, whereas concentration of NEFA declined (P < .001). Blood urea nitrogen decreased during early realimentation despite a large increase in diet protein intake and in protein storage, suggesting an increased efficiency of nitrogen use for protein synthesis. During realimentation, IGF-I levels rose above those of control steers and remained higher at the final weight of 510 kg (P < .05). Levels of IGF-I paralleled growth rate and may explain why GH levels can be elevated during periods of reduced growth rate in beef cattle. Copyright © 1989. American Society of Animal Science . Copyright 1989 by American Society of Animal Science