Serum Insulin-Like Growth Factor I Profiles in Beef Heifers with Single and Twin Pregnancies

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Abstract
Serum samples and BW were obtained from 2-yr-old beef heifers, pregnant with either single (SF, n = 12) or twin (TF, n = 7) fetuses, at 7-d intervals from d 190 of gestation until calving. Serum insulin-like growth factor I (IGF-I) concentrations of SF heifers gradually declined from d 190 (69.9 ± 1.0 ng/ml) to d 263 (55.6 ± .8 ng/ml), then exhibited a slight increase by d 277 (63.4 ± 1.1 ng/ml). Serum IGF-I concentrations of TF heifers essentially paralleled, yet were lower (P < .05) than, concentrations in SF heifers for all days tested except d 197 and 205. The SF heifers pregnant with heifer fetuses (n = 6) had higher IGF-I concentrations (P < .1) than heifers pregnant with bull fetuses (n = 6) for all days tested except d 214 and 235. Instantaneous absolute growth rate (IGR) of SF heifers declined from 1.485 kg/d at d 190 to .257 kg/d by d 277. Rate of decline in IGR of TF heifers was much greater (P < .0001). Correlations between serum IGF-I concentrations and IGR for SF and TF heifers were .79 (P < .001) and .59 (P < .05), respectively. These data suggest that number and sex of fetus influence maternal concentrations of IGF-I and that the combined growth rate of the dam and conceptus during gestation is related to serum IGF-I concentration. Copyright © 1988. American Society of Animal Science . Copyright 1988 by American Society of Animal Science