Effect of energy balance and somatotropin on insulin-like growth factor I in serum and on weight and progesterone of corpus luteum in heifers.

Holstein heifers (approximately 1 yr old) were fed to be in negative energy balance (n = 14; NEB) or positive energy balance (n = 15; PEB) during four estrous cycles. Each heifer was given PGF2 alpha to control the timing of fourth estrus. From 1 d after PGF2 alpha injection until 10 d after estrus, half of the heifers were injected daily with 14 mg of somatotropin (bST), and the corpus luteum (CL) was collected on d 10. Heifers in NEB had less IGF-I in serum (P < .01), smaller CL (P < .01), and less progesterone in CL (P < .01) than heifers in PEB. Exogenous bST increased (P < .01) IGF-I in serum, but this increase was greater in heifers in PEB than those in NEB (P < .01 for interaction of energy balance and bST). Nonetheless, bST did not alter weight or progesterone content of the CL in either energy balance group. In addition, serum IGF-I during the first 10 d after estrus was not correlated to luteal weight and only weakly correlated (r = .3) with luteal progesterone content. We conclude that exogenous bST given during luteal development does not alter growth or progesterone content of the CL in heifers. Reduced concentration of IGF-I in serum during luteal development does not account for suppressed luteal growth and progesterone synthesis of heifers in NEB.