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 (approximate to 1 yr old) were fed to be in negative energy balance (n = 14; NEB) or positive energy balance (n = 15; FEB) during four estrous cycles. Each heifer was given PGF(2 alpha) to cont rol the timing of fourth estrus. From 1 d after PGF(2 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 w as greater in heifers in FEB than those in NEB (P < .01 for interactio n of energy balance and bST). Nonetheless, bST did not alter weight or progesterone content of the CL in either energy balance group. In add ition, serum IGF-I during the first 10 d after estrus was not correlat ed to luteal weight and only weakly correlated (r = .3) with luteal pr ogesterone content. We conclude that exogenous bST given during luteal development does not alter growth or progesterone content of the CL i n heifers. Reduced concentration of IGF-I in serum during luteal devel opment does not account for suppressed luteal growth and progesterone synthesis of heifers in NEB.