THE EFFECT OF MANIPULATION IN ENERGY ALLOWANCE DURING THE REARING PERIOD OF HEIFERS ON HORMONE CONCENTRATIONS AND MILK-PRODUCTION IN 1ST LACTATION COWS

Fifteen Holstein heifers that were 175 +/- 4.0 d old and at BW of 175 +/- 4.9 kg were used to determine the effect of three feeding regimens from 6 to 12 mo of age on growth, blood concentration of several hormones, and milk production during first lactation. The feeding regimens consisted of two periods, the first lasting for 4 mo and the other for the subsequent 2 mo. For group A (restricted) heifers, the diet during period 1 was restricted to 85% of NRC (1988) recommendations (a daily BW gain of .7 kg); during period 2, a high energy, high protein diet was provided for ad libitum intake. Group B (control) heifers received a diet that corresponded to 100 and 90% of the NRC (1988) recommendations in periods 1 and 2, respectively. Group C (ad libitum) intake heifers received a high energy, high protein diet throughout both periods. Daily BW gains of heifers of groups A, B, and C were, respectively, .625, .768, and 1.100 kg for period 1 and 1.162, .705, and .797 kg for period 2. The different feeding regimens influenced the age at which the heifers achieved puberty but did not affect BW at puberty. Milk production during 250 d of lactation was 7056, 6070, and 5975 kg for groups A, B, and C, respectively. A statistical model that included serum-derived mitogenic activity and serum prolactin of period 2 accounted for 63% of the difference in milk production at first lactation.