EFFECTS OF ENERGY ALLOWANCE AND CIMATEROL FEEDING DURING THE HEIFER REARING PERIOD ON GROWTH, PUBERTY AND MILK-PRODUCTION

Twenty-one 6-month-old Israeli Holstein heifers were used to determine the effect of three 6-month feeding regimes on growth, blood concentration of insulin, prolactin and insulin-like growth factor-1 (IGF-1), attainment of puberty and milk production during first lactation. Feeding regimes were as follows. (1) Restricted: the heifers were given food during months 1,2 and 4,5 of an experimental period, to support live-weight (LW) gin of 0.35 and 0.50 kg/day, respectively and during months 3 and 6 the heifers were given food to support compensatory growth. (2) Control: the heifers were given food to support LW gain of 1.0 kg/day (3) Control + C: the heifers were given the same as the control, with a daily supplement of 0.05 mg/kg LW cimaterol (C) for 4 months. The total LW gain of the restricted heifers during the 6 months of the trial was significantly lower than that of the control heifers. Cimaterol improved growth rate only during the first 2 months of its application and its withdrawal was associated with severe LW gain retardation. The feeding regime employed in the restricted treatment was associated with a significant reduction in serum concentrations of insulin, prolactin and IGF-1 during the first restricted phase, followed by an evaluation in the first compensatory phase. During the second restriction-compensation cycle, only the serum concentration of prolactin was significantly reduced. Cimaterol addition was also associated with a reduced blood concentration of the hormones. The animals in the restricted, control and control + C groups attained puberty at LW of 249.2, 277.6 and 304.9 kg (P<0.05), respectively. No effect of the treatments on milk yield was observed. The effect of the feeding regimes on skeletal growth and on metabolizable energy efficiency for growth is discussed.