EFFECTS OF LOW-ENERGY DIETS FOLLOWED BY A COMPENSATORY DIET ON BODY-WEIGHT GAIN AND PLASMA-HORMONE CONCENTRATIONS IN BULL CALVES

Holstein bull calves at 138.4 d of age were fed one of four diets that contained 2.28, 2.43, 2.61, or 2.8 Meal of metabolizable energy/kg of dry matter and 10.8, 11.7, 12.8, or 13.9% crude protein, respectively , for 77 d followed by a diet that contained 2.80 Meal of metabolizabl e energy/kg of dry matter and 13.9% CP. During the energy restriction period, the metabolizable energy of the diets was positively correlate d with the plasma concentration of insulin-like growth factor-I, which was positively correlated with daily body weight (BW) gain during thi s period and the plasma concentration of total thyroxin. During the fi rst 37 d of the realimentation period, compensatory growth occurred, a nd the rate of increase in plasma concentrations of insulin-like growt h factor-I was positively correlated with that of daily BW gain. At d 37 of the compensatory period, the mean plasma concentration of total thyroxin in calves in three of the four groups did not differ signific antly; only the concentration of total thyroxin in the plasma of calve s fed the highest energy restricted diet was significantly higher. The mean BW of calves in groups fed the high energy diets during the rest riction period tended to be heavier even after 158 d of the realimenta tion period.