SUMMARYThe aim of the current study was to determine how cooling and supplemental recombinant bovine somatotropin (rbST) affect body function with respect to digestion kinetics, digestibility and other variables relevant to milk production in cross-bred Holstein cattle. Ten primiparous cross-bred dairy cattle (0·875 Holstein Friesian×0·125 Red Shindi) were used and divided into two groups of five animals each that were housed in a normal shaded barn (NS barn; non-cooled cows) and in a shaded barn with mist-fan cooling (MF; cooled cows). The cows in each group were supplemented with rbST in early, mid and late stages of lactation with three consecutive subcutaneous injections of 500 mg rbST every 14 days. All cows were fed the same total mixed ration twice daily at approximately 1·1 of assumed ad libitum intake and water was offered ad libitum. During the experimental periods, values of ambient temperatures and temperature humidity index (THI) in the NS barn were significantly higher than in the MF barn, whereas the relative humidity in the MF barn was significantly higher than in the NS barn (P<0·01). The respiration rate and rectal temperature were significantly higher for non-cooled cows than for cooled cows during the daytime whether there was or was not rbST supplementation. Supplementation of rbST for either cooled or non-cooled cows significantly increased dry matter intake (DMI), the efficiency of feed utilization and milk yields (P<0·05). Digesta kinetics using chromic oxide as an external marker showed a high digesta passage rate constant and low mean retention time of digesta in cows either by cooling or supplementation of rbST, whereas no changes were seen for the digestibility of dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent fibre (NDF) and acid detergent fibre (ADF). The half-time of Cr2O3 in the whole digestive tract of cooled cows was lower than those of non-cooled cows and significantly decreased (P<0·05) during rbST supplementation in both groups in all stages of lactation. The magnitude of responses for the digesta passage rate and efficiency of feed utilization were larger in animals supplemented with rbST than in animals under MF cooling only.The main effect of cooling and supplemental rbST was to improve digestion by an increase in the rate of passage of digesta and in turn an increase in feed intake. Digestibility was not influenced by changes in passage rate of digesta either by cooling or rbST supplementation. Milk production in response to rbST supplementation is probably enhanced with cooling. The increased milk production induced by rbST supplementation was mediated by increased efficiency of feed utilization without changes in diet digestibility.
Effect of Growth Hormone on Milk Production and Feed Utilization in Dairy Cows