Assessment of thermal status of somatotropin-injected lactating Holstein cows maintained under controlled-laboratory thermoneutral, hot and cold environments

Two experiments, with a single-reversal design and 12 cows per experiment, were conducted to study the effects of somatotropin on thermal status of lactating cows under laboratory thermoneutral (18 to 22°C, 30-50% relative humidity), hot (24-35°C, 30-50% relative humidity) and cold (-5-+5°C, 50-55% relative humidity) conditions. Throughout the experiment six cows were injected with somatotropin (25 mg/d) and six with sodium bicarbonate. All injections were started the first day of the 30-d natural environmental period (22-35°C and 41-97% relative humidity for summer experiment, and -5-+5°C and 4-25% relative humidity for winter experiment) before the cows were exposed alternatively to laboratory thermoneutral and hot or cold environments. The somatotropin-treated cows had higher energy intake (13 and 17%), heat production (19 and 25%), heat loss (36 and 24%) and milk energy (40 and 49%), then control cows under thermoneutral and hot conditions, respectively. The somatotropin-treated cows had higher heat production (18 and 10%), respiratory heat loss (27 and 17%) and milk energy (14 and 25%) than control cows under thermoneutral and cold conditions, respectively. The increased heat production associated with somatotropin was accompanied by increased heat losses. Part of the increased heat production was attributed to the increased milk energy and energy intake. These results suggest that increased heat production during somatotropin treatment was within the range that could be dissipated by the cows.