Effects of high environmental temperature and high humidity on the physiological and nutritional status of prepubertal Holstein heifers

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A series of studies were carried out to investigate the effects of increasing environmental temperature (20°C, 28°C and 33°C) under a constant (80%) relative humidity (RH) on the physiological responses, utilization of nutrients and energy and nitrogen metabolism of Holstein heifers (n = 4, initial body weight = 251 ± 19kg, initial age = 10.1 ± 1.0 months). All the heifers were fed Italian ryegrass silage, Alfalfa hay cubes and concentrate to meet the heifer’s energy and protein growth rate of 0.8kg/day. The energy balance was measured daily by means of indirect calorimetry on the last 4 days of each temperature treatment. The results were as follows ; 1) The relatively high environmental temperature of 28°C with RH 80% increased the rectal temperature by 1.3°C and the respiration increased by 57 breaths per minute compared to the rectal temperature and respiration at 20°C and RH 80%. At 33°C with RH 80%, the rectal temperature and respiration rate increased by 2.2°C and by 73 breaths per minute, respectively, compared to those at 20°C and RH 80%. 2) Dry matter intake decreased at 28°C and above because of reduced roughage intake. As high environmental humidity reduced the latent heat dissipation from heifers even at 28°C, the increase in body temperature contributed to the decreased dry matter intake. 3) The effects of high environmental temperature on the dry matter digestibility of feed have been attributed to the reduction in the passage rate of digesta and the changes in feed composition because of the decreased roughage intake. 4) Blood alanine aminotrans-ferase, total cholesterol, glucose, alkaline phosphatase and triiodothyronine were significantly decreased as environmental temperature increased ; however, blood urea nitrogen increased. 5) Nitrogen and energy retention significantly decreased at 28°C and above, whereas nitrogen and energy digestibility increased. 6) At 20°C and 28°C, energy retention of body fat was higher than the energy retention of body protein, however, energy retention at 33°C was negative and body fat of the heifers was mobilized. These results show that the detrimental effects of high environmental temperature at 80% RH on the performance of Holstein heifers appeared at 28°C and above, and that the effects of high humidity were even more severe at 33°C.

Effects of high environmental temperature and high humidity on the physiological and nutritional status of prepubertal Holstein heifers

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