Effects of high environmental temperatures on physiological and nutritional status of prepubertal Holstein heifers

A series of studies were carried out to determine the effects of increasing environmental temperature (20 to 28 to 33 °C) at a constant (60%) relative humidity on the physiological responses, energy and nitrogen balance of prepubertal Holstein heifers (n = 4, initial body weight = 198 ± 3 kg). The relatively high environmental temperatures of 28 and 33 °C increased rectal temperature by 0.2 and 1.2 °C and increased respiration by 23 and 58 breaths per min, respectively. At 28 °C, dry matter intake was similar to that at 20 °C, whereas at 33 °C dry matter intake was lower (P < 0.10) by about 9%. However, the dry matter digestibility was higher (P < 0.05) at 33 °C than at 20 °C, hence the thermal treatments did not affect daily weight gain. At 33 °C blood urea nitrogen increased whereas hematocrit and plasma glucose concentrations decreased. At 33 °C energy and nitrogen intake were lower (P < 0.10), while energy and nitrogen digestibility were higher at 33 °C than at 20 °C. Urinary nitrogen was elevated at 33 °C resulting in lower nitrogen retention. Although, energy retention as fat did not differ (P > 0.10) among the treatments, energy retention as protein was lower (P < 0.05) at 33 °C than at 20 °C or 28 °C. These results confirmed that the effects of environmental temperature on prepubertal heifers were severe above 28 °C.