Effects of heat exposure on nutrient digestibility, rumen contraction and hormone secretion in goats

The present study investigated changes in nutrient digestibility, rumen fermentation, behavior and hormone (growth hormone (GH), insulin and insulin-like growth factor-1 (IGF-1)) secretion among goats (three male goats) in a hot environment (H, 35 ± 1.2°C; relative humidity (RH), 80 ± 7.2%; 13 days), in a thermoneutral environment (T, 20 ± 0.6°C; RH, 80 ± 3.4%; 20 days), and in a thermoneutral environment accompanied by the same restricted diet as provided in the hot environment (TR, 20 ± 0.6°C; RH, 80 ± 3.4%; 20 days). The following results were obtained: rectal temperature and water intake were higher in the H treatment than in the T treatment or TR treatment, while hay consumption was lower. Crude protein, neutral detergent fiber and acid detergent fiber digestibility was highest in H treatment. The concentrations of acetic acid and butyric acid in the rumen was also highest in the H treatment. Time spent eating in the H treatment was also the highest, followed in order by T treatment and TR treatment. Ruminating time was lower in H treatment than in T treatment or TR treatment, and reposing time was highest in the TR treatment. When eating and ruminating, the amplitude values of the rumen contraction were lowest in the H treatment, as was the frequency of rumen contraction. Excretion of plastic particles was faster in T treatment and TR treatment than H treatment. Heat exposure was associated with world lowered concentrations of total volatile fatty acids and acetic acid in plasma. The plasma glucose concentration was highest in the T treatment, followed in order by TR treatment and H treatment. The plasma GH concentration was lowest in the H treatment, while the plasma insulin was highest in the H treatment. The IGF-1 concentration was highest in the H treatment, followed in order by T treatment and TR treatment. In conclusion, heat exposure in goats decreased feed intake and rumen contraction, but increased digestibility. However, when goats in a thermoneutral environment received the same restricted feeding as they received in the hot environment, digestibility increased without a change in rumen contraction. Between the H treatment and TR treatment, the changes in digestibility were accomplished by coordinate changes in hormone secretion in order to maintain body homeostasis.