Effects of heat stress on in vivo and in vitro ruminal metabolism in fat-tailed ewes

Context Interest in studying heat stress (HS) has increased significantly due to the problems associated with increasing global warming. Heat stress has very destructive effects on the health and performance of livestock. Aims Our objective was to investigate the effects of heat stress on in vivo and in vitro ruminal metabolism in fat-tailed Iranian sheep. Methods Fourteen intact non-lactating and non-pregnant mature fat-tailed Makoei ewes (67.5 ± 2.5 kg BW) were kept indoors for 24 h/day and randomly assigned to HS (33.0–41.0°C and a temperature–humidity index (THI) of =83 for 24 h/day) or thermoneutral (TN; 24.5 ± 2.3°C and a THI of 66.1 ± 2.5) condition in two consecutive experimental periods. At the end of first experimental period, the animals in each group were exchanged with another group. The ewes were fed a total mixed ration two times a day, composed of lucerne hay (33%) and corn silage (1:2) to meet their maintenance metabolisable energy and protein requirements. Key results HS ewes had lower dry-matter (DM) intake than did TN ewes (P < 0.05). HS increased the in vivo DM, organic matter (OM) and neutral detergent fiber digestibility (P < 0.05), but crude protein digestibility was not affected. Total volatile fatty acid concentration and pH were not affected by HS. However, propionate molar percentage was increased and N-NH3 concentration was decreased by HS. In vitro gas production of three different tested feeds was lower in rumen fluid collected from HS than that from TN group, but DM and OM digestibility and methane emission were decreased only in the case of Orchard grass (P < 0.05). Conclusions and implications In general, HS had detrimental effects on DM intake and in vitro nutrient digestibility but increased in vivo nutrient digestibility, and changed microbial population.