Effects of heat stress on rumination activity in Holstein-Gyr dry cows

The objective of this study was to evaluate the effects of heat stress (HS) on: i) time and daily pattern of rumination and ii) in situ degradability in Holstein-Gyr crossbred dry cows. Nine cows, weighing 556.2 +/- 97.1 kg were housed in a climate chamber (average THI=83.1) or a freestall barn (average THI=66.5) randomly assigned to treatments in a crossover design. Cows were fed corn silage ad libitum and 1 kg of concentrate as a maintenance diet. To measure rumination, we used a sensor attached to a collar. The in situ ruminal degradability was evaluated by incubation of concentrate, silage, and diet samples into the rumen of a fistulated cow for 6, 24, 48 and 96 h. The RT was affected by HS (P<0.001) and time of the day (P<0.001). The rumination pattern for control and HS cows had a first peak at approximately 04:00h and the magnitude of this peak in HS was 22.9% less than controls. The second RT peak occurred in the evening (similar to 20:00h), when RT in HS was 27.8% less compared with controls. We observed a reduction in dry matter digestibility (DMD) of concentrate (85.1% vs 77.8%, control and HS, respectively; P<0,05) and also in the slow-degradable fraction of concentrate (93.9% vs 85.1%, respectively; P<0,05). Finally, DMD of the diet reduced from 73% to 70% for animals exposed to HS compared to controls. In conclusion, HS negatively affected e rumination time and in situ ruminal degradability in dry dairy cows.