Heat stress modulates rumen fermentation and milk yield in Holstein dairy cows: a randomized crossover study
Background: Rising ambient temperatures threaten the productivity and welfare of high-yielding dairy cows.
Methods: Sixteen multiparous Holstein cows were enrolled in a randomized crossover design with two 21-day periods: thermoneutral (temperature–humidity index, THI < 68) and heat stress (THI 78–82). Rumen fluid, milk yield and composition, and physiological variables were measured.
Results: Heat stress reduced dry matter intake by 12% and milk yield by 3.9 kg/day (P < 0.01). Total volatile fatty acid concentration decreased while ruminal pH and the acetate-to-propionate ratio increased.
Conclusions: Heat stress impairs rumen fermentation independently of intake, supporting nutritional strategies that target rumen function during hot seasons.