Changes in the composition of milk and rumen contents in cows exposed to a high ambient temperature with controlled feeding.

Two pairs of Jersey cows were exposed to either 15 or 30 degrees C air temperature; intake of dried grass was controlled, to be equal at both temperatures. Exposure to 30 degrees C caused increases in rectal temperature and in respiratory rates, and decreases infood intakes in both cows. Milk yield decreased by similar amounts at both temperatures, in association with the decreases in food intakes. The fat and protein content of milk decreased significantly at 30 degrees C; the proportion of shorter chain fatty acids (C6-C14) IN THE milk fat also decreased at 30 degrees C. The proportion of acetic acid in the rumen contents decreased significantly at 30 degrees C, in association with a small decrease in pH. The results indicate that changes in the metabolism of cows occurred at 30 degrees C, independently of changes in food intake.