Thermoregulation in intensively lactating cows in near-natural conditions.

1. Thermoregulatory reactions of lactating cows (33 kg milk/day) have been measured in summer (25-39 degrees C) and in winter (9.5-24 degrees C) at 3 hr intervals, during four nychthemeral (24 hr) cycles in each season.2. The rectal-to-tympanic temperature gradient increased with rising body temperatures. The seasonal changes were larger for the rectal temperatures than for the tympanic membrane temperatures. These and the significantly lower correlations between rectal temperatures and regulatory responses suggest that in the ruminant rectal temperatures are considerably affected by rumen metabolism and do not represent a reliable index of the regulated temperature.3. In the winter the regulatory responses were correlated with skin temperatures only. In the summer responses were correlated with both skin and tympanic temperatures, excepting for skin water loss.4. The seasonal difference in the effects of skin and tympanic temperatures on regulatory responses was associated with a small change in mean tympanic temperature. The twofold larger nychthemeral fluctuation in the summer tympanic temperature is suggested to cause the seasonal difference in the thermoregulatory mode.5. The winter range of nychthemeral fluctuations in tympanic temperature suggests a range of permitted core thermolability. A wider permitted core thermolability in summer is unlikely to be a mechanism of acclimatization in the cow.