Nychthemeral patterns of thermoregulation in high-yielding dairy cows in a hot dry near-natural climate

Abstract

The thermoregulatory responses of Israeli Holstein cows (33 kg milk/day) were determined during 4 days at 3-hr intervals in summer and in winter in their normal environment. The mean ambient conditions in summer and winter were 31.8 and 16.8¦C black globe temperature, 32 and 43 % relative humidity, and 1.6 and 1.2 mlsec wind velocity respectively. Seasonal changes in cows from winter to summer were: 0. 11¦C increase in tympanic membrane temperature, 0.37 and 2.6¦C increments in rectal and mean trunk skin temperatures respectively, 57 % reduction in non-evaporative heat loss, 47 % increase in sweating rate, 47 % increase in respiratory frequency, and 23 % reduction in total heat production. This indicates that the lower summer heat production is not necessarily associated with depressed milk production. Distinct nychthemeral (24 hr) cycles in vasomotor, respiratory, and sweating activity were found in both seasons. Similar maximal values were attained in both seasons. This suggests a considerable seasonal shift in thermal sensitivity. Heat-dissipating functions were activated at lower levels than in comparable climatic chamber studies. These results cast doubt on the validity of climatic chamber studies for the assessment of thermoregulatory capacity.