Upper critical temperatures and forced ventilation effects for high-yielding dairy cows in a subtropical climate.

Upper limit of thermal stability and subsequent rise of thermoregulatory functions as affected by forced ventilation were examined. Rectal temperature, respiratory frequency, ear skin temperature, body weight, and milk yield were recorded biweekly July to March over 2 yr for 170 Israeli-Holstein cows (305-day milk yield 9000 kg/cow) at air temperatures 10 to 36 degrees C. Cows were in an open shelter. One side was force ventilated over 2.5 m along the stanchions (air velocity 1.5 to 3 m/s) from 0500 to 2200 h. Control side mean air velocity was .5 m/s. Within the 10 to 24 degrees C range, rectal temperature was not affected by air temperature or forced ventilation but increased by .02 degrees C/kg fat-corrected milk in animals producing above 24 kg/day. Between 26 and 36 degrees C rectal temperature increased with air temperature in both groups; rate of rise was halved by forced ventilation. In this range of air temperature, rectal temperature increased with rising milk yield, as in the lower air temperature range, in both high-producing and lower-producing cows in forced ventilation. Body weight or parity did not have significant effects. Mean ear skin temperature was higher for control animals, but its rate of increase with air temperature was similar in both groups. Forced ventilation reduced mean respiratory rate. An upper critical temperature is 25 to 26 degrees C and is independent of milk yield or acclimatizational state of cows exposed to the natural sequence of climate.