Nychthemeral and seasonal patterns of thermoregulation in cattle

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Abstract
1. The nychthemeral (24 hr) and seasonal cycles in thermoregulation at high (Fh) and standard (Fs) levels of feeding were studied in Holstein heifers during spring (mean daily air temperature 18.2°C) and late summer (mean daily air temp. 27.5°). 2. In the two seasons the percentage respiratory evaporative cooling (% RC) and rectal temperature (Tr) displayed marked nychthemeral cycles in both feeding groups, while heat production (Hp) cycles were evident in the Fh group only. 3. In both seasons the effect of the 22.4% larger Hp in the Fhgroup was to prolong the duration of higher % RC; Tr was relatively increased in the summer only. 4. The seasonal changes were: a 20% reduction in Hp in both groups; a larger amplitude of Hp cycling (19% of the mean in summer and 12.9% in spring) in the Fh group; a higher Tr (+ 0.40°C in the Fs group and + 0.63° in the Fh group), cycling with a larger amplitude (0.33° v. 0.52°); a 21° depression in feed intake in the Fh group and 9° in the Fs group. 5. Within each season Tr and the rate of change in Tr were significantly correlated with Hp in the Fh group only. On a between-season basis Hp and Tr were significantly correlated in both feeding groups. These suggest that the relationship between body temperature and heat production follows different patterns in nychthemeral cycles from those in seasonal changes. 6. The nychthemeral patterns of respiratory, %RC, Tr, and Hp responses indicated that the animals were acclimatized to the seasonal change in Tr, possible through a shift in the set-point for temperature control. The depression in heat production and feed intake did not prevent the maintenance of a normal rate of growth.