EFFECTS OF WINTER ACCLIMATIZATION ON RESTING METABOLISM OF BEEF COWS

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Abstract
Twelve pregnant beef cows in average or fat body condition were overwintered in either heated housing (18 C) or outside and exposed to the naturally occurring cold winter conditions (5 to −10 C during early winter, 0 to −48 C during mid-winter). The housed and half of the outside cows were provided with woodshavings as bedding. In November (early winter) and again in January (mid-winter) metabolic measurements were made on each cow (22 h postfeeding) while exposed to test temperatures of −30, 0 and + 30 C. After adjusting for differences in body weight (kg3/4 basis), metabolic rate was not significantly influenced by either the body condition or by the availability of bedding. Metabolic rates were elevated in all cows when exposed to −30 C but, as evident from the decreases in rectal temperature, the housed cows were unable to maintain homeothermy during the tests at −30 C. Metabolic acclimatization occurred in the cows kept outside resulting in an increased resting metabolism and downward shift in their thermoneutral zone. Resting metabolic rate (22 h postfeeding) and the temperature of its occurrance were estimated by regression analysis to be 3.13 kcal/h.kg3/4 at about 30 C for the housed cows, and for the cows kept outside 3.71 and 4.29 kcal/h.kg3/4 at 17.4 and 12.7 C during early and mid-winter, respectively.

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