The relationship between the rate of heat production and the level of milk production in a subtropical climate

Abstract

Heat production as determined by the open circuit mask method, thermoregulatory responses, fat-corrected milk (F.C.M.) yield, and feed intake were recorded in two groups of 11 Israeli-Holstein cows each from the seventh day after calving up to 104 days of lactation. One group was fed according to the Fredricksen standard and the other group ad libitum, its feed intake being 125% of the Fredricksen standard. The experiment was carried out from January to mid July. No apparent heat increment of lactation was observed in the group fed according to the Fredricksen standard. A highly significant heat increment was found (10.3 kcal/hr/kg F.C.M.) at lower air temperatures (16–22°C) in the group fed ad libitum, while at higher air temperatures (23–28°C) a smaller, non-significant heat increment (5.4 kcal/hr/kg F.C.M.) was found. The hourly rate of heat production, the rectal temperature, respiration rate, respiratory volume, and respiratory vaporization changed only by small non-significant amounts with increasing temperatures. From these data it is inferred that heat production did not increase above heat requirements, except for the group fed ad libitum during the period at lower temperatures. This is explained by an increase in the critical temperature of the animals through a gradual adaptation to the summer conditions. This adaptation involves a decreased peripheral insulation by the summer coat and a lower basal metabolic rate. It is , suggested that these adaptational processes occurring in nature should be considered in studies on heat increments of feeding and of lactation.