Heat stress in dairy cattle : physiological responses and variations in milk composition and equilibrium

Dairy cattle were exposed to air temperatures from comfort to heat stress levels within the range 70°F to 105°F in which relative humidity changed from 71.0 ± 2.0 percent at 70°F to 36.0 ± 1.0 percent at 105°F. The following reactions were studied under conditions of constant and alternating day/night temperatures in which different feed treatments were used (dry feeds, balanced and unbalanced for milk production) :- rectal temperature; respiration rate; pulse rate; rumenal movement; animal behaviour; milk production and composition (solids-not-fat and butterfat content); inherent acidity and freezing point of milk; water and feed consumption; body weight. Temperature increase above an apparent threshold (85°F) resulted in rectal temperature increase and severe disturbance in respiratory and cardiac function. The respiration rate increased prior to body temperature and both showed a significant correlation with air temperature. The pulse rate was initially stimulated but declined to minimum rates when body fever was maximum. The cycle of rumination was not influenced by ambient temperature. Distress and nervous tension was evident at temperatures greater than 95°F but sweat-gland activity was strictly limited. Changes in behaviour (restlessness, panting, tongue protrusion, frequent visits to water, salivation, sliming of nostrils and change in faecal texture) were most apparent in cows which experienced greater body temperature increase. During active heat dissipation, important differences between cows were recorded in the relative emphasis placed on the different physiological responses involved in maintenance of homeostasis. Milk composition and equilibrium changes were related to increase in rectal temperature. There were decreases in solids-not-fat, acidity and the freezing point depression which were unrelated to feed treatment. Changes in butterfat were extremely variable and did not conform to a common pattern. The degree of animal response was influenced by the intensity and duration of heat stress. Constant heat stress conditions effected a progressive decrease in milk yield and production of fatty and nonfatty solids. Animals under heat stress for shorter periods (alternating day/night temperatures) did not show this trend. In each trial, the cow with the higher milk production level was least able to cope with a hot environment as shown by greater body temperature increase, more intense depression of solids-not-fat and acidity and higher elevation of the freezing point. This response was independent of feed treatment and suggested a probable relationship to a higher heat increment resulting from greater mammary gland metabolism and the difficulty of dissipating extra heat. Decline of feed intake paralleled and reflected decline in milk production and was associated with body weight loss when heat stress effected a significant increase in body temperature. (3° to 4°F rise above normal). Conditions in which feed intake and milk production remained relatively constant were associated with lower body temperatures and body weight gains. Body weight decreased under constant temperature exposure and increased under alternating day/night temperatures. In each case the balanced diet minimised weight loss and increased weight gain. Water consumption greatly increased with air temperature increase but in one cow was reduced to a level below maximum intake when there was a drastic decrease in milk production and feed intake. The chance occurrence of
atural\" body fever in the field permitted observation of concurrent changes in milk and body temperature. Changes in milk production composition acid-base balance and osmotic pressure were similar to those associated with increase in body temperature induced by ambient air temperature increase. The pulse rate but not the respiration rate was similarly affected. Improved animal performance at higher temperatures within the comfort zone (indicated by stability of rectal temperature) was considered to be due to greater efficiency of feed utilisation and the lower energy requirement for body heat maintenance and general metabolism. This trend was independent of the effect of feed treatment. Marked milk changes occurred during controlled conversion from field to trial feeding of the unbalanced ration which were similar to those effected under heat stress conditions. This suggested that in the field environment the effect of change in the qualitative character of seasonal feed could either be supplementary or complementary to that of the direct effect of temperature on the dairy cow. The contribution by solar radiation and humidity to the heat load placed on the lactating cow is discussed. The evidence suggested that in the field a much lower shade-temperature than that established in this study would initiate and sustain similar changes in milk composition and equilibrium. The changes recorded in milk when cows were in heat stress may be related to changes in blood composition and acid-base balance."