By Regan, W. M. and Richardson, G. A., Journal of Dairy Science, 1938
Research Paper Web Link / URL:
http://www.sciencedirect.com/science/article/pii/S0022030238956176
http://www.sciencedirect.com/science/article/pii/S0022030238956176
Description
Summary The effect of environmental temperature on high producing dairy cows was studied in a large psychrometric room in which the temperature was increased from 40 to 100° F., while air movement and relative humidity were maintained at the constant values of 50 feet per minute and 60 per cent respectively. It was found that, as the room temperature was increased, there was a uniform increase in the respiration rate, which approximately doubled for each increment of 18° F.; that there was a decrease in pulse rate; and that at 80 or 85° F., depending upon the breed, a pyrexial point was reached where the animals were no longer able to maintain heat balance. As the room temperature was elevated above this pyrexial point, anorexia developed, milk flow declined, and an alteration occurred in the characteristics of the milk produced, which included a lowering of the casein and solids-not-fat content, and an increase in the percentage of butterfat. The pH of the milk was raised, the freezing point depression lowered, and a longer rennet coagulation time was required. The butterfat secreted was lower in volatile acids and higher in unsaturated components. The cow's principal avenue for heat dissipation is the respiratory system. Her breathing varies directly with the environmental temperature, and her skin has a high insulating value. She may, therefore, adapt herself with comfort to conditions of extreme cold; a fact that helps to explain why, as found by certain research workers, exposure to zero weather brings about neither lowered production not increased maintenance requirement. It is also evident that the cow is not especially well fitted to withstand hot climates, though our studies show a distinct breed difference in this regard. Because the cow's heat regulating mechanism functions in such a radically different manner from that of man, we are not justified in applying the comfort standards worked out for man to the management of cattle. The changes in com-position and physico-chemical characteristics of milk when a state of positive heat balance is reached probably result from blood changes instituted to facilitate heat disposal.
Summary The effect of environmental temperature on high producing dairy cows was studied in a large psychrometric room in which the temperature was increased from 40 to 100° F., while air movement and relative humidity were maintained at the constant values of 50 feet per minute and 60 per cent respectively. It was found that, as the room temperature was increased, there was a uniform increase in the respiration rate, which approximately doubled for each increment of 18° F.; that there was a decrease in pulse rate; and that at 80 or 85° F., depending upon the breed, a pyrexial point was reached where the animals were no longer able to maintain heat balance. As the room temperature was elevated above this pyrexial point, anorexia developed, milk flow declined, and an alteration occurred in the characteristics of the milk produced, which included a lowering of the casein and solids-not-fat content, and an increase in the percentage of butterfat. The pH of the milk was raised, the freezing point depression lowered, and a longer rennet coagulation time was required. The butterfat secreted was lower in volatile acids and higher in unsaturated components. The cow's principal avenue for heat dissipation is the respiratory system. Her breathing varies directly with the environmental temperature, and her skin has a high insulating value. She may, therefore, adapt herself with comfort to conditions of extreme cold; a fact that helps to explain why, as found by certain research workers, exposure to zero weather brings about neither lowered production not increased maintenance requirement. It is also evident that the cow is not especially well fitted to withstand hot climates, though our studies show a distinct breed difference in this regard. Because the cow's heat regulating mechanism functions in such a radically different manner from that of man, we are not justified in applying the comfort standards worked out for man to the management of cattle. The changes in com-position and physico-chemical characteristics of milk when a state of positive heat balance is reached probably result from blood changes instituted to facilitate heat disposal.
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