KLEIBEP 



temperature , the animal may eat only enough for maintenance, 

 as illustrated in Figure 21 (Kleiber and Dougherty, 1934). 



This temperature (T max) is the highest environmental 

 temperature for animal production. As the environmental tempera- 

 ture is decreased, the animal will eat more, and the net energy, 

 appearing in the animal product, N, will rise. Below the critical 

 temperature of the full fed animal, Tc , the heat production will 

 be determined by the thermostatic heat requirement. Since the 

 capacity for food intake is limited, whereas the heat requirement 

 continues to increase with decreasing environmental temperature, 

 less and less energy is available for production, and at the tempera- 

 ture, T . , the maximum food intake of the animal provides just 

 enough heat for maintaining the animal's body temperature. Below 

 this temperature the animal will eat all it can and yet starve to 

 death because it will have to burn up its own body substance in 

 addition to all the food it can eat in order to maintain its body 

 temperature. This situation may be less significant for practical 

 purposes than the lack of food in a cold environment which calls 

 for human action such as operation "Hay Lift". Between the low 

 temperature, at which the animal eats a lot but needs most of 

 the food for fuel for keeping warm, and a high temperature at 

 which it loses appetite to such an extent that it burns up all it 

 takes in for maintenance, there should be an optimal environ- 

 mental temperature at which the efficiency of animal production 

 is at a maximum. This is illustrated on the lower part of Figure 21. 



An indication, though not too obvious, of such a temperature 

 optimum has been obtained in respiration trials with lactating 

 cows fed to capacity with alfalfa hay, beet pulp, and grain, and 

 kept alternately for weekly periods at 7 C, 18 C, and 30 C 

 (Kleiber, 1961). Total carbon and nitrogen balance was determined 

 over a three day period during each week. 



The results are shown in Figure 22. The decline in food 

 intake at a high environmental temperature is most conspicuous. 

 The milk production was little affected, but the loss of body sub- 

 stance was greater at the low and at the high temperature, than 

 at 18 C where the net energy was at a maximum. 



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