MILK PRODUCTION 497 



that the digestibility of the rations in Haecker's experiments 

 was estimated from average figures which, according to Eckles' 

 results (722), are probably too high for cows in milk, although 

 on the other hand Haecker's estimate for the maintenance re- 

 quirement also seems high. 



591. Net energy values for milk probably greater than for 

 fattening. A comparison of Kellner's results (589) with those 

 obtained by the same author 1 and by Armsby and Fries 2 for 

 the utilization of metabolizable energy in either maintenance, 

 growth or fattening seems to indicate clearly that the net 

 energy values for milk production are distinctly higher than 

 those for the latter purposes, although no direct comparisons 

 on the same feeding stuff or ration can be made. 



Both Jordan's and Eckles' results tend to confirm this con- 

 clusion, which is further strengthened by the fact, to which 

 Eckles calls attention, that with one exception the actual energy 

 content of the milk in his experiments was greater than the net 

 energy value available in the ration producing it as computed 

 by the use of Kellner's factors. 



Unfortunately, no results upon the net energy values of 

 single feeding stuffs or nutrients for milk production have yet 

 been reported, so that it is impossible at present to make any 

 exact quantitative comparisons. 



592. Cause of higher net energy values for milk production. 

 The apparently higher net energy values for milk produc- 

 tion as compared with tissue production may be plausibly as- 

 cribed to the difference in the composition of the products. 

 As shown in Chapters X and XI, the organic matter of the in- 

 crease in fattening consists chiefly of fat (441-443) and even in 

 the case of growth fat makes up a considerable proportion of it 

 (458) except in extreme youth. In average milk, on the com- 

 trary, protein and milk sugar constitute two-thirds of the total 

 organic matter and carry over one-half of the total energy. 



It seems not improbable that the conversion of digestible 

 protein into milk protein, or of digestible carbohydrates into 

 milk sugar, may involve a comparatively small expenditure 

 of energy as compared with the synthesis of fat from carbo- 

 hydrates or protein. If such be the case, the organic matter 



1 Landw. Vers. Sta., 53 (1900), i. 



2 Jour. Agr. Research, 3 (1915), 435; 7 (1916), 379. 



