MILK PRODUCTION 493 



If the probable requirement for protein maintenance be 

 deducted from the total resorbed nitrogen, the utilization of 

 the remaining protein, calculated as in the experiments of the 

 previous paragraph, was notably higher, approaching or reach- 

 ing 100 per cent in several instances. 



The differences observed were largely due, however, to fluc- 

 tuations in the gain or loss of body protein, the formation of 

 milk protein being quite uniform from period to period, and 

 this fact seems to render the results of somewhat questionable 

 relevance as regards the special question of comparative values 

 as sources of milk protein, although they do show marked 

 differences in total efficiency. 



The utilization of energy 



588. Net energy values for milk production. The net 



energy value of a feeding stuff or ration for milk production is 

 identical in conception with that for fattening (448) or for 

 growth (472) already considered. It is that part of the feed 

 energy supplied in excess of the maintenance requirement which 

 is recovered in the product. For example, if a cow produces 

 per day 20 Ib. of four per cent milk, Containing (604) 336 

 Cals. of energy per pound, the total of 6720 Cals. would be the 

 net energy value which must be supplied in the ration in ad- 

 dition to that required for maintenance. 



As pointed out in Chapter VIII (371), it cannot be assumed 

 that the net energy values for maintenance, fattening or growth 

 apply to milk production, but the values for the latter purpose 

 must be determined by direct experiment. As yet, very scanty 

 data are available on this point, the only results yet reported 

 being three contained in a brief preliminary paper by Kellner. 



589. Complete energy balances. Kellner 1 reports the 

 nitrogen, carbon and energy balances, determined as in his ex- 

 periments on oxen, of three cows receiving mixed rations and 

 varying considerably in their milk yield. By the method de- 

 scribed in Chapter XVII (768-772), it is estimated that the net 

 energy values of the rations and the percentage utilization of 

 their metabolizable energy for fattening would be : 



1 s ter Internal. Kongress fur Milchwirtschaf t, 191 1 . 



