J. Alan Murray 175 



equivalent to I lb. of fat. Digestible fat in the food does not, however, 

 produce its own weight of body fat. When the increase consists entirely 

 of fat : 



N = 2-3 (M - m) X 100/y, 



where ■// is the percentage of nietabolisable energy recovered in the gain 

 and N is the amount of digestible nutrients (starch) required to produce 

 the increase (M — »w). 



When the increase consists entirely of non-fatty matter, of which 

 about 22 per cent, is protein : 



N = 22/100 {M - m) x 100/y, 



where iV is the amount uf digestible nutrients (protein) required to 

 produce the increase (M — ni). 



It will be seen that the amount of food required to produce a pound 

 of fat is more than ten times as much as is required for an equal in- 

 crement of non-fatty matter. These however are particular and extreme 

 cases, though not perhaps of infrequent occurrence in practice. Com- 

 monly the increase comprises both fat and non-fatty matter, due to 

 simultaneous growth and fattening, and it must be considered as a 

 whole. 



If til and M be the live weights and/ and F the percentages of fat 

 in them respectively the amount of fat in the increase is {31F — »?/)/100 

 and the amount of non-fatty matter is [M (100 - F) - m (100 -/)]/100. 

 The nutrients required to produce the same are: 



,,, ^ 2-2,{MF-mf) 100 

 &tarch= -^-j^Q-^-'x-; 



, ^ , . 22 [M (100 -F)- m (100 -/)] 100 

 and Protem = — |-^ ^^ ^| x -^ . 



This formula is perfectly general ; it covers both of the cases previously 

 considered. Of course, if the nutritive ratio of the dietary so calculated 

 is more than about 12 to 1 — in some cases it may be much less — it 

 must be reduced by substitution of protein for starch in equivalent 

 quantity. On this \inderstanding the formula may be simplified to: 



N = 0-23 110^(:^_™i+ 9 (^^ -»^ ^ 



where N is the total digestible nutrients (starch) required to produce 

 the increase {M — ni). 



N divided by T (duration of the fattening period) gives the average 

 amount required per day. For example, if the data were, as in the 



