368 VETERINARY PHYSIOLOGY 



the undigested nitrogen in the faeces decreases in amount, as 

 occurs on a low nitrogen intake, the relative proportion of 

 excretory nitrogen becomes greater, so that on a very low 

 protein intake the apparent amount digested may become a 

 negative quantity. 



FOOD REQUIREMENTS. 



Food is the source of (1) the energy, and (2) tlte material 

 (p. 283) which the animal kept for profit transforms into 

 the products desired by the feeder. Thus, the horse changes 

 the chemical energy of oats and hay into work. Grass and 

 other food-stuffs are changed by the dairy cow to milk, by 

 the bullock to meat, by the sheep to mutton and wool. The 

 problem of the feeder is to secure the greatest return of 

 these products with the most economical consumption of food. 



Nutritive Ratio. — Protein is of special importance in the 

 food. This constituent can replace fats and carbohydrates as 

 a source of enercrv, but none of the other constituents of the 

 food can replace protein as the source of material for growth 

 and repair (p. 275). In comparing the values of different food- 

 stuffs or combination of food -stuffs, therefore, it is necessary 

 to know what proportion of the energy value of the digestible 

 portion of the food consists of protein. This is expressed by 

 the nutritive ratio, sometimes called the " albuminoid ratio." 

 It might be written — 



Protein : fats + carbohydrates = nutritive ratio. 



In calculating the ratio it must be remembered that fats 

 can liberate, weight for weight, about two and a quarter 

 times as much energy as carbohydrates (p. 257). Fats are 

 therefore multiplied by 2 "2 5. For example, if a sample of 

 meadow hay contained the following percentage of digestible 

 nutrients — nitrogen free extract 28, crude fibre 15, fats 1"2, 

 protein 5 — the nutritive ratio would be — 



5 : 28 + 15 + 1-2 X 2-25, i.e. 5 : 45*7, or 1 : 9-14. 



The ratio gives useful information as to the suitability of 

 a food for purposes that require much protein, e.g. growth 



