THE FEEDING OF ANIMALS 179 



approximately Is. 3d " per unit " per ton. Thus the digestible 

 fat and protein become worth about 3s. IJd "per unit'* per 

 ton. To calculate the value per ton of a food-stuflf the per- 

 centages of digestible fat and protein should be added together, 

 their total multiplied by 2*5 and added tp the percentage of 

 digestible carbohydrates, thus giving the number of " food 

 units." The food units multiplied by Is. dd. will then give the 

 value. In practice, it will be found that if the total carbohy- 

 drates be taken as worth Is. per unit, and the total fat and 

 protein as worth 2s. Qd. per unit, the value per ton, calculated 

 on this basis, will be roughly correct. 



Manurial Value of Foods.— Another factor of importance 

 in determining the relative values of foods is their effect in 

 enriching the excreta of animals fed upon them, in the valu- 

 able manurial constituents — nitrogen, phosphates and potash. 

 Many of the rich nitrogenous food-stuffs are rich too in 

 phosphates and potash ; oil-cakes, for example, have a marked 

 effect upon the manure of animals fed upon them, especially if 

 the animals are not growing rapidly or producing milk. A 

 variable proportion of the nitrogen, a smaller proportion of 

 the phosphates, but practically none of the potash is retained 

 "by the animal in order to build up new tissue ; the remainder 

 •eventually finds its way into the excreta. 



Great importance is rightly attached to the maDurial value 

 ■of food-stuffs consumed on the farm, but it would be well for 

 the farmer to remember that combined nitrogen can often be 

 more cheaply purchased in the form of nitrates or ammonium 

 salts than as cake or other concentrated food-stuff. 



Lawes and Gilbert made elaborate experiments upon the 

 manurial value of the common feeding-stuffs when supplied to 

 fattening sheep and oxen. Voelcker and Hall have published 

 a revised table, embodying the results of Lawes and Gilbert. 

 In this table it is assumed that half the nitrogen, three-quarters 

 of the phosphoric acid, and all the potash are voided in the 

 excrement, and that nitrogen is worth 12s. per unit per ton, 

 phosphoric acid Ss. per unit, and potash 4s. per unit. 



