4 THE ECONOMICS OF FEEDING HORSES 



and prevents the digestion to a greater or lesser extent 

 of other soluble constituents, the digestibility of a food- 

 stuff by the horse may be said to vary inversely as the 

 amount of crude fibre present. Most of the herbivorous 

 domesticated animals can digest a considerable part of 

 the soft fibre of green plants, but only ruminants (ox, 

 sheep, and goat) are able to extract any nourishment 

 from the hard, woody fibre of foods like straw and the 

 coarser leguminous plants. 



The nitrogenous constituents of a food occur in one 

 of two forms, either as proteins or as amides. The 

 proteins, albuminoids, or, as they are often called, flesh- 

 formers, are essential constituents of a diet, because, 

 besides being broken down in the body so as to give up 

 heat or energy, they are the only constituents of food 

 which can build up and repair the muscular tissue which 

 has been subjected to the wear and tear of work. After 

 digestion these bodies are assimilated — i.e., they are in- 

 corporated into the proteid tissues of the body. 



When a working animal is neither gaining nor losing 

 weight, protein is being assimilated to the same extent as 

 the proteid tissues have been broken down by wear and 

 tear, and this is the minimum amount of albuminoid 

 matter that must be supplied in the food. When broken 

 down in the body by combination with oxygen, proteid 

 tissue is not oxidised to its simplest form, but splits into 

 several bodies, the ultimate products of which are carbon 

 dioxide, water, and urea. Urea is an incompletely 

 oxidised constituent of the urine eliminated from the 

 blood by the liver and kidneys, and it contains practi- 

 cally all the nitrogen taken in as food. The amount of 

 urea excreted is therefore an index to the amount of 

 proteid decomposition which has taken place in the body, 



