100 AGRICULTURAL RESEARCH AND THE FARMER : 



food, or, as it is usually called, the protein. This is the most 

 necessary and at the same time the most expensive constituent 

 of all feeding stuffs. Until a few years ago protein was regarded 

 as a definite unit, which was used directly by the animal in restor- 

 ing the waste of tissue which goes on continually in the body, and 

 in providing material for growth. But it is now recognised 

 that the animal cannot make use of protein as such ; the protein 

 has first to be split up by the digestive ferments into nitrogenous 

 substances known as amino-acids. This discovery would not be 

 so important if all proteins contained the same amino-acids in 

 the same proportion; but actually the proteins in the various 

 feeding stuffs vary considerably in their amino-acid content^ 

 and some idea of the complexity of the whole question may be 

 gained from the fact that at least i8 amino-acids have already 

 been found to be present, in varying proportions, in the proteins 

 of ordinary feeding stuffs. 



Perhaps the importance of the subject to the stock feeder 

 will be better realised if a rough indication is given of the changes 

 that go on inside the body of the animal. The animal has to 

 produce, say, muscle, which consists of proteins containing 

 certain amino-acids combined in a definite and constant propor- 

 tion. As raw material for the manufacture of this protein, the 

 animal is supplied by the farmer with feeding stuffs containing 

 various other proteins, and each of these proteins contains its 

 own mixture of amino-acids in a definite proportion. The first 

 thing the animal does is to dissolve these various proteins in its 

 digestive juices, and separate out all their amino-acids, so that 

 from these amino-acids it can build up the new proteins of which 

 its muscle is composed. But the new proteins must contain 

 certain amino-acids in definite proportions, and any amino-acids 

 which are left over, being of no use to the animal for flesh formation, 

 are converted into urea or uric acid and excreted in the urine. 

 The aim of the stock feeder, therefore, must be to provide his 

 animal with feeding stuffs in such a proportion that their proteins 

 supply amino-acids in the proportion required for flesh formation, 

 for only by so doing can feeding be carried out without a consider- 

 able loss of the most expensive ingredient, protein. In other 

 words, in compiling rations regard should be had not so much to 

 the protein content of the various food stuffs, as to their amino- 

 acid content, and it is this that the research worker is now engaged 

 in finding out. The chief difficulty in making an attempt to 

 adjust the amino-acid supply of the food to the amino-acid require- 

 ments of the growing tissues of the animal is that the composition 

 of the proteins of both the tissues and the food are not fully 

 known. At the Rowett Institute work on the compositon of flesh 



