The synthesis of lysine by the lactating goat.

In planning the present experiment it was assumed that, if the diet of a lactating ruminant is devoid of lysine and the animal is in positive nitrogen balance, the lysine in the milk must have been synthesized in the animal by the micro-organisms in its paunch. Lysine synthesis by rumen bacteria has been demonstrated by McDonald (1944) and by McNaught (1951). The former fed sheep on a diet in which zein represented 82% of the total protein and it was estimated that 40% of the zein was converted into lysine-containing microbial protein. McNaught's experiments showed a 12% increase in the lysine content of bacterial preparations from the rumen, the material being incubated under conditions shown by Smith & Baker (1944) to cause very little change in the facies of the rumen flora. It thus remained to be seen whether such a synthesis could provide an animal with sufficient lysine to allow the production of milk protein and to maintain the animal in good health. Preliminary experiments (unpublished) carried out at this Institute were unsatisfactory because the beet pulp, used in the basal diet and fed as roughage, contained more lysine than had been originally thought. In addition, the microbiological assay used for lysine estimation gave results which, though reproducible, did not agree with the figures published by other authors. A new experiment was therefore devised with a diet much lower in lysine and a more reliable method for estimating lysine. This experiment is described in the present paper.