The effect of the plane of energy nutrition of the cow on the secretion in milk of the constituents of the solids-not-fat fraction and on the concentrations of certain blood-plasma constituents.

An increase in the plane of energy nutrition of the cow is known to increase both the yield and the solids-not-fat (s.N.F.) content of milk: a decrease in fat content also occurs but there is little effect on the yield of fat (for review of literature, see Burt, 1957 a). Though small depressions in milk lactose content occur with underfeeding (Rowland, 1946), any change in S.N.F. content is largely due to a change in the protein content of the milk, and thus an increase in the net energy supply to the cow gives rise to a specific increase in the synthesis of protein by the mammary gland. The metabolic pathways by which this effect and the increase in milk yield are achieved are, however, as yet unknown. The experiments described here were designed to investigate the effect of the plane of energy nutrition of the cow on the amounts of the individual proteins in milk and on the concentrations of certain blood-plasma constituents that are possibly of importance in the synthesis of proteins within the mammary gland. Two experiments have been made. In the first, the variations in plane of energy nutrition imposed were within the range of feeding levels normally experienced in practice, whereas in the second extreme differences in plane of energy nutrition were compared in an attempt to exaggerate any changes in blood composition associated with change of level of feeding.