Interrelations of Milk-Fat, Milk-Protein and Milk-Energy Yield

This article grew out of an inquiry by the American Dairy Cattle Club.In the registration of dairy cattle the Club has taken the progressive step of requiring an estimate of milk-protein yield (in addition to the usual estiffiate of milk and milk-fat yield) of individual cows.The yield estimates are based on monthly tests covering the 3d-307th days of each lactation, or 305-day partial lactation system.A recent article by Dr. Goodale (1), geneticist for the Club, represents that protein is the most valuable of all the milk components and that it is desirable, by breeding, to increase the ratio of protein yield to yield of other milk solids (comparable to increasing the proportion of high-priced cuts in meat animals).1 A breeding program along this line requires a practical field test for protein.Pending the development of such a test the question arises as to the possibility of estimating protein yield from the milk and fat yield.This question may be put in the form of the relation between fat percentage and protein percentage of the milk of individual cows.If we know the 305-day fat percentage for a given cow, how accurately can we estimate her corresponding protein percentage ?The relation between fat and protein (as well as other milk components) for 3-day samples has been heretofore reported (2, 3).It is the purpose of this paper to apply the analyses of these 3-day samples to the appropriate milk yields to secure an estimate for individual cows for a partial lactation approximating the above 305-day period, and to present the interrelation between yields of various milk components, particularly milk fat, milk protein and milk energy. DATA AND ]YIETttODSThe chemical analyses (2, 3) were made on 3-day composite samples of the milk of individual cows in the University of Illinois herd.For the most Received for publication January 8, 1938. 1 This is a very free version of Goodale's paper, which in fact contains a mixture of legal, economic and biologic points, and unfortunately some misinformation as to milk composition, lie states that as we pass from milk of 3 per cent fat to milk of 6 per cent fat the percentage of lactose increases from 4.0 to 4.4 and the percentage of ash increases from .6 to .9.Aside from the gross errors involved in his absolute lactose and ash values it is quite contrary to the well-known principle of osmotic equilibrium in milk secretion that lactose percentage and ash percentage should vary so markedly in the same direction: To maintain a constant osmotic pressure of milk (equal to that of the blood) it is inevitable that variations in lactose percentage and ash percentage will tend to be in opposite directions.261