545. Studies of the secretion of milk of low fat content by cows on diets low in hay and high in concentrates: II. The effect of the protein content of concentrates

Abstract

1. Twenty cows were used in an experiment to compare the effect of five different diets on the fat and solids-not-fat content and yield of milk.2. During initial and final control periods all the cows received 18 lb. hay daily and about 4 lb. of a balanced concentrate mixture (crude protein 16·5%) per 10 lb. of milk produced. In the experimental period, which lasted 7 weeks, a control group of four cows remained on this diet. Two similar groups received daily only 4 lb. hay and about 6 lb. extra concentrates; in these groups the concentrates contained 22·3% crude protein. The remaining two groups received similar treatment except that the concentrates contained only 11·6% crude protein. In addition, 10 1b. macerated delignified straw pulp (containing 1 lb. crude fibre) were given to half the cows receiving each type of concentrate. At the end of the experiment the cows were turned out to grass.3. With the diets low in hay and high in concentrates there was a fall in the fat content of the milk, with some variability in the response of individual cows (see 4 below). With the concentrates high in protein, the fall appeared to be rather slower than with those low in protein, but by the sixth and seventh weeks of experimental treatment it was estimated to be 0·67% fat with high and 0·72% fat with low protein concentrates. The experiment showed, therefore, that serious falls in fat content may occur in cows early in lactation with diets of 4 lb. hay and concentrates containing 22·3 or 11·6% protein. The addition of a limited amount of crude fibre as straw pulp brought about no recovery in milk fat content.4. The extent of the depression brought about by the change to diets low in hay diminished with advancing stage of lactation.