1. Three comparable groups of cows were used to investigate the effect, on milk composition and yield, of diets consisting of 4 lb. hay daily and concentrates containing 35% weatings, 15% decorticated groundnut cake and 50% of either flaked maize, maize meal or dredge corn (crushed oats and barley) respectively.

2. During initial and final control periods all the cows received 16 lb. hay daily and, per 10 lb. of milk produced, they received about 4 lb. of the concentrate mixture containing flaked maize. During the experimental period of 6 weeks all the cows received 4 lb. hay, concentrates according to milk yield, and an extra 7 lb. of concentrates daily. One group of cows received the concentrate mixture containing flaked maize, a second group the mixture with maize meal and the third group that with dredge corn. The mean daily intakes of starch for the different groups were respectively 6·0, 6·4 and 5·8 lb. per cow.

3. The diet containing flaked maize caused a marked, and that containing maize meal a small fall in the fat content of the milk, but the diet containing dredge corn was without effect. In the last 2 weeks of experimental treatment the adjusted mean fat percentage for the group of cows receiving flaked maize was 0·51 and 0·71 below the percentages for the groups receiving maize meal and dredge corn respectively. The yields of milk and of milk fat were also lower in the group receiving flaked maize than in either of the other groups, but the experimental treatments produced no changes in the milk solids-not-fat percentage.

4. This experiment has shown that when diets low in hay and high in concentrates are given to lactating cows, the type of starch in the concentrates is of great importance in determining the extent of the depression in milk fat percentage. The greater depression with flaked maize than with maize meal or dredge corn is thought to be associated with the effect of the starch on the flora of the reticulo-rumen.