1. In an experiment with twenty-four Ayrshire cows, four treatments were compared in a change-over experiment in which each cow completed four periods each of 4 weeks. In all the treatments the cows received daily 4 lb of concentrates and roughage according to their initial live weight. For milk yields above 10 lb the cows received 4 lb of a concentrate mixture per 10 lb of additional milk produced in the previous week in one treatment, and 3 lb in the other three treatments. Two of the treatments with the lower rate of feeding were supplemented with 0·75 lb of sodium acetate daily, given in either one or three doses.
2. Digestion trials were made with the hay and concentrates to aid in the estimation of their starch equivalents. The allowance made for the maintenance requirements of the cows appears to have been close to that recommended by Woodman (1957).
3. On one treatment the cows in fact received 4·29 lb of concentrates/10 lb of milk produced, and gave on average 36·6 lb of milk (35·2 lb of 4% fat-corrected milk) in the last 2 weeks of the experimental period with a daily decline in milk yield of 0·14 lb/day. On the other treatments the cows in fact received 3·48 lb of concentrates/10 lb of milk produced, and averaged 34·4 lb of milk daily with an average daily decline of 0·21 lb/day; this differed significantly from the rate of decline observed with the higher rate of feeding.
4. The addition of 0·75 lb of sodium acetate to the daily diet, in one or three doses, exerted no detectable effect on milk yield or composition, apart from a small effect on the solids-not-fat (S.N.F.) percentage which was of doubtful meaning.
5. At the higher rate of concentrate feeding the S.N.F. content was 0·10% higher than at the lower rate of feeding. This difference and the resulting difference in the yield of S.N.F. (0·21 lb daily) were significant.