460. Studies of the secretion of milk of low fat content by cows on diets low in hay and high in concentrates: I. The effect of variations in the amount of hay

Abstract

1. Twelve Shorthorn cows were used for an investigation into the effect of diets low in roughage and high in concentrates on the fat content of milk. 2. An experiment was conducted with initial and final control periods. During the control periods all the cows received daily 17 to 21 lb. of hay, 30 lb. of mangolds and 4 lb. of a balanced concentrate mixture per 10 lb. of milk. In the period of experimental treatment, lasting 4 weeks, a control group of four cows remained on this diet. Two other similar groups received daily only 6 and 2 lb. of hay, but this was supplemented with 9 lb. of a concentrate mixture low in fibre and protein. The same allowances of mangolds and of concentrates for production were given to these as to the control group. 3. The mean fat content of milk produced in the last 2 weeks of the period of experimental treatment by cows receiving the diets low in roughage was 0·49 % (group receiving 6 lb. of hay) and 0·59% (group receiving 2 lb. of hay) below that produced by the cows receiving the control diet, these differences being statistically significant. In the group receiving daily 6 lb. of hay the drop in fat content was accompanied by an increase in the yield of milk and there was only a small fall in the yield of milk fat, but in the group receiving daily 2 lb. of hay there was a significant fall in the yield of fat. The latter fall represented a loss of 18·9% of the fat as compared with the control group. The fat content of the milk and the yield of milk rapidly returned to normal when the cows reverted to 18 lb. of hay per day. 4. The butterfat produced by the cows receiving diets low in roughage had a higher iodine value and lower softening point than that from the control cows. The fat from the group receiving daily 2 lb. of hay, but not that from the group receiving 6 lb. of hay, had a distinctly lower Reichert value than fat from the control group. The treatments caused no change in the saponification value. 5. The level of solids-not-fat in the milk was not affected by reduction in the roughage intake.