A note on the influence of stage of lactation on the response in the lactose content of milk to a change of plane of energy nutrition in the cow

Summary Three groups of Friesian cows, one of cows in early lactation, one of cows in mid-lactation and one of cows in late lactation, received over successive 4-week periods according to a double changeover Latin square design, rations providing a low, a normal and a high plane of energy nutrition. With the group of cows in early lactation the treatments were repeated again in mid-lactation and in late lactation. In all experimental groups there were increases and decreases in the treatment mean values for milk yield and solids-not-fat, protein and lactose contents, in response respectively to overfeeding and to underfeeding. The responses in lactose content in early and mid-lactation were small (overfeeding, 0·00 to +0·06 percentage units; underfeeding, −0·01 to −0·08 percentage units) and less than those in protein content, whereas in late lactation they were larger (overfeeding, +0·09 to +0·14 percentage units; underfeeding, −0·13 to −0·19 percentage units) and of the similar size to those in protein content.