The composition of sow's milk during lactation with particular reference to the relationship between protein and lactose

Data are given on the changes which occurred in the composition of sow's milk during the course of 24 lactations. The mean percentages of the major constituents (192 samples) were: total solids, 20·0%: crude protein, 5·7%: lactose, 4·7%; fat, 8·6% and ash, 0·89%. 2. There were differences in milk composition between sows and between successive lactations, but these were only appreciable for milk fat percentage between sows. 3. A highly significant negative correlation (P < 0·001; r = –0·62) was found to exist between the percentages of protein and lactose in the milk, quite apart from the general trend for protein content to fall with advance in lactation. Possible reasons for this are discussed.