The effect of varying periods of energy undernutrition on milk yield and composition was determined in 30 British Friesian cows receiving a typical winterstall diet. An absence of steaming-up, and underfeeding from the beginning of lactation, depressed both milk yield and solids-not-fat (SNF) content, especially in early lactation. Initially the reduction in milk SNF content was accounted for mainly by a loss in lactose content, but as lactation progressed this effect lessened and an effect on milk protein content was established. Protein content was similarly depressed by underfeeding in mid-lactation. The restoration of normal feeding in mid-lactation after varying periods of underfeeding was associated with only small responses in SNF and protein contents.

The effect of the type of basal diet and supplementary concentrates on the extent of the increase in SNF and protein contents in response to improved feeding was examined in 2 further experiments. Responses were in all instances small and it was concluded that with the diets commonly in use in Great Britain the provision during the winter period of supplementary concentrates, whatever their composition, is unlikely to give a marked improvement in milk SNF or protein contents.