Nutritional factors affecting milk quality, with special regard to milk protein: a review

This paper reviews some of the recent scientific literature that relates nutritional management to protein concentration in the milk of dairy cows. Nutritional management of the dairy cattle was organized under the headings of nutrition and body condition before calving, forage to concentrate ratio, type and treatments of forage, dietary fibre, dietary protein, dietary amino acid supplementation, dietary fat, dietary carbohydrates, concentrate composition, vitamins, feed additives, and feeding frequency. The level of feeding and rate of liveweight change prior to calving within "normal" limits seems to have relatively little effect on milk yield and composition. Cows which are thin at calving, are biologically more efficient because they produce more milk directly from food rather than from mobilization of body reserves. Increasing the forage content of the diet leads to increased milk fat and depressed milk protein as well as milk yield, and vice versa. Modification of forage type and the treatments of forage seem to exert a relatively weak effect on milk composition. Increasing neutral detergent fibre (NDF) concentration of the diet from 25% to 37% on dry matter (DM) basis corresponds to a linear decrease in milk production, milk protein and a linear increase in the fat content of milk. When data were analysed by the source of roughage, a significant interaction of effects was found between NDF content of the diet on milk yield. Increasing the protein concentration of the diet up to 22% was found to raise milk yield and milk fat concentration, but an increase beyond that level had no consistent effect. A moderate amount of undegradable protein, however, can improve the milk yield and milk protein content of high-lactating cows. Milk protein concentration may be improved by various combinations of ruminally protected methionine and lysine if these amino acids are limiting in the diet. The inclusion of free lipids in diets generally increases the milk yield, but decreases milk protein concentration. By-pass lipids improve milk yield. Levels of dietary fat above 6-7 per cent lead to a depression of both total milk yields and fat and protein content. The source of carbohydrate in the concentrate has little effect on milk production when the concentrates are of similar metabolizable energy (ME) content. Dietary beta carotene has no significant effects on milk production or milk components, while vitamin A may be more effective. Niacin supplementation may correct a milk protein depression induced by dietary oil.(ABSTRACT TRUNCATED AT 400 WORDS)