Effects of stage of lactation, milk protein genotype and body condition at calving on protein composition and renneting properties of bovine milk.

The lactational variation in milk protein composition and renneting properties and their relationship to the cow's body condition at calving were investigated in 39 Danish Holstein first lactation cows fed on a well balanced standard diet. All milk characteristics measured were significantly affected by stage of lactation (P < 0.01). Casein as a proportion of total milk nitrogen reached a maximum in mid lactation. The proportion of alpha s- and kappa-casein in total casein decreased and the proportion of beta-casein increased systematically during lactation while the proportion of gamma-casein was lowest in mid lactation. The alpha-lactalbumin content of milk and its proportion of total whey proteins decreased during lactation. Renneting time was highest and curd firmness lowest in mid lactation. These results appeared to reflect a low degree of proteolysis in late-lactation milks compared with several other investigations, probably because of the good nutritional state of the cows. The body condition at calving affected proteolysis and the renneting properties of milk. A good body condition increased the content of whey protein in total milk nitrogen and of gamma-casein in total caseins (P < 0.05); in addition, curd firmness was improved (P < 0.01) and aggregation time was reduced (P < 0.05). We suggest that these effects were related to the fat metabolism and energy status of the cows during lactation. The interrelationships between the milk characteristics were evaluated by factor analysis to support the interpretation.