Factors contributing to variation in the proportion of casein in cows’ milk true protein

Abstract

The aim of this study was to identify and rank the various factors, in particular feeding ones, affecting the proportion of caseins in milk true protein. Twenty nine feeding trials involving 821 lactations were used. Lactoprotein genetic variants were known for 551 of these lactations. The main factor affecting the casein/protein ratio was the genetic polymorphism of beta-lactoglobulin: once corrected for other factors, BB type animals exhibited a ratio nearly 3 g/100 g higher than AA animals. kappa-casein variant B also had a positive effect (+1.2 g/100 g in favour of BB animals relative to AA animals). Except in the first week of lactation and in the two last weeks of pregnancy, the casein/protein ratio varied little during lactation. It was significantly reduced when milk cell count exceeded 200 000 cells/ml, even without clinical mastitis. It also decreased slightly with parity. Among the various dietary factors studied (level and type of nitrogen and energy supplies, forage type and preservation method), none had any significant effect on the milk casein/protein ratio, except in drastic dietary situations. That ratio increased very slightly in parallel with the animals’ milk yield and milk protein content. In practice, measuring the milk protein content in animals free of clinical mastitis remains a very precise predictor of casein content, accounting for 93 % of its variation.