Search

The aim of this study was to identify and rank the various factors, in particular those involving feeding, that affect the proportion of caseins in milk true protein. Twenty-nine feeding trials involving 821 lactations were assessed, and lactoprotein genetic variants were known for 551 of these. The main factor affecting the casein: protein ratio was the genetic variant of β-lactoglobulin: once corrected for other factors, the milk of BB type animals had a ratio nearly 30 g/kg total protein higher than AA animals. κ-Casein variant B also had a positive effect (+12 g/kg in favour of BB relative to AA animals). Except in the last weeks of pregnancy and the first weeks of lactation, the casein[ratio ]protein ratio varied little during lactation. It was significantly reduced when milk cell count exceeded 200000 cells/ml, even in the absence of 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[ratio ]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 from clinical mastitis remains a very precise predictor of casein content, accounting for 93% of its variation.

SummaryOne hundred and thirty-seven (Holstein (41), Montbéliarde (42) and Tarentaise (54)) dairy cows in first or second lactation received during winter one of two levels (high, H; low, L) of energy intake, and were later fed identically at pasture. Thrice in winter and twice at pasture, the chemical composition and coagulation properties of individual milks were measured. Milk from Holstein cows had lower casein and calcium contents, and poorer coagulation properties (curd firming time and curd firmness) than that from Montbéliardes and Tarentaises (P< 0·01). These differences practically disappeared when taking into account the distribution of the differentκ-casein variants and milk casein content.κ-BB milks had coagulation properties 20–50% superior, according to characteristic, to those of AA milks. In the three breeds, animals from the H group had casein contents higher by 1·4 g/l than those of the L group, which induced a significant improvement in curd firming time, curd firmness and cheese yield. Turning out to pasture induced an increase of 0·02 in milk pH, and improved milk coagulation properties. These changes did not appear to result entirely from the parallel increase in milk casein content.