Thrice-daily milking throughout lactation maintains epithelial integrity and thereby improves milk protein quality

Cows managed for extended lactations of 16 months duration were milked on a half-udder basis twice or thrice daily, commencing in lactation week 9. Ma mmary epithelial integrity (assessed by milk sodium:potassium ratio) was gr eater in the half-udder which was milked thrice daily. This difference was evident throughout the lactation but became greater after week 41. Milk pro tein composition was assessed during late lactation (52+/-3 weeks). Casein number (casein as a proportion of total protein) was significantly higher i n half-udders milked thrice daily, as were the relative amounts of alpha- a nd beta -caseins, whilst those of kappa- and gamma- caseins were reduced. T wo days of inverted milking frequency (i.e. thrice-milked udder halves now milked twice, and vice versa) only partly reversed these differences. We co ncluded that thrice-daily milking will help to prevent or ameliorate the us ual decline in milk processing quality associated with late lactation. Pert of this effect is due simply to reduced exposure to proteolytic enzymes as a result of decreased storage time in the udder, but part is due to a bett er maintenance of epithelial tight junction integrity as lactation advances , which restricts leakage of proteolytic enzymes from serum into milk.