Relation of Milk Fat-Depressing Rations and Subclinical Mastitis to Milk Proteins

Abstract

Two continuous feeding trials involving 28 cows were used to study the influence of a pelleted, high-grain ration, of dietary supplementation with unsaturated oils (safflower and cod liver oil), and of subclinical mastitis under these conditions, on milk total proteins, casein, whey proteins, solids-notfat (SNF), and fat. Milk samples from 512 udder quarters were analyzed. Total milk protein and casein contents were elevated by each nutritional regime. Whey protein and SNF contents increased while feeding the high-grain ration, but both showed only minor changes during the oil supplementation. A net gain in milk protein synthesis of 15% was obtained during the high-grain regime, but protein yields during the oil regime were lower because of reduced milk production. Milk fat percentages and fat yields were depressed by each regime.Ratios of casein content to total protein content of milk were significantly related to Wisconsin Mastitis Test (WMT) scores (r = —0.45). Intraclass correlation coefficients within udder quarters for WMT scores on whey protein per cent, SNF per cent, casein yield, milk yield, total protein yield, fat per cent, casein per cent, whey protein yield, and total protein per cent were, in decreasing order, 0.45, —0.41, —0.34, —0.33, —0.29, —0.19, —0.18, —0.14, and 0.08, respectively.