Estimates of Genetic Parameters for Milk Constituents and Yields

Abstract

Genetic parameters were estimated and selection alternatives for milk yield and its major constituents examined, using 3,841 lactation records from Holstein cows in the North Carolina Institutional Breeding Association. Hcritability estimates from the daughter-dam regressions were 0.28, 0.17, 0.24, 0.21, and 0.21 for milk, fat, PLM, (protein-lactose-mineral), TS (total solids), and protein yields, respectively, and 0.62, 0.53, 0.60, and 0.47 for fat, PLM, TS, and protein percentages, respectively. Within herd-year-season phenotypic correlations between milk yield and the percentages all were negative, whereas correlations among constituents were strongly positive. Genetic correlations were --0.61 between milk and fat per cent, --0.43 between milk and PL~[ per cent, --0.45 between milk and protein per cent, 0.69 between fat and PL~I percentages, 0.74 between fat and protein percentages, and 0.82 between PLM and protein percentages. Direct selection for PL~I yield would give an expected correlated genetic change in milk yield close to that expected from direct selection for milk yield. Selection for fat or protein yield would be expected to result in a substantial reduction in milk yield compared with direct selection for milk yield. Selection for milk or PLM yield would result in moderate to large expected decreases in the percentages. Efforts to increase, prevent declines in, or change the proportions of tl~e constituents appear to be costly in terms of selection pressure which can be applied to the yield traits.