Genetic and Environmental Parameters of Milk Constituents for Five Breeds. I. Effects of Herd, Year, Season, and Age of the Cow

Open Access
Publisher Website
Google Scholar
Abstract
The influence of herd, year, season, and age on milk production and composition was investigated on 4,253 lactation records of cows of the 5 major dairy breeds. A least-squares analysis of the data showed significant effects of herd, year, season, and age on all milk traits studied. The averages of the variance component estimates expressed as per cent of the total variance were: for herd, 15.7% and 11.6% for constituent yields and percentages respectively; age, 14.6% and 7.3% for yields and percentages; season, 1.9% and 1.4% for yields and percentages; and year, 1.0% and 1.6% for yields and percentages. For younger cows, the least-squares constants for age showed an upward bias of +.10 to +.03% for percentages fat and protein; +.23to+.04% for per cent solids-not-fat (SNF); and +.33 to +.05 for per cent total solids (TS); whereas, for older cows, the downward age bias was -.04 to .20% for per cent SNF; -.07 to -.26% for per cent TS; and was negligible for percentages fat and protein. A set of multiplicative age factors was derived for each yield trait, and the results suggest 2 sets of factors, one for milk yield and one for yields of fat, protein, SNF, and TS.