A. TIJANI,* G. R. WIGGANS, C. P. VAN
TASSELL, J. C. PHILPOT, and
*Unité de Zootechnie, Faculté Universitaire des Sciences Agronomiques, B-5030 Gembloux, Belgium
Animal Improvement Programs Laboratory and
Gene Evaluation and Mapping Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350
§Fonds National Belge de la Recherche Scientifique B-1000 Brussels, Belgium
ABSTRACT (Co)variance functions for milk, fat, and protein yields during first lactation were developed from (co)variance components for four lactation stages of 75 d each that had been previously estimated using test day data from 17,190 Holstein cows from 37 herds in Pennsylvania and Wisconsin. The (co)variance functions were evaluated at 18, 43, 68, 93, 118, 143, 168, 193, 218, 243, 268, and 293 d in milk. Residuals were subdivided into time-dependent (permanent) and temporary environmental effects for estimation of (co)variance functions. Mean relative variance (portion of total variance) for time-dependent environmental effects was 0.50 for milk yield and 0.51 for fat and protein yields. Heritability estimates generally were lower at the start and end of lactation and were highest for milk yield; mean heritability estimates were 0.20 for milk, 0.16 for fat, and 0.17 for protein yields. Phenotypic and genetic correlations were higher between milk and protein yields than between milk and fat yields. Within yield traits, genetic correlations declined from >=0.93 for adjacent lactation stages to 0.52 for milk, 0.58 for fat, and 0.60 for protein between initial and final lactation stages. Within lactation stage, mean genetic correlations were 0.40 between milk and fat yields, 0.78 between milk and protein yields, and 0.55 between fat and protein yields; corresponding mean phenotypic correlations were 0.65, 0.92, and 0.67. The (co)variance function methodology allowed interpolation and extension of (co)variance components over the entire lactation.
Key Words: (co)variance function, test day model, variance component estimation
1999 J. Dairy Sci. 82:226
© 1999, by the American Dairy Science Association. All rights reserved.