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Genetic Aspects of Lactation Curves¹

Department of Dairy Science, University of Illinois, Urbana 61801

P. J. Berger and A. E. Freeman

Department of Animal Science, Iowa State University, Ames 50011

F. N. Dickinson

AIPL, ASI, AR, SEA, USDA, Beltsville, MD 20705

ABSTRACT

Parameters of lactation curves were defined by a gamma-type function for 113,705 lactations from the California Dairy Herd Improvement Association. Heritabilities and genetic correlations were estimated from a mixed-model analysis of paternal half sisters for first, second, third, and later lactations. Daily milk production on day 6 postpartum was estimated from production at first test-day to reduce the percentage of atypical lactations to less than 1% instead of an expectation of 30% without estimating production for day 6. The natural logarithm of the gamma-type function was ln(Y_t) = ln(a) + b ln(t) - ct, where Y_t was daily milk production in the t^th week and ln(a), b, and c were coefficients. Persistency (c^—(b+1)), week of peak yield (b/c), peak yield (a(b/c)^be^-b), and coefficient of determination were estimated from coefficients for each lactation. Heritabilities in first lactation were: ln(a) .10 ± .01, b .06 ± .01,c .14 ± .02, persistency .02 ± .01, week of peak yield .02 ± .01, peak yield .23 ± .02, and determination .03 ± .01. Heritabilities for later lactations were similar. Cows in first lactation had lower ln(a), lower peak yield, lower c, greater persistency, and took longer to reach peak yield than did cows in later lactations. Genetic and phenotypic correlations indicated that selection for increased peak yield would be associated with increased ln (a), b, and c without changing persistency or week peak yield.

¹ Journal Paper No. J-9976 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project No. 1053, a contributing project to North Central Regional Project, NC-2, Improvement of Dairy Cattle Through Breeding.

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