Genetic aspects of lactation curves.

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(yt) = ln(a) + b ln(t) - ct, where yt was daily milk production in the tth 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 of peak yield.