Selecting for Lactation Curve and Milk Yield in Dairy Cattle

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Abstract
Knowledge of genetic relationships between characteristics of lactation curves and lactation yields is essential for joint selection for both. An equation, yt = atbexp(-ct), was chosen to depict individual lactation curves for 5927 1st lactations by Holsteins in 557 herds in Michigan [USA] Dairy Herd Improvement where yt was daily milk yield at day t in lactation, a was yield at time zero, b was ascent to peak and c was declined after peak. Genetic correlations for 305 day milk yield with initial production (a), ascent to peak (b), descent after peak (c) and peak yield were -0.37, 0.40, 0 and 0.91. From empirical results from applied selection indexes, selecting for increase of ascent to peak and peak yield did not decrease 305 day milk substantially. Rankings of sires on these indexes were similar to their rankings on milk yield alone. Attempts to decrease peak yield and increase persistency decreased milk yield greatly.