Maximization of Lactation Milk Production Without Decreasing Persistency*

This study treats each daily estimated breeding value (EBV) of the lactation as a separate trait to modify the lactation curve on a daily basis. Six selection strategies for improving lactation milk without decreasing persistency were compared: 1) index IR1, subject to the restriction of equal genetic gains at days in milk (DIM) 60 and 280, 2) IR2, subject to the restriction of zero gain at DIM 60, 3) desired gains index Id, designed to increase lactation milk without altering the lactation curve, 4) index Iu, comprising lactation EBV and persistency without standardization, 5) index Iw, consisting of lactation EBV (EBVL) and persistency with standardization, and 6) conventional selection on EBVL and used as a basis for comparison. Of the 6 selection strategies compared, IR2 yielded the greatest persistency, but achieved the smallest response in EBVL, suggesting that it is impractical to increase persistency by inhibiting change in the peak yield. Index Iu showed the same response in lactation milk as conventional selection on EBVL, but resulted in the same decreased persistency. Although both IR1 and Id achieved constant persistency, the former produced a greater lactation response (669 kg EBV) than the latter (560 kg EBV). Thus, IR1 is a viable strategy for improving EBVL while holding persistency constant. None of the 6 selection strategies excelled in both lactation milk and persistency. Index Iw appears to be a reasonable choice for improving both traits, although responses would depend on the relative economic importance of the 2 traits. Differential responses between Iu and Iw emphasize the need to weight the EBV of different traits by the inverse of their standard deviations in index construction when the EBV vary widely in variance. The general formula developed here provides a useful genetic means of modifying the lactation curve by restricting differential genetic gains among different days of the lactation.