Reproductive Performance in Genetic Lines Selected for High or Average Milk Yield

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Abstract
Genetic lines were created by selection of service sires differing by approximately 450 kg of milk for estimated transmitting ability. High line sires were selected from the best available proven sires. Selection continued over 24 yr with up to eight generations of selection. Records from 708 nulliparous, 575 first parity, and 437 second parity animals were analyzed. High milk yield was associated with longer days open and calving intervals in both first and second parities. A 1000-kg increase in 305-d milk production was associated with average increases in both days open and calving interval of around 7 d in first parity and 13 d in second parity and with average increases in days to first detected estrus of 4.5 d in first parity. Difference between genetic lines for milk yield was 804 kg in first parity and 772 kg in second parity. Days open and calving interval were less for the average line in both parities and differed by 10 d in second parity. Other reproductive differences were small or insignificant. Selection for yield has affected reproductive fitness modestly.