Effects of Early and Late Breeding of Dairy Cows on Reproduction and Production in Current and Subsequent Lactation

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Abstract
Effects of early and late breeding post partum on reproduction and production were investigated in a Holstein herd. Cows were assigned randomly to 2 groups at calving: an early group bred at the 1st observed estrus following 50 days postpartum and a late group bred at 1st observed heat 80 days postpartum. Early vs. late breeding resulted in intervals to 1st service of 72 .+-. 18 or 93 .+-. 17 days; conception at 88 .+-. 33 or 121 .+-. 40 days postpartum; and 1.50 (range 1-3) or 1.96 (range 1-5) services per conception. Postpartum reproductive activity was monitored with milk progesterone analyses. Average duration of 1st estrous cycle was 17 .+-. 7.5 days. The 1st estrous detected by milk progestrone occurred 33 .+-. 10 days postpartum compared to the 1st observed heat at 48 .+-. 26 days postpartum. Calving problems and calving in pasture season delayed conception in the late bred group but did not influence conception in the early bred group. The late bred group produced more fat-corrected milk in 305 day lactation and in the 1st 150 days of the subsequent lactation . Average daily milk production from beginning of current lactation to day 150 of subsequent lactation (including the dry period) was the same for both early and late bred groups.