The present study was conducted to investigate the effect of seasonal stress on economic traits with special emphasis on postpartum resumption of ovarian activities. Reproductive and productive records of crossbred dairy cows were analyzed at a large-scale state dairy farm from 1997 to 2013. Genetic groups consisted of HF 50.0%, HF 62.5%, HF 75.0% and HF 87.5%, having the respective levels of Holstein Friesian blood were studied in different seasons of the year. No significant differences were found between the groups with different exotic blood level in calving interval, age at first calving, postpartum ovulation interval and dry period. Calving interval, postpartum ovulation interval and dry period were significantly longer in the spring season while age at first calving was significantly longer in autumn. Significantly high milk production was found in HF 50%. Peak daily milk yield and lactation length were significantly higher in autumn and spring respectively. Animals with highest exotic blood levels showed the trend for prolonged calving interval probably due to reduced resistance to thermal stress. Cows with 50.0% exotic blood and those born in spring showed a slightly shorter age at first calving. The crossbred cows of genetic group HF 62.5% tended to show shortest ovulation intervals and the summer calvers showed significantly highest intervals. Based on results of this study it is suggested that cows calving in summer show lowest calving interval. Crossbred cows with 50% exotic blood showed earliest age at first calving, although non-significant. Crossbred dairy cows showed a constant upward trend in peak daily milk yield from first through sixth parity with a change of 18%. Peak mssilk yield declined up to 23% from parity 7 to 8. The finding may help the farmers in culling of the lactating cows to maintain an optimum herd average for milk production.
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