Genotype x environmental interactions on reproductive traits of bovine females. III. Seasonal variation in postpartum reproduction as influenced by genotype, suckling and dietary regimen.
We examined the effect of season of calving (winter = October to March, summer = April to September) on postpartum reproductive traits by analyzing six data sets in which cows calved throughout the year. In two experiments, with suckled and nonsuckled cows, animals that calved in winter had longer (P less than .01) intervals from parturition to first estrus (IE) than those that calved in summer. Multiple regression analysis [model; IE = y + b1 (calving date) + b2 (calving date)2] indicated that the relationship between calving date and IE was less for nonsuckled cows than for suckled cows because there was heterogeneity of regression (P less than .005) between groups. A similar seasonal difference and calving date X suckling status interaction (P less than .10) was observed in a third experiment. Another experiment involved Holstein cows ranked for genetic ability for milk production. Cows with genetic ability for high milk production and calving in summer tended (P less than .12) to have shorter IE than winter-calving cows. Genetically low producing cows did not show this tendency and there was heterogeneity of regression (P less than .10) between groups. In two experiments cows were fed diets either low or high in energy and data were analyzed for three consecutive postpartum periods (PP1, PP2, PP3) beginning after first calving. In PP1 of one experiment, intervals to first ovulation, estrus and conception were longer in winter than in summer. The multiple regression equation between calving date and IE was significant (P less than .10) only for cows fed diets low in energy. There was no significant relationship between calving date and IE in PP2 or PP3. In the other experiment, there were not enough winter-calving cows to make seasonal comparisons in PP1 and season did not significantly influence IE in PP2 or PP3. We concluded that, in general, winter-calving cows appeared to take longer to regain reproductive competence after calving than summer-calving cows. Differences between seasons tended to be greater for suckled cows, cows with genetic ability for high milk production, primiparous cows and cows fed diets low in energy.
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