Effect of milk yield, number of calving and calving season on the resumption of postpartum ovarian activity in Holstein cows

The objective of the study was to evaluate the effect of accumulated milk yield from calving to day 50 of lactation (MY50), number (NC) and season (SC) of calving on the intervals from calving to the appearance of the first dominant follicle (DF), the appearance of the first corpus luteum (CL), the time to completion of uterine involution (Ul), and the probability of ovulation of the first DF in Holstein cows calving in spring or winter in a commercial dairy farm in Tlahualilo, Durango, Mexico. Neither SC nor NC affected the appearance of the first DF (alpha = 0.05). However, cows with higher MY50 presented the first DF two days latter than cows with low MY50 (13.6 vs 11.9 days; P<0.05). The effects of NC, SC, and MY50 did not affect the probability of ovulation of the first DF (alpha=0.05). However, cows with higher body condition score at calving (BCS > 3.6 units) were more likely (OR = 9.0; P<0.05) to achieve ovulation in the first DF than cows with lower BCS. The appearance of the first CL after calving was not affected by MY50; however, cows with four or more calvings showed their first CL later (P<0.05) than younger cows. Likewise, cows calving in winter showed their first CL later than those calving in spring (20 vs 16 days; P<0.01). However, cows calving in spring required longer time intervals to complete their Ul (36 vs 32 days; P<0.05). It is concluded that follicular growth is not the main limiting factor to the resumption of pospartum ovarian activity in Holstein cows managed under the conditions of the present experiment.