Short communication: A positive relationship between the first ovulation postpartum and the increasing ratio of milk yield in the first part of lactation in dairy cows.

The aim of the present study was to examine the relationship between characteristics of the lactation curve, on the basis of daily milk yield, and ovulation within 3 wk postpartum as an indicator of early return to luteal activity in dairy cows. Lactation records from 46 lactating Holstein cows between calving and 305 d postpartum were studied. Milk samples were collected twice weekly between d 7 and 100 for later determination of progesterone concentrations. Occurrence of an early first ovulation was determined by an increase in milk progesterone by 3 wk after calving. Milk yield was recorded daily until 305 d postpartum, and average yield was calculated weekly. The lactation curve was characterized by 8 indices on the basis of the weekly average of milk yield as follows: a) first-week milk yield; b) peak milk yield; c) actual 305-d milk yield; d) peak week; e) difference in milk yield between the first week and peak week; f) difference in milk yield between the peak week and last week (43rd week postpartum); g) ratio of increase in milk yield between wk 1 and the week of peak yield; and h) ratio of decline in milk yield between the week of peak yield and the last week. Indices g and h were calculated as linear. The number of cows having ovulated by 3 wk postpartum was 22 (47.8%). The resumption of ovarian cycles with normal luteal phases occurred earlier in ovular cows than in anovular cows (32.0 d vs. 57.1 d). Although total milk yield did not differ between ovular and anovular cows, the ratio of increase in milk yield from the first week to the peak week (index g) in ovular cows was smaller compared with that of anovular cows (1.71 vs. 2.54). In addition, the ratio of increase in milk yield from the first week to the third week postpartum was greater in anovular cows by 3 wk postpartum (ovular = 1.43 +/- 0.23 vs. anovular = 2.32 +/- 0.29). In conclusion, the present study demonstrates that a greater increasing ratio of milk yield during early lactation may delay resumption of ovarian cycles after parturition. Therefore, this study is the first to demonstrate statistically that a smaller increasing ratio of milk yield (index g) during early lactation may have a beneficial effect on the first ovulation by 3 wk postpartum.