# Factors Associated with the Occurrence of the First-Follicular-Wave Dominant Follicle on the Ovary Ipsilateral to the Corpus Luteum in Dairy Cattle

## TL;DR

This study found that the location of a dominant follicle in dairy cows is influenced by the position of a previous follicle and milk production levels.

## Contribution

The study identifies for the first time that milk production levels and follicle location relationships affect dominant follicle occurrence in dairy cattle.

## Key findings

- The IG occurrence rate was higher when the preovulatory follicle was contralateral to the regressing CL.
- Milk production levels positively correlate with the IG occurrence rate in lactating dairy cows.

## Abstract

The determination of the factors associated with the occurrence of the first-wave dominant follicle (DF) in the ovary ipsilateral to the corpus luteum (CL) (IG) in lactating Holstein dairy cows and dairy heifers was conducted in this study. The IG occurrence rate was significantly higher when the preovulatory follicle (PF) was located contralateral to the regressing CL rather than ipsilateral in lactating dairy cows and heifers. In addition, the IG occurrence rate increased with the increasing of milk production. From these results, the first-wave DF and CL locational development was associated with the PF and CL locational relationship and with the level of milk production in dairy cattle.

This study aimed to determine the factors associated with the occurrence of the first-wave dominant follicle (DF) in the ovary ipsilateral to the corpus luteum (CL) in lactating dairy cows and dairy heifers. A total of 505 estruses were investigated (lactating dairy cows, n = 361; dairy heifers, n = 144). The locations of the preovulatory follicle (PF) and regressed CL were examined at the estrus, and the locations of the first-wave dominant follicle (DF) and newly formed CL were examined seven days after estrus using transrectal ultrasonography. Then, cows were classified into two groups: the first-wave DF in the ovary ipsilateral to the CL (IG) and the first-wave DF in the ovary contralateral to the CL (CG). To evaluate the factors which affect the occurrence of IG and CG, binominal logistic regression analysis was conducted; the location of the PF and regressing CL, season (warm: June–September; cool: October–May), live weight, days in milk at estrus, daily milk production, and body condition score were used as independent variables. The occurrence rate of IG was significantly higher when the PF was located contralateral to the regressing CL (lactating dairy cows, 63.4%; dairy heifers, 58.6%) rather than ipsilateral (lactating dairy cows, 44.9%; dairy heifers, 35.1%). The IG occurrence rate was significantly higher with an increase in daily milk production (<30 kg, 47.3%; 30–40 kg, 55.2%; >40 kg, 60.5%) in lactating dairy cows. In conclusion, the occurrence of IG was associated with relative locations of the PF and regressing CL in lactating dairy cows and dairy heifers and with the level of milk production in lactating dairy cows.