# Follicular Dynamics and Pregnancy Rates during Foal Heat in Colombian Paso Fino Mares Bred under Permanent Grazing

## TL;DR

This study examines how follicular growth and timing of ovulation during foal heat affect pregnancy rates in Colombian Paso Fino mares under tropical grazing conditions.

## Contribution

The study is the first to evaluate peripartum follicular dynamics leading to foal heat in tropical environments, specifically in Colombian Paso Fino mares.

## Key findings

- Mares that became pregnant had slower-growing dominant follicles and ovulated later (11th–13th postpartum day) compared to non-pregnant mares.
- Pregnant mares had larger pre-ovulatory follicles (48.57 mm) and lower uterine edema scores than non-pregnant mares.
- A 71% pregnancy rate was observed when mares were inseminated during foal heat under tropical grazing conditions.

## Abstract

A high percentage of mares (92–94%) exhibit foal heat in the first 21 days postpartum, and insemination during this estrus is critical to ensure a higher fertility rate per breeding season. A retrospective descriptive study evaluated follicular dynamics peripartum and at foal heat, uterine edema, and endometrial cytology between mares that did or did not become pregnant when inseminated. Mares became pregnant if inseminated when the foal heat started from the second postpartum week, had a lower growth rate of the larger diameter follicle and had an adequate degree of edema compared to non-pregnant mares.

No studies have evaluated the peripartum follicular dynamics resulting in foal heat under tropical environments. We aimed to assess retrospectively the peripartum follicular dynamics in Colombian Paso Fino mares that were inseminated at the foal heat, becoming pregnant or not. Records including follicular dynamics of pregnant mares prepartum and from foaling until foal heat ovulation were assessed in Colombian Paso Fino mares (CPF, n = 24) bred under permanent grazing in a tropical herd in Colombia. The number of ovarian follicles >10 mm before foaling and the largest follicle (F1) growth rate (mm/day) from foaling until the F1 reached the largest diameter (pre-ovulatory size) at the foal heat were assessed. Mares were inseminated at foal heat with 20 mL of semen (at least 500 million live spermatozoa) with >75% motility and 80% viability from a stallion of proven fertility. Ovulation was confirmed the day after follicles had reached the largest diameter. Quantitative data from follicular growth, the day at ovulation, from mares that became pregnant (PM) or not (NPM) at 16 days post-insemination were compared by one-way ANOVA, repeated measures ANOVA (follicle growth rate data) or Chi-square test (edema and cytology scores data). Epidemiological data, gestation length, and the number of follicles on third prepartum days did not significantly differ between PM and NPM (p > 0.05). Seventy-one percent of mares (17/24) got pregnant. Ovulatory follicles grew faster in the NPM group (n = 7), which ovulated between the seventh and ninth postpartum days, compared to PM (n = 17), which ovulated between the 11th and 13th postpartum days. Pre-ovulatory follicle diameter in PM (48.57 ± 0.8 mm) was significantly larger than in NPM (42.99 ± 1.0 mm) (p < 0.05). In addition, the PM edema score (2.93 ± 0.32 mm) on ovulation day was significantly lower (p < 0.05) than NPM (4.47 ± 0.05 mm). First postpartum ovulation occurred at 12.6 ± 0.3 and 8.5 ± 0.4 days (p < 0.05) in PM and NPM, respectively. Colombian Paso Fino mares bred under permanent grazing under tropical rainforest conditions with no foaling or postpartum complications showed a 71% gestation rate when inseminated at foal heat when ovulation occurs between the second and third postpartum week.