Introduction:

The low fertility is one of the greatest problems in mare reproduction. Preovulatory follicle diameter could be used as estrus predictor.

Methods:

Forty-three mares, 550 ± 34 kg live weight, 6.00 ± 2.30 years old, and 4.42 ± 0.39 body condition score (CC), without reproductive or sanitary problems were used to study the evolution of follicle diameter (DF) during an estrus cycle. The estrus detection and measurement of DF were done daily, mares with DF > 38 mm were inseminated, pregnancy was tested 16 d post insemination. Variables evaluated were estrus length (Dcelo) and estrus cycle (Dciclo); DF at 21 d (DF21), at start of estrus (DFcelo), and at artificial insemination (DFIA); corpora luteum diameter lúteo (DCL); and pregnancy rate (TP16). The fixed effects included in the model were age of mare, CC, Dcelo, Dciclo, number of follicles, time to estrus appearance after day 21 (Celoh), DFcelo, and DF21. The statistical analyses were carried out with the MIXED and GLIMMIX procedures of SAS.

Results:

Age (p ≤ 0.0221) and Dcelo (p ≤ 0.036) were significant for most of the variables analyzed. Mares 9-11 years old had the greatest Dciclo, DCL, and DF. DFcelo was significant (p ≤ 0.0056) for DCL and DF21; while DF21 was (p ≤ 0.0001) for Celoh, DFcelo, and DFIA. The follicle growth showed a cubic trend, with a reduction in growth rate days before ovulation.

Conclusion:

Age of mare, DF21, and DFcelo, help to predict estrus appearance. Knowledge of ovarian dynamics helps to breed mares at the right time.

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