Endometrial biopsy in mares under nonuniform breeding management conditions: Prognostic value and relationship with age.

Endometrial biopsy plays an important role in fertility examination of the mare. The purpose of this study was to examine the prognostic usefulness of endometrial biopsy under nonuniform breeding management conditions, and to clarify any relationship with age of the mare. Based on endometrial histological changes, 192 mares from different farms were classified using a four-category grading system. There were significant associations between categories and fertility. Mean pregnancy rates in the four categories were 79, 49, 33 and 0% and mean foaling rates were 70, 42, 18 and 0%, respectively. Mares older than the median age (=11 years) had, on average, higher category scores and significantly lower fertility than younger mares. Within categories, there was a tendency towards higher fertility in younger mares. Although the precise prediction of fertility of the individual mare remains difficult, it is evident from this investigation that endometrial biopsy has considerable prognostic potential even under nonuniform breeding management conditions. Furthermore, the age of a mare must be considered when estimating her fertility potential.