LAUSR

ABSTRACT This study was conducted to better understand factors contributing to lifelong high fertility and fecundity in a herd of ponies kept under natural social organization and environmental conditions, where most fillies begin foaling as 2‐year‐olds and continue to foal annually as long as they remain in the herd. Endometrial biopsy samples were procured from 23 mares on Day 6 postpartum for histologic evaluation. Kenney classification as well as degrees of lymphocytic endometritis and periglandular fibrosis of mares aged 2–9 years (after one to eight pregnancies, n = 14) were compared with those of mares aged 10 years and older (after 9–14 pregnancies, n = 9). Periglandular fibrosis ranged from none to moderate. As expected postpartum, lymphocytic endometritis was reported as moderate for 22 mares. The one exception was reported as moderate to severe. Differences in the proportion of younger versus older mares with classifications of I, IIA, and IIB, the proportion of mares with greater than mild lymphocytic endometritis, and the proportion of mares with greater than mild periglandular fibrosis were not significant (Fisher's exact tests, P > .10). HIGHLIGHTSSemi‐feral herd since 1994 for study reproductive physiology and behavior.Fillies typically conceive as yearlings and continue to foal annually until natural death.Herd mares show no apparent decline in reproductive efficiency.Endometrial histology characterized comparing younger versus older mares.Endometrial histologic findings similar in younger versus older mares.