Age-Specific Pregnancy Rates in Feral Horses

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Abstract
Blood samples were collected from 137 feral female horses (E. caballus) captured near Salmon, Idaho [USA] in Oct., 1980. Serum samples were analyzed for progesterone, luteinizing hormone or pregnant mare serum gonadotropin [PMSG] and estradiol-17.beta. for evaluation of reproductive status. The mares were age classified, observed for lactation and the presence of a foal noted. A diagnosis of pregnancy was made on the basis of progesterone > 2.4 ng/ml and either PMSG > 13 ng/ml or estradiol-17.beta. > 69 pg/ml. Mares 1 yr and older were classified as anestrus = 31, cycling = 12, possible abortions = 3, uncertain status = 6 and pregnant = 85. Age-specific pregnancy rates increased with age from 36% in the 2-yr-olds to about 85% in the 6-15-yr-olds. No yearlings were pregnant. Sixty-two mares, 56 with foals, were lactating. No 2-yr-olds, 25% of the 3-yr-olds, 43% of the 4-yr-olds and 50-75% of the 5-yr and older animals were lactating. Fifty-two animals were pregnant and were lactating or had a foal at the side. Thus, at least 50% of the eligible mares were pregnant in consecutive years.