Seasonal ovarian function in feral mares: Seasonal patterns of LH, progestins and estrogens in feral mares

Abstract

The reproductive physiology of feral mares in North America is largely unstudied, thus it was of interest to examine seasonal ovarian function in these animals. Four adult mares of proven fertility from the Pryor Mountain National Wild Horse Range (Montana) were captured, and maintained in a corral on alfalfa hay. After 3 months acclimation, blood was collected every 3 days for 13 months (Sept. 1978–Sept. 1979). A teaser stallion was kept in an adjacent corral and the mares were tested for behavioral estrus. Plasma LH and total estrogens (E) were measured by RIA and progresterone (P) by CPB assay. In the captive mares average basal plasma LH levels (Ng/ml) were greater from Apr. through July (8.1+/−0.5) than from Nov. through Jan. (2.2+/−0.2). A total of 21 LH peaks occurred between Apr. 13 and Aug. 31 among the four mares. Many peaks exceeded 20X basal levels, and a trend of higher LH levels in each succeeding peak was observed. Basal to peak LH increases were greater than those reported for domestic mares. In all instances except one, LH peaks were associated with P levels of 0.5 ng/ml or less and with concomitant elevations of E (peak average=43.1+/−12.1 pg/ml). Basal P levels from Apr. through July (1.5+/−1.2 ng/ml) did not differ significantly from levels for Oct. through Jan. (1.1+/−0.7 ng/ml), nor did basal E levels differ significantly between these two time periods (8.4+/−3.2 pg/ml versus 12.9+/−4.6 pg/ml respectively). Behavioral estrus always occurred with LH and E peaks from Apr. through July. However, behavioral estrus was occasionally observed from Aug. through Oct., when LH peaks no longer occurred, suggesting that behavioral estrus is not a reliable indicator of ovarian cycling in feral mares.