Effects of norgestomet, altrenogest, and/or estradiol on follicular and hormonal characteristics of late transitional mares

Two experiments were conducted to investigate the effect of norgestomet and altrenogest, alone or in combination with estradiol, on late transitional mares. In the first experiment, 32 mares were assigned to four treatment groups: controls (C), those treated with 1.5 mg of norgestomet (N1), 3.0 mg norgestomet (N2) or 26 mg altrenogest (AT). Treatments were initiated during the months of April and May and given daily for 15 d. During treatment, altrenogest suppressed estrous behavior and diameter of the largest follicle, whereas norgestomet had no effect at either dose. The rise in serum luteinizing hormone (LH) levels following the withdrawal of altrenogest treatment was significantly greater than that for the other three groups. In the second experiment, 24 late transitional mares were assigned to three treatments: controls (C), those receiving 26 mg altrenogest (AT) daily, or 26 mg altrenogest plus 10 mg estradiol (AE) daily for 16 d. Both altrenogest treatments suppressed estrous behavior and follicular growth compared with controls. However, suppression of follicular activity was significantly greater for the combined steroid treatment. Following treatment, the interval to ovulation and estrus was longer for the combined steroid group. We concluded that: 1) norgestomet at a dose up to 3.0 mg per day had no effect on follicular activity, estrous behavior or serum LH levels in late transitional mares, 2) estradiol combined with altrenogest had greater suppressive activity on follicular growth than altrenogest alone, and 3) the greater suppression by the combined steroid treatment had no advantage over altrenogest alone on induction of estrus and ovulation in late transitional mares.