Efficacy of oral altrenogest for postponing ovulation in the mare
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The horse industry relies heavily on the breeding manager's ability to get a large number of mares bred each season. Therefore, it is beneficial to optimize use of the stallion and number of cycles per conception, both of which are related to the time of ovulation. Currently, methods exist to induce ovulation of a Graafian follicle; however there is no widely accepted method for delaying ovulation. Considering the main factors that lead to ovulation it could be hypothesized that the short-term administration of a progestin, such as altrenogest, would have the potential to postpone ovulation of a follicle that has reached ovulatory size. Twenty-six mares ranging in age from three to 23 y were paired by age and randomly assigned to a 2X4 blocked experiment. The factors were 1) stallion (Groups A-D) and 2) hormonal treatment (.044mg/kg BW altrenogest for 2 d) or control (Neobee M5 oil for 2 d). Hormonal administration for each mare was initiated upon detection of a follicle with a 35 mm or greater diameter. Mares were artificially inseminated starting on the first day of treatment and continuing every other day until ovulation was confirmed by ultrasonography. Blood samples were collected daily throughout the estrous cycle and at 12 h intervals from the detection of a 35 mm follicle until three days post-ovulation. Samples were analyzed for luteinizing hormone using a double antibody RIA, for progesterone and estradiol using RIA kits (Coat-A-Count® and double antibody, DPC®). Short-term altrenogest administration increased the number of days to ovulation (P<0.05). Mean days to ovulation from d 1 of treatment for the control versus treated mares were 3.15 d and 6.12 d, respectively. There was no difference (P=0.65) between control and treated groups with respect to size of follicle at the time of ovulation. Luteinizing hormone, progesterone and estradiol concentrations were analyzed during treatment until 3 d post-ovulation. Altrenogest treatment had no effect on LH, progesterone and estradiol concentrations as demonstrated by the lack of difference between control and treated mares (P=0.27, P=0.56, P=0.67). There was no difference in pregnancy rates (P=0.62) between the control 10/13 and treated 11/13 mares.
Murrell, Sandra Lee (2003). Efficacy of oral altrenogest for postponing ovulation in the mare. Master's thesis, Texas A&M University. Texas A&M University. Available electronically from