Use of Etonogestrel Implants to Suppress Estrous Beahvior in Mares

Abstract

The objective of this study is to evaluate a synthetic progestin (etonogestrel) as a reliable method to suppress behavioral estrus in mares. Healthy mares between the ages of 6 and 20 years with normal estrous cycles were randomly assigned to 4 groups (n=5). Group C was the control group (no treatment), group T1 received one Implanon® subdermal implant (68mg etonogestrel), group T2 received two Implanon® implants (136 mg etonogestrel), and group R was the positive control, receiving 0.044 mg/kg altrenogest orally daily (Regumate®). Behavioral estrus response to teasing with a stallion was evaluated twice weekly by a blinded observer. Estrous cycles were monitored for three months by weekly progesterone levels and twice weekly transrectal examinations. Interestrus interval (IEI) was measured based on both behavioral estrus (teasing scores) and plasma progesterone concentration (below 1.0 ng/ml). Mean IEI per group, based on teasing and progesterone levels respectively, were as follows: group C (control) 21±0.3 and 21±0.4 days (± SEM); group T2: 34±8.2 and 31±6.4 days; group T2: 42±14.1 and 41±14.4 days; and group R: 111±1.3 and 48±0.9. Group T2 had an IEI twice longer than the control group, however, no statistical difference was found between groups C, T1 and T2. Group R (positive control) was different from all other groups (P<0.05) based on teasing observations, and estrous iii behavior in this group was suppressed during the entire study period. Based on progesterone levels, IEI was different only between groups R and group C. The IEI determined by teasing and progesterone levels were highly correlated (r=0.911). The high correlation between teasing and progesterone levels may validate teasing score as a reliable tool to determine estrus in mares by an experienced observer in animals with regular estrus behavior. Etonogestrel was not consistently effective for estrus suppression in mares at this dose (136mg), however, it did produce an IEI twice as long as the negative control. Future studies with a higher dose would be necessary to determine whether or not etonogestrel can be used to fully suppress estrus in mares.