Use Of Etonogestrel Implants To Suppress Estrous Behavior In Mares

Abstract

The objective of this study was to evaluate a human contraceptive subcutaneous implant (Implanon®) as a method to suppress behavioral estrus in mares. Twenty mares were randomly assigned to 4 groups (n=5). Group C was the control group, group T1 received one Implanon® implant (68 mg etonogestrel), group T2 received two Implanon® implants (136 mg etonogestrel), and group R was the positive control, receiving 0.044 mg/kg altrenogest daily. Estrous behavior was evaluated twice weekly and was graded by a blinded observer. Estrous cycles were monitored during three months using progesterone levels and transrectal examinations. Interestrus interval (IEI) was measured based on both behavioral estrus (teasing scores) and serum progesterone concentration (below 1.0 ng/ml). Mean interestrus interval ± SEM per group, based on teasing and progesterone levels, respectively, were as follows: group C 21±0.3 and 21±0.4 days; group T1: 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 Interestrus interval twice as long as 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. Etonogestrel was not consistently effective for estrus suppression in mares, however, it did produce an IEI twice as long as the negative control at the highest dose used in this study (136mg). Further studies with a higher dose would be necessary to determine whether or not etonogestrel can be used to fully suppress estrus in mares.