The effects of oral altrenogest on two-year-old stallions in training

Despite widespread use of oral altrenogest (Regu-Mate®, Hoescht-Roussel Vet, Warren, NJ) by trainers throughout the performance horse industry to suppress sexual/aggressive behavior in the stallion, few studies have been performed to document both short and long term physiological effects. Further, variable doses of altrenogest are currentiy in extra-label use, including 0.044mg/kg BW per day (current label dose for the suppression of estrus in mares) and 0.088 mg/kg BW per day. This study examined the suppressive effects of the single mare dose of altrenogest (0.044 mg/kg BW daily) on the steroidal hormone profiles, seminal quality, and sexual/aggressive behavior in two-yearold stallions. Ten two-year-old Quarter Horse stallions in early training were exercised in a training protocol consistent with methods to produce a working ranch horse. Prior to the study, horses were put through a training period for semen collection behavior. Horses were stratified across treatments according to breeding performance and sire, then randomly assigned to treatment (n=5) and control (n=5) groups. The treatment group was administered a daily oral dose of 0.044 mg/kg BW daily for a period of 67 days. No significant differences were found between treatment groups in weight or body condition scores at the experimental dose given. Altrenogest also had no significant effect on any behavioral parameters measure by standardized teasing tests or at time of semen collection. No significant differences were seen for any seminal parameter, at any point in this study. Averages of total scrotal width (TSW) across control and treatment groups were lower (P<. 03) for treatment animals at the end of treatment time period (d 67). By day 157 mean TSW for both treatment and control stallions were not different from each other; however, both were significantiy higher from pre-trial and end of treatment period values. At castration, comparisons of trimmed testicular weight (testicle minus the epididymis) between control and treatment groups were not different. Histological analysis of testicular tissue revealed no significant difference between treatment groups for the average number of spermatids per seminiferous tubule. Altrenogest treatment significantly reduced serum level estrogen (estradiol 17-P) by day 67. Estrogen levels remained suppressed (P = .02) after three months of recovery in these treatment animals. There were no effects on serum testosterone levels throughout the trial. Data from the current study suggest that a daily dose of 0.044 mg/kg BW of altrenogest suppressed estrogen production and scrotal width in the two-year-old stallions, however, stallion sexual/aggressive behavior and seminal parameters were not significantly affected by treatment. This study also showed large variation in libido and sexual/aggressive behavior between these young stallions. Further research is needed to determine various doses and age effects of altrenogest in the stallion.