Effects of short-term oral altrenogest dosages on behavioral and testicular parameters of post-pubertal stallions

Overly sexual/aggressive stallions within the performance horse industry create a hazardous working environment and several training challenges To suppress these behaviors without surgical castration, many trainers are currently administering oral altrenogest (Regu-Mate®, Intervet International B V., Boxmeer, Netherlands) to stallions in an off-label apphcation. Though few studies have been performed on stallions, previous work suggests the effectiveness of altrenogest in reducing the levels of reproductive hormones, spermatogenesis, and sexual/aggressive behavior varied depending on the dosage size, the duration of treatment, and the age of the stallion. However, to date no one has examined the efficiency of a short term, repetitive dosage with practical application, such as when behavior is unwanted only for a temporary period (during a performance presentation; i.e., show or race). Therefore, the objective of this study was to evaluate the testicular, hormonal, and behavioral parameters of three year old stallions receiving a short term, repetitive dosage of oral altrenogest. Further objectives were to etermine if the sexual/aggressive behavioral could be suppressed while reducing the longterm negative effects on reproduction. Nine, three-year-old Quarter Horse stallions were initially blocked by sire, and then randomly designated to two groups, treatment (n=5) and control (n=4). The treatment group received an oral daily dose of 0.088 mg/kg BW Regu-Mate® followed by ten days of recuperation (no treatment). This process was repeated for a total of three times over 60 days, followed by a recovery period of 36 days. The altrenogest dosages had no significant effects on body weight, but did increase body condition scores (P=0.052). Body condition scores remained higher after Day 30. Of the testicular parameters analyzed on Day 96, scrotal width and gross testicular parameters were unaffected, however, mean spermatid counts were decreased (P<0.05). While no differences were due solely to treatment for either estrogen or testosterone concentrations, there was a significant time by treatment interaction involving estrogen levels(P<0.05). No significant differences were seen for any of the behavioral actions analyzed based on frequency, latency, or duration, at any point of the study. Several traits assessed in this study showed large variations between individual stallions. Further research of altrenogest in the stallions is needed to determine a practical dosage amount and sequence to produce a manageable animal, which takes age and fertility effects into consideration.