EFFECTS OF ORAL ALTRENOGEST ON SEXUAL AND AGGRESSIVE BEHAVIORS AND SEMINAL PARAMETERS IN YOUNG STALLIONS

This study examined the suppressive effects on sexual and aggressive b ehavior and seminal parameters in young stallions as a result of daily administration of altrenogest(a) for a period of eight weeks. These p arameters were also examined after a recovery period of eight weeks (d ay 111). At three times in the study (days -9, 60 and 111), measuremen ts were recorded including weight, scrotal circumference and body cond ition score of the stallions. Stallions in the two treatment groups di d not differ in weight, body condition score or metacarpal measurement s throughout the study. However, scrotal circumferences decreased (P < .05) in treated stallions at week eight and remained lower (P < .05) after eight week post cessation of altrenogest (week 16). Values for s crotal circumference were different (P < .05) between control and trea tment groups at week 16. Decreases in estimated daily sperm production (DSP) were also found in stallions within the treatment group at both eight weeks (P < .01) and 16 weeks (P < .05). This decrease in estima ted DSP was most likely correlated with decreased scrotal circumferenc e. Spermatozoal abnormalities were significantly increased within the treatment stallions, with head abnormalities (P < .009) and tail abnor malities (P < .001) increasing between the pre-trial collection period and the eight-week collection period. Stallions were scored for sexua l and aggressive behaviors using a defined teasing test three times du ring the study (days -12, 57 and 111). Behavior was evaluated by expos ing the stallion to a hand-held mare in estrus for a three-minute obse rvation period. Flehmen frequency and duration decreased (P < .05) amo ng altrenogest-treated stallions at the eight-week collection period. The duration of dropping of the penis tended to decrease (P < .08) at week eight in the treated stallions. Erection duration (P < .05) and f requency (P < .005) were also reduced in the treated group at eight we eks. These data suggest that the treatment of young stallions with alt renogest for eight weeks has marked effects on sexual and aggressive b ehaviors and sperm production. Several of these parameters did not ret urn to pre-trial values by eight weeks after cessation of treatment. F urther research is needed to investigate the capacity of young stallio ns to recover over time from the adverse effects of altrenogest admini stration as demonstrated by reduced libido, scrotal measurements and d aily sperm production documented in this study.