SEMEN PARAMETERS AND HORMONE CONCENTRATIONS IN STALLIONS SUBJECTED TOLONG-TERM ESTROGEN ADMINISTRATION

Eight pony stallions were paired by estimated daily sperm output (DSO) and randomly assigned to one of two treatments in a randomized block experiment. Stallions received 44 mu g/kg BW estradiol cypionate (ECP) or an equivalent volume of physiological saline solution on alternate days during the breeding season. Blood samples collected immediately preceding each injection were assayed for luteinizing hormone (LH), es tradiol-17 beta (E-2) and testosterone (T). Semen was collected twice weekly, 3.5 days apart, to evaluate sperm motility and total number of sperm per ejaculate. Prior to and after 4, 8 and 12 weeks on treatmen t. semen was collected once daily for 7 days to determine DSO. Data we re separated into 9 periods (10 days each) for statistical analysis an d subjected to analysis of variance for a randomized block design to d etermine treatment effects. There were no differences (p>.05) between groups for DSO or LH prior to initiation of treatment. Testosterone wa s higher (p<.05) in ECP stallions compared with C stallions prior to t reatment and at all time points measured. As expected, E-2 was higher (p<.05) in the ECP stallions compared to C stallions after 20 days (pe riod 2) of treatment and for the remainder of the experiment. However, E-2 was higher (p<.05) in the C group prior to treatment, but there w as no difference between the groups after 10 d of treatment (period 1) . ECP stallions had higher (p<.05) DSO than C stallions after 30 d on treatment. After 40 and 50 d (periods 4 and 5), ECP stallions demonstr ated higher (p<.05) total sperm per period than C stallions. This was preceded by higher (p<.05) LH values for ECP stallions than for C stal lions after 10 and 20 d (periods 1 and 2). No differences were found b etween the ECP and C groups for LH between 30 and 60 d. Although numer ically higher, no significant differences (p>.05) were seen after 60 d ays for DSO or after 60, 70 or 80 days for total sperm per period. ECP stallions had higher (p<.05) DSO and total sperm per period after 90 d than C stallions. Additionally, LH remained significantly higher (p< .05) in the ECP group after 60 days (periods 7, 8, and 9). Elevated LH concentrations in ECP stallions demonstrated that estrogen treatment did not inhibit LH secretion in this study.