Progesterone Supplementation Day 3 to 5 Post-Ovulation in AI-Bred Mares H. Richard 1, P. Appleton 2, D.J. Kesler 3, C.E. Ferguson 1 1 Department of Agricultural Sciences, McNeese State University, Lake Charles, LA 2 Department of Theriogenology, Delta Equine Center, Vinton, LA 3 Department of Animal Sciences, University of Illinois, IL (a)(b) On day 3 to 5 post-ovulation, treatment mares received 15 mg of altrenogest, orally, and jugular blood samples were collected on all mares from day 0 to 6 post-ovulation. Plasma was collected from blood samples and frozen until Radioimmunoassay (RIA) at Louisiana State University. RIA was performed using a P 4 kit (Diagnostic Systems Laboratory, Webster, TX) with a sensitivity of 0.1 ng/ml and an intraassay and interassay coefficient of 7.3 and 9.3, respectively. Pregnancy data was collected by ultrasound evaluations on day 14, post- ovulation. Statistical analysis was performed in SAS using the GLM procedure to determine differences in plasma P 4 concentrations between pregnant and open control mares. Differences in pregnancy rates were determined using the GLIMMIX procedure for binomial data. Independent variables evaluated included; stallion, mare age, BCS, lactation status. MATERIALS & METHODS Since an increase P 4 from day 3 to 5 failed to improve pregnancy rates in treated mares yet pregnant mares had a significant increase in P 4 from day 1 to 5 it is possible that the increase in P 4 in pregnant mares is the result of a luteotropic substance of embryonic origin. This would explain an increase in P 4 in pregnant mares over open mares without P 4 supplementation having a significant effect. DISCUSSION RESULTS The objective of this study was to determine supplementing a low dose of progestin from day 3 to 5 post-ovulation in AI-bred mares improved pregnancy rates and to determine if there were differences in plasma progesterone between early pregnant and non- pregnant mares. OBJECTIVES Due to the increase in racing purses as a result of incorporating profits from some forms of casino style gaming allowed at horse racing tracks the use of artificial insemination (AI) has been increasing in the American Racing Quarter Horse, in states with this type of gaming systems. In an effort to increase the number of racing Quarter Horses from proven sires, post-AI treatment s that could improve pregnancy rates were evaluated. Previous research in repeat breeder cattle demonstrated that progesterone (P 4 ) from day 3 to 5 improved pregnancy rates (Ferguson et al., 2012). Additionally, pregnant cattle had a greater increase in plasma P 4 from day 3 to 5 of the estrous cycle (Ferguson et al., 2012). Therefore, the natural increase in P 4, in the mare was evaluated and found to be highest from day 1 to 3 post-ovulation, but possible disruption of embryo transport due to exogenous P 4 was wished to be avoided. INTRODUCTION A total of 61 Quarter Horse and two Thoroughbred mares were evaluated via ultrasonography to identify a follicle ~ 30 mm in diameter. Based on visual signs of estrus and follicle size mares were administered Desorelin Acetate. Mares were AI bred with fresh (n = 57) or cooled (n = 6) semen from four different Quarter Horse Race Stallions. Ovulation occurred ~ 12 to 24 h post-AI and was determined via ultrasonography. All ultrasound evaluations and AI-breeding was performed by a single experienced veterinarian. After AI mares were randomly allotted to one of two treatments (control or altrenogest, Regumate ® ). MATERIALS & METHODS RESULTS There were no significant difference in pregnancy rate between control and altrenogest treated mares (table 1). There was a sire effect where sire B had a higher pregnancy rate in the control group compared to his pregnancy rate in the treatment group and to the pregnancy rate of sire A in either group (table 2). A significant (P<0.05) increase in P 4 occurred from day 1 to 5 post-ovulation in pregnant mares compared to open mares (table 3). Although there were no differences in P 4 on any day between treatments there was a trend (P<0.10) for P 4 to be higher in pregnant mares on day 5 compared to open mares (figure 1). The authors would like to thank Delta Equine Center and all horse owners who consented to participate in this research project. ACKNOWLEDGEMENT Figure 1. A comparison of plasma P 4 concentrations from day 0 to 5 in pregnant and open control mares.