NORGESTOMET AND ESTRADIOL VALERATE INDUCED LUTEOLYSIS IS DEPENDENT UPON THE UTERUS C. A. Peterson, J. C. Huhn, and D. J. Kesler SUMMARY Beef heifers were assigned to three groups: 1) untreated controls (n= 4), 2) Syncro-Mate B® (SMB) treated (n= 5), and 3) hysterectomized and SMB treated (n= 4). SMB was administered eight or nine days after estrus, approximately 30 days after hysterectomy. This study was conducted to determine if the uterus was necessary for SMB to induce luteolysis. SMB induced premature luteolysis as only 20% of the intact SMB treated heifers had ≥ .75 ng/mL of progesterone seven days after the time of SMB treatment compared to all (100%) of the untreated heifers (P <.05). By nine days after the time of SMB treatment, 25% of the untreated heifers and none (0%) of the intact SMB treated heifers had ≥ .75 ng/mL of progesterone; however, all (100%) of the hysterectomized SMB treated heifers had ≥ .75 ng/mL of progesterone (P <.05). Therefore, SMB-induced luteolysis required the involvement of the uterus. The luteolysin, prostaglandin F2α, is probably the secretion from the uterus that mediates the SMB-induced luteolysis. SMB treatment, however, required 7-8 days to induce luteolysis. INTRODUCTION Syncro-Mate B® (SMB) is a commercially available procedure to synchronize estrus in beef and dairy cattle. The procedure consists of a norgestomet implant and an intramuscular injection containing norgestomet and estradiol valerate administered at the time of implantation. SMB has three known mechanisms of action. First, an estrus suppression dosage of norgestomet diffuses from the implant during the nine days in situ (Kesler and Favero, 1995). Secondly, the injection causes atresia of antral follicles and recruitment of a new cohort of follicles four to five days after administration (Vasconcelos et al., 1997). Thirdly, the injection causes regression of corpora lutea (Kesler and Favero, 1995). Since the implant is left in place for nine days, the injection is needed to induce regression of corpora lutea in cows during the first half of the estrous cycle. Estradiol-17β, the active metabolite of the estradiol valerate contained within the SMB injection, has been demonstrated to hasten corpus luteum regression (Thatcher et al., 1986). Thatcher et al. (1986) reported spikes of 15-keto- 13, 14-dihydro-prostaglandin F2α (PGFM) in the peripheral blood before luteolysis ensued and concluded that estradiol-17β induced luteolysis by provoking a release of PGF2α from the uterus; however, Thatcher et al. (1986) administered estradiol-17β during the second half of the estrous cycle. Progesterone treatment during metestrus has also been reported to shorten the estrous cycle, but only by four days (Woody et al., 1967; Harms and Malven, 1969; Ginther, 1970; Battista et al., 1984; Garrett et al., 1988). The objective of this study was to determine if the hypothesis that SMB induced luteolysis is dependent upon uterine involvement was correct. MATERIALS AND METHODS Three groups of purebred Angus beef heifers from the University of Illinois beef research unit (Urbana, IL) were included in this study. The control group (n=4) was selected from a larger group of estrus-cycling females administered prostaglandin F2α (25 mg Lutalyse®; Pharmica and Upjohn, Kalamazoo, MI, USA) due to their similar timing of estrus (detectable estrus within 48 hours of each