CHANGE IN MORPHOLOGY OF CORPORA-LUTEA, CENTRAL LUTEAL CAVITIES AND STEROID-SECRETION PATTERNS OF POSTPARTUM SUCKLED BEEF-COWS AFTER MELENGESTROL ACETATE WITH OR WITHOUT PROSTAGLANDIN-F2-ALPHA

Change in morphology of the corpus luteum (CL) and patterns of progest erone and estradiol secretion after treatment with melengestrol acetat e (MGA) were monitored in postpartum beef cows. Twenty Angus cows were randomly assigned to MGA or MGA+prostaglandin F-2 alpha (PGF) treatme nts. All cows were fed 0.5 mg of MGA per cow per day for 14 d. The MGA -treated cows (n=10) were allowed to return to estrus spontaneously at the second estrus after withdrawal of MGA from the feed. The MGA+PGF- treated cows (n=10) received an injection containing 25 mg of PGF(2 al pha) 17 d after the last feeding of MGA. Cycle 1 was defined as the fi rst luteal phase after MGA feeding and Cycle 2 represented the subsequ ent cycle or luteal phase after PGF. Blood sampling and transrectal ul trasonography of the ovaries was done daily through the completion of 2 estrous cycles upon removal of MGA from the feed. Blood samples were analyzed for plasma progesterone and estradiol concentrations. Area o f CL and fluid-filled cavities within each CL were determined by ultra sonography. Concentrations of progesterone and area of CL were similar between cycles and treatments. Estradiol concentrations were higher ( P<0.05) in Cycle 2 than in Cycle 1. Fluid-filled cavities were larger (P<0.001) in Cycle 1 than in Cycle 2 for both mid-luteal (Days 5 to 9) and late-luteal (Days 10 to 14) phases. Multiple CL (2 or more during 1 cycle) were observed in 5 cows. Progesterone concentrations and tot al area of luteal tissue did not change with respect to treatment or c ycle, but CL morphology was altered in the first cycle after MGA treat ment. Of the 19 cows that ovulated after withdrawal of MGA, 3 experien ced a short luteal phase. These data characterize changes that occur a mong cows that are fed melengestrol acetate during the postpartum peri od and enhance observations from prior studies regarding MGA use.