Effect of Interval from Melengestrol Acetate to Prostaglandin F_2α on Timed and Synchronized Pregnancy Rates of Beef Heifers and Cows

The objective of this experiment was to determine the optimal interval from the last day of melengestrol acetate (MGA) feeding to prostaglandin F_2α (PGF_2α) treatment on pregnancy rates of beef heifers and cows. All females (149 heifers and 399 postpartum cows) were fed MGA (.5 mg) daily for 14 d and then administered PGF_2α (25 mg Lutalyse®) 13, 15, and 17 d (Groups 1, 2, and 3, respectively) after the last day of MGA feeding. Females not in estrus the first 52 h after PGF_2α treatment were artificially inseminated 72 h after PGF_2α treatment. Females in estrus 0 to 52 h and 78 h to 6 d after PGF_2α treatment were inseminated at estrus. Blood sera (collected immediately before and 3 d after PGF_2α treatment) were assayed for progesterone concentrations. Pregnancy was determined 44 to 47 d after the 72-h AI by rectal examination. The intervals from MGA feeding to PGF_2α that had the highest 72-h AI pregnancy rates were 17 d for heifers (43%) and 15 d for cows (43%). Heifers with a 17-d interval had a higher (P < .05) 72-h AI pregnancy rate than heifers with 13-d and 15-d intervals, and cows with a 15-d interval had a higher (P < .05) 72-h AI pregnancy rate than cows with a 17-d interval. The 4-d synchronized pregnancy rates (the 72-h AI and the succeeding 3 d inseminations) for both heifers (44%) and cows (53%) were not different (P > .05) among groups. Fewer (P < .05) cows with a 17-d interval from MGA to PGF_2α had corpora lutea regression (by 72 h) after PGF_2α treatment than cows with 13-d and 15-d intervals. We interpret the results to indicate that the interval from MGA to PGF_2α treatment may influence 72-h AI pregnancy rates, that optimal intervals are 17 d for heifers and 15 d for cows, and that pregnancy rates are improved by insemination for 3 d after the 72-h AI.