Estrous Synchronization in Beef Cows: Partitioning of Melengestrol Acetate and 48-H Calf Removal

Abstract

Spring calving, Shorthorn x Angus x Hereford beef cows (N = 287) nursing Simmental-sired calves were allotted to four treatments: 1) .5mg of Melengestrol Acetate (MGA) ·hd-1 ·d-1 in 2.27kg of ground ear corn for 7 d, followed by a 25-mg injection of prostaglandin F2X on d 8 (PGF), and 48-h calf removal (CR) d 8 and 9 (MGA/PGF/CR); 2) MGA/PGF; 3) PGF/CR; and 4) PGF. Treatment was followed by a 72-d breeding season (42 d artificial insemination (Al) and 30 d natural service). Blood was collected on d -10, 0, 7, 14, 28, 35, 42, and 49 (d 1 = 1st d of MGA feeding) for progesterone analysis. A majority of cows (89%) were anestrus before initiation of treatments. Treatments did not differ in postpartum interval (PPI, 76 ± 4 d) or percentage of anestrus cows induced to ovulate (30%). PGF cows showed a greater percentage of standing heat in the first 21 d of the breeding season than MGA/PGF cows (65 vs. 47%). In addition, MGA/PGF cows had a 30% reduction in first service conception rate if they were induced to ovulate. Treatment groups did not differ in conception rate (81%) when bred Al. However, on a whole herd basis, a greater percentage of MGA/PGF/CR cows conceived to Al (77, 65, 63, and 68% for MGA/ PGF/CR, MGA/PGF, PGF/CR, and PGF, respectively). Treatment groups did not differ in overall pregnancy rate (84%) or average calving date the following year. In cows induced to ovulate, greater fertility resulted from CR than MGA. However, MGA must be combined with CR to exceed the reproductive advantages resulting from a single prostaglandin injection alone.