Synchronization of estrus in suckled postpartum beef cows with melengestrol acetate, 48-hour calf removal and PGF 2α

Abstract

One hundred and sixty-five suckled postpartum beef cows were utilized to evaluate the effectiveness of 2 estrus synchronization systems for the initiation and synchronization of estrus. The treatment groups consisted of 1) melengestrol acetate (MGA)-PGF 2α (cows were given 0.5 mg MGA/head/day for 14 d with 25 mg PGF 2α injected 17 d after the last day of MGA administration); 2) MGA-48-h calf removal (CR)-PGF 2α (cows were given 0.5 mg MGA/head/day for 14 d with 48-h calf removal starting on the second day after completion of the MGA regimen plus 25 mg PGF 2α administered 17 d after the last day of MGA); and 3) unsynchronized controls. Cows were assigned to treatments by the numbers of days post partum, body condition, age, and breed of sire. The cows were observed for estrus at 12-h intervals for 5 d after PGF 2α administration and were artificially inseminated 12 to 18 h after the observed estrus. Both the MGA-PGF 2α and MGA-CR-PGF 2α treatments (64.8 and 61.8%) had greater (P < 0.05) 5-d estrus rates than the control treatments (34.5%). The synchronized pregnancy rate was greater (P < 0.05) for the MGA-CR-PGF 2α than the control treatment.(52.7 vs 30.9%, respectively). The MGA-CR-PGF 2α cows had a higher 25-d pregnancy rate than either the MGA-PGF 2α (P < 0.05) or control cows (P < 0.08). Of the anestrous cows at the beginning of treatment, more MGA-CR-PGF 2α (P = 0.1) and MGA-PGF 2α cows were cyclic posttreatment than control cows (58.7 and 55.1 vs 44.7%, respectively), suggesting that treatment initiated estrous cycles in only a small number of the anestrous cows. Both MGA-PGF 2α and MGA-CR-PGF 2α treatments appear to be effective methods of synchronizing estrus in suckled postpartum beef cows. However, MGA-CR-PGF 2α was more effective in establishing pregnancy earlier in the breeding season than MGA-PGF 2α.