SYNCHRONIZATION OF ESTRUS IN SUCKLED POSTPARTUM BEEF-COWS WITH MELENGESTROL ACETATE, 48-HOUR CALF REMOVAL AND PGF(2-ALPHA)

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 con sisted of 1) melengestrol acetate (MGA)-PGF(2 alpha) (cows were given 0.5 mg MGA/head/day for 14 d with 25 mg PGF(2 alpha) injected 17 d aft er the last day of MGA administration); 2) MGA-48-h calf removal (CR)- PGF(2 alpha) (cows were given 0.5 mg MGA/head/day for 14 d with 48-h c alf removal starting on the second day after completion of the MGA reg imen plus 25 mg PGF(2 alpha) administered 17 d after the last day of M GA); and 3) unsynchronized controls. Cows were assigned to treatments by the numbers of days post parium, body condition, age, and breed of sire. The cows were observed for estrus at 12-h intervals for 5 d afte r PGF(2 alpha) administration and were artificially inseminated 12 to 18 h after the observed estrus. Both the MGA-PGF(2 alpha) and MGA-CR-P GF(2 alpha) 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(2 alpha) than the contro l treatment (52.7 vs 30.9%, respectively). The MGA-CR-PGF(2 alpha) cow s had a higher 25-d pregnancy rate than either the MGA-PGF(2 alpha) (P <0.05) or control cows (P<0.08). Of the anestrous cows at the beginnin g of treatment, more MGA-CR-PGF(2 alpha) (P=0.1) and MGA-PGF(2 alpha) 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(2 alpha) an d MGA-CR-PGF(2 alpha) treatments appear to be effective methods of syn chronizing estrus in suckled postpartum beef cows. However, MGA-CR-PGF (2 alpha) was more effective in establishing pregnancy earlier in the breeding season than MGA-PGF(2 alpha).