Estrus synchronization of beef cattle with a combination of melengestrol acetate and an injection of progesterone and 17 beta-estradiol

Our hypothesis was that estrus synchronization in beef cattle using melenge strol acetate (MGA) and an injection of progesterone (P-4) and 17 beta-estr adiol (E-2) to regress dominant ovarian follicles would improve pregnancy r ate (number conceived/number in group) to AI compared with feeding only MGA or injecting PGF(2 alpha). During 2 yr, peripubertal heifers (n = 52) and cows (n = 327) received either 1) MGA for 18 d (d 0 = 1st d of MGA) plus an injection of P-4 and E-2 in sesame oil (vehicle) on d 11 to regress persis tent ovarian follicles (MGA+P-4), 2) MGA for 18 d plus vehicle on d 11 (MGA ), or 3) two injections of PGF(2 alpha) 10 d apart (d 7 and 17, PG). Concen tration of P-4 was assessed in blood samples obtained on d 0, 7, and 17 to indicate estrual status (anestrual or estrual) during treatment to induce e strus synchrony. Observations for detection of estrus occurred every 6 h fo r 180 h following treatment cessation. Females showing estrus were insemina ted 6 to 12 h after estrus detection. Conception to AI was determined by ul trasonography 35 to 40 d later. Conception rate was greater (P < .05) in fe males in the PG than in those in the MGA group but did not differ from conc eption rate of females in the MGA+P-4 group. Among anestrual females, estru s synchrony rates were greatest (P < .10) among females treated with MGA+P- 4. Among females that were estrual before treatment cessation, estrus synch rony rates were greater (P < .10) among females treated with MGA+P-4 or PG than among those given MGA. Pregnancy rates were greater (P < .05) among fe males that were anestrual before treatment cessation and treated with MGA o r MGA+P-4 than among those treated with PG. Estrus synchronization using MG A+P-4 and E-2 differentially improves estrus synchronization and pregnancy rates among anestrual and estrual beef cattle while maintaining conception rates similar to those of PGF(2 alpha)-treated females.