SYNCHRONIZATION OF ESTRUS IN YEARLING BEEF HEIFERS WITH THE MELENGESTROL ACETATE PROSTAGLANDIN F-2-ALPHA SYSTEM - EFFICIENCY OF TIMED INSEMINATION 72 HOURS AFTER PROSTAGLANDIN TREATMENT

摘要

Two experiments were conducted to determine the conception rates of heifers time-inseminated following melengestrol acetate/prostaglandin F-2 alpha (MGA/PG) estrous synchronization treatment. In Experiment 1, timed insemination of heifers at 72 h after the PG injection, without regard for behavioral estrus, tended to improve (P < 0.15) the percentage of heifers pregnant to artificial insemination (AI) compared with that of synchronized heifers bred 12 h after they were first detected in estrus. In the timed-insemination treatment, heifers exhibiting behavioral estrus 48 to 72 h after PG tended to have an increased (P < 0.15) conception rate to AI compared with heifers exhibiting estrus within 48 h of PG administration. In Experiment 2, the number of heifers conceiving to AI following the MGA/PG estrous synchronization regimen was increased by mass insemination of all heifers not exhibiting estrus by 72 h after PG. The pregnancy rate to AI was higher in heifers with serum progesterone (P-4) concentrations higher than 1 ng/ml compared with that of heifers with concentrations lower than 1 ng/ml. Of heifers with serum P-4 greater than 1 ng/ml, the pregnancy rate to AI tended to be higher when concentrations exceeded 2 ng/ml than when concentrations were 1 to 2 ng/ml. In cyclic heifers, timed insemination can increase the percentage of heifers pregnant after being synchronized with MGA/PG.