Experimental Procedures
Yearling heifers from four herds (n = 709) were used in the study: Herd A, 439 head of black and black baldy heifers; Herd B, 100 head of Angus cross, Angus x South Devon and Angus x Limousin heifers; Herd C, 83 head of Angus and Angus x Simmental heifers; and Herd D, 87 head of Angus and Angus x Simmental heifers. All heifers were fed melengestrol acetate (MGA; 0.5 mg/head) daily for 14 days (days _32 to day _19 of the experiment). Nineteen days after the last feeding of MGA, all heifers were administered 25 mg of PGF2a (Lutalyse®; day 0). Heifers receiving no further treatment served as the untreated controls (Control; n = 253) and were artificially inseminated 12 hours after detection of estrus. In the second treatment, heifers received an injection of GnRH (Cystorelin®; 100 mg) on day _7 (GnRH, n = 260) and were artificially inseminated 12 hours after detection of estrus. Heifers in the third treatment received GnRH on day _7 and day 2 (2XGnRH, n = 196) and were time bred at the second GnRH injection. Heifers in the 2XGnRH treatment that were detected in estrus early (before day 1.5) were bred 12 hours after detection. For purposes of data summary, we defined the synchrony period as days 0 to 7 after PGF2a.