Two age groups [14 to 15 mo, younger heifers (YH), n = 291; 21 to 22 mo, older heifers (OH), n = 194] of Brangus and Brangus crossbred heifers were used to compare conception rates to AI at 48, 72, 96, or 120 h after two injections of prostaglandin F 2 α (25 mg −1 heifer; PGF) given 14 d apart. Estrous detection was not conducted, but a heat mount detector was placed on each heifer at the second PGF injection. Heifers with activated detectors at 48 h after a second injection of PGF were artificially inseminated between 0930 and 1100 h. This procedure was repeated for heifers with newly activated detectors at 72, 96, and 120 h after the second PGF injection. Mean BW was heavier (P 2 = 7.2; df = 3) in YH. Conception rates in YH were 52.9 and 31.5% at 48 to 72 h and 96 to 120 h, respectively (P χ 2 = 7.4; df = 1). Within age group, mean BW of heifers that conceived did not differ among the four intervals to insemination. Mean BW did not differ between heifers that did or did not conceive. It is concluded that conception rates decline in YH, but not in OH, as the interval to response and subsequent insemination after the second injection of PGF increases.
