Conception Rates in Beef Heifers at Four Time Intervals After Prostaglandin F2
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2001 American Registry of Professional Animal Scientists 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 120h after two injections of prostaglandin F2 (25mg1 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 48h after a second injection of PGF were artificially inseminated between 0930 and 1100h. This procedure was repeated for heifers with newly activated detectors at 72, 96, and 120h after the second PGF injection. Mean BW was heavier (P<0.01) for OH compared with YH (448 5 and 394 3 kg, respectively). A total of 259 heifers (53.4%) had activated heat mount detectors, and 29, 107, 80, and 43 heifers were inseminated at 48, 72, 96, and 120 h, respectively. Conception rate did not differ between age groups but was affected by interval to insemination (P=0.07; 2= 7.2; df = 3) in YH. Conception rates in YH were 52.9 and 31.5% at 48 to 72h and 96 to 120 h, respectively (P<0.01; 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.
