Assessment of estrus detection by visual observation and electronic detection methods and characterization of factors associated with estrus and pregnancy in beef heifers

One hundred and sixty-four beef heifers representing Angus, Brahman and the ir crosses were subjected to estrus synchronization treatment following eva luation of weight. body condition score and reproductive tract. Heifers wer e assigned to 1 of 2 methods of estrus detection, either visual observation for signs of sanding estrus or a rump-mounted pressure-sensitive detection device. All heifers were artificially inseminated during a 25-d inseminati on period and then bred by a bull. The effectiveness of estrus detection an d timely insemination were evaluated by the type of detection method, breed and breeding event resulting in a pregnancy. Although there was not a sign ificant difference in first service conception for method of detection, at the end of a 25-d insemination period, 60.5% of thc visually observed heife rs were pregnant, while only 45.8% of the heifers detected by the mount det ection device were pregnant (P = 0.05). The reduced 25-d conception rate in the pressure-sensitive detection group suggested that insemination of dete cted heifers may not have hm optimal for pregnancy. The mean time to estrus after implant removal and the time of insemination were evaluated. Heifers pregnant at the first service had a shorter time to estrus (32.11 +/- 1.6 h, P=0.13), a longer mean interval from the start of estrus to insemination (12.10 +/- 1.2 h. P = 0.16) and a positive mean interval between the end o f standing estrus and the time of insemination (3.17 +/- 1.7 h, P=0.19) tha n heifers not conceiving at the synchronized estrus (38.5 +/- 2.1, 7.42 +/- 1.9 h and -2.04 +/- 2.1 h, respectively). Breed differences were observed in estrus durations (Angus 8.52 +/- 12 h, Brahman 6.65 +/- 1.2 h, crossbred 11.90 +/- 1.2 h: P = 0.03), number of mounts (19 +/- 3.6, 25 +/- 5.4, 37 /- 5.5, respectively; P=0.02) and gestation length (281 +/- 1.2, 291 +/- 1. 8, 286 +/- 1.1 d, respectively; P = 0.001). (C) 1999 by Elsevier Science In c.