DETECTION OF ESTRUS BY VISUAL OBSERVATION AND RADIOTELEMETRY IN PERIPUBERTAL, ESTRUS-SYNCHRONIZED BEEF HEIFERS

The effectiveness of two estrus-detection methods (visual observation and radiotelemetric, pressure-sensitive, rump-mounted devices) was com pared in peripubertal, crossbred yearling beef heifers. Heifers (n = 5 0) were fitted with a pressure-sensitive device affixed to their rumps to which a battery-operated radio transmitter was connected. Activati on of the sensor sent a radiotelemetric signal to a microcomputer via a fixed radio antenna. Heifer identification, date, time, and duration of standing events were recorded. Estrus was synchronized by feeding melengestrol acetate (MGA) for 14 d and then injecting PGF(2 alpha) 17 d after the last daily dose of MGA. Following the injection of PGF(2 alpha), heifers were observed visually for signs of estrus at 0730 and 1630 (45 min each). Estrus was detected in 41 of 49 heifers (one lost the device) by visual observation and(or) the radiotelemetric device. Number of standing events during estrus, determined by the radiotelem etric device, averaged 50.1 +/- 6.4 per heifer, with the duration of e strus ranging from 2.6 to 26.2 h (average = 14 +/- .8 h). Visual obser vation failed to detect 11 of 41 heifers (37%) that were detected by t he radiotelemetric device. Heifers with fewer standing events (19.3 vs 60.5; P <.001) and estrus of shorter duration (8.4 vs 15.6 h; P <.001 ) were those not identified by visual observation. Based on intervals between the first detected standing event of estrus and time of insemi nation, a wide window of fertility seems to exist in some heifers. Rad iotelemetric devices were useful in identifying a greater(P <.05) prop ortion (100%) of heifers in estrus (increased efficiency) compared wit h visual observation (73%). However, accuracy of both methods was simi lar.