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The optimal time of artificial insemination (AI) was determined from data for 2661 AI in 17 herds utilizing a radiotelemetric system for estrus detection that has the potential for continuous 24-h surveillance to monitor behavioral events associated with estrus. The system consisted of pressure-sensitive radio frequency transmitters affixed over the sacrum region of cows. The activation of the sensor sent a radiotelemetric signal to a microcomputer via a fixed antenna. Cow identification, date, time, and duration of each standing event were recorded in the software program provided with the system. Each farm selected a 3-h interval to AI for cows that were identified in estrus during the previous 24h. Pregnancy status was determined from data for return to estrus and palpation of the uterus 35 to 75 d following AI. Standing events during estrus averaged (± SD) 8.5±6.6 per cow, and the number of events per estrus across …