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1 Department of Dairy Science, Virginia Polytechnic, Institute and State University, Blacksburg 24061-0315
Time of ovulation was determined for Holstein cows (n = 51) for estruses occurring spontaneously (n = 33) or those induced by PGF2
(n = 86). Ultrasound examination of ovaries was conducted 42 to 49 d postpartum, followed by administration of 25 mg of PGF2 if a corpus luteum was observed. In the absence of a corpus luteum, ovaries were reexamined weekly, and PGF2 was administered upon observation of the presence of a corpus luteum. Onset of estrus was determined by Heatwatch®, an electronic pressure-sensing system that recorded each mount associated with estrus. To determine ovulation in relation to first detected mount, ultrasound examinations were conducted at 12, 20, and 24 h after the initial mount and then every 2 h until 40 h. Cows were assigned randomly to receive one of two treatments: 1) the cow received 25 mg of PGF2 8 to 13 d later or 2) the cow was allowed to cycle spontaneously and then was switched to alternate treatment at a third cycle. The mean estrus period, determined from mounting activity recorded by Heatwatch®, consisted of 10.1 mounts over 9.5 h (6.0 mounts were 
2 s) for a total 24.1 s of mounting activity. Estrus characteristics were extremely variable and were not different for estruses induced by PGF2 or for those occurring spontaneously. Mean ovulation time relative to first mount was 27.6 ± 5.4 h and was not different between spontaneous and induced estruses. Knowing the time of ovulation in reference to the first mount of estrus and being able to identify the first mount consistently and accurately with the HeatWatch® system allows for accurate timing of AI.
Key Words: artificial insemination • estrous behavior • ovulation • radiotelemetry
Submitted on September 21, 1995
Accepted on February 20, 1996
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