Estrous behavior in confined beef cows.

Forty-two postpartum beef cows were continuously monitored for behavioral signs of estrus until 100 d postpartum. Twice-weekly blood samples were collected and assayed for plasma progesterone concentrations. The duration of standing increased, recumbency decreased and intra-pen ambulation increased significantly on the day of estrus in comparison to the average of the 4 d prior to and after estrus. The average duration of the receptivity phase (standing when mounted) was 4.4 h, with a range of individual variation between 1 and 18 h. The highest occurrence of standing when mounted was observed during the first hour of the receptivity phase, gradually declining thereafter. Also, other recorded estrual activities, particularly being chin-pressed, being sniffed and sniffing other cows had their highest peak during the first hour of the receptivity phase. Plasma progesterone concentrations revealed a gradual increase in the postpartum estrus sequence. Based on the progesterone profile up to 100 d postpartum, 95% of the herd started to cycle regularly, 93% displayed mounting or being mounted during estrus, and 86% displayed receptivity consistently after initiation of regular cycling. The interval from parturition to first occurrence of a behavioral sign of estrus was 52.3 d (SD = 17.1 d).