Effect of number of cows in estrus and confinement area on estrous behavior of beef cows1

Mature Angus and Angus x Hereford cows were used to determine the effects of confinement area and number of cows in estrus on estrous behavior. During each of 2 replicates, 32 nonpregnant, nonlactating cows were maintained in a drylot (60 x 100 m) or in a pasture (12 ha). Estrous cycles were synchronized with 2 injections of PGF(2alpha) 10 to 14 d apart at the initiation of the experiment. Thereafter, PGF(2alpha) was administered between d 6 and 18 of the estrous cycle so that 1, 2 to 3, 4 to 6, or > or = 7 cows were in estrus at the same time. Concentrations of progesterone were quantified in plasma at treatment to ensure cows had a normal response to PGF(2alpha). Duration of estrus and the number of mounts received during estrus were recorded by HeatWatch. If any part of estrus for a cow occurred at the same time as estrus for another cow, the cow was considered to have concurrent estrus with the other cow. Cows in the drylot had a shorter (P < 0.02; 61.8 +/- 3.1 h) interval to estrus after PGF(2alpha) compared with cows on pasture (72.8 +/- 3.3 h). The interval to estrus was longer (P < 0.07) when cows were treated with PGF(2alpha) on d 10 to 13 (76.7 +/- 3.3 h) of the estrous cycle than when treated on d 6 to 9 (62.3 +/- 4.7 h) or d 14 to 18 (62.9 +/- 3.6 h). Increasing the number of cows concurrently in estrus increased the number of mounts each cow received per estrus (P < 0.001) and the duration of estrus (P < 0.01). When only 1 cow was in estrus, she received 11.0 +/- 6.2 mounts during 11.6 +/- 1.5 h. When > or = 7 cows were in estrus at the same time, each cow received 50.4 +/- 3.2 mounts during 17.3 +/- 0.8 h. Cows in drylot were in estrus longer (P < 0.04; 16.4 +/- 0.8 h) than cows on pasture (14.2 +/- 0.7 h). Duration of the longest interval between mounts received decreased (P < 0.002) as the number of cows in estrus at one time increased (5.3 +/- 0.7 h for 1 estrous cow; 2.6 +/- 0.3 h when > or = 7 cows were in estrus). We conclude that increasing the number of beef cows in estrus at the same time will increase the number of times a cow is mounted and the duration of estrus. The increase in estrous behavior associated with more cows in estrus could increase the number of estrous cows detected with infrequent visual observation.