Onset of estrus in dairy cows maintained indoors year-round

During 6 mo two active testosterone-treated females fitted with chin-ball markers and paint were used to ascertain the time of onset of estrus in 56 cows of three genetic lines and at various stages of lactation. The two treated females were rotated every 24 h, and two colors of paint were used, one color for night, 1800 to 0600 h, and the other for day, 0600 to 1800 h. The incidence of behavioral signs of estrus, standing, mounting activity, and mucus discharge, also was recorded. More cows (65%) came into estrus during the night, and only 78% of the total marked cows, assumed to be in estrus, were observed standing by the herdsman, indicating that 22% of the cows would not have been observed in estrus by early morning and late afternoon observation by the herdsman. Similarly, of the marked cows a mucus discharge was observed for 60% and mounting activity by 62%. Neither parity nor days postpartum affected either time of onset of estrus or other overt signs of estrus. Differences among the genetic lines for standing and mounting activity were unexplained.