Weight changes prepartum and presence of bulls postpartum interact to affect duration of postpartum anestrus in cows.

Our hypothesis was that cows with greater body condition at parturition would respond to the presence of bulls (biostimulation) by having shorter periods of postpartum anestrus than cows with a lesser body condition. Multiparous cows in good body condition at the initiation of the study were either maintained on a high dietary regimen or switched to a low dietary regimen during the 90 d preceding parturition. After parturition, half of the cows from each group were placed in pastures with sterile bulls, and the remaining half were placed in pastures without bulls. This study was replicated during a 2nd yr. Blood samples were collected twice weekly and assayed for concentrations of progesterone to estimate time of onset of luteal function after parturition. There was a significant interaction (P less than .05) between body condition at parturition and presence of bulls postpartum on the duration of postpartum anestrus. Cows of lesser body condition that were in the presence of bulls after calving initiated estrous cycles 14 d earlier than cows with lesser body condition that were isolated from bulls. In cows with a greater body condition, presence of bulls after calving only shortened postpartum anestrus by 6 d. Thus, we reject our initial hypothesis.