We always figured it was when the bulls were turned out. Angus cattle can have a short gestation period and calves can come 10 days early. Many ranchers have a lot of calves on the ground before their due date. I don't know about other breeds, I just know Angus. We were always glad to see those little vigorous black calves. We didn't want big, slow
calves that can cause extra work, so we had a lot of early calves.