Normally a cow produces a single offspring, which is plenty for her to raise. Occasionally, however, twins are born. Since the cow will only raise a single calf, she rejects one of the twins. This rejected calf is called a bummer calf.
If it is considered worth the trouble for the farmer to raise a motherless calf, the bummer may be brought into a barn and bottle fed. These bottle fed calves become quite tame, almost pets. Bottle feeding bummer calves is a good chore for children. Bummers also make good 4-H project animals. If the bummer calf is a heifer, a free-martin, chances are it will not breed.
Sometimes in a herd a cow will lose a calf. If a bummer calf is available, the farmer will try to transfer the rejected calf to the calf-less cow. This saves trouble for both the farmer, having to deal with a rejected calf, and the cow, who grieves for her lost calf and suffers from not being suckled. The usual gruesome but effective method is to skin the dead calf and tie the calf skin onto the bummer calf, leaving it there until the cow accepts the bummer calf as her own.