A cull cow is any cow that has left the herd. The term "culling" is most often associated with the deliberate killing of undesirable cattle, but it also refers to animals that are sold off alive for any reason and those that die naturally while still on the farm.
Culling is performed whenever it makes the most economic sense to remove cows from the herd. Older dairy cows are often sold off and slaughtered as cull because after a certain age, they do not produce as much milk. Cattle may also be culled from the herd during times of unfavorable weather conditions, such as a prolonged drought. Some cull cows are in good condition when they are sold or slaughtered, but some are sickly, possibly due to age or chronic illness. Very ill cull cattle that are unable to walk are called downers.