Horns on cattle are used primarily for defense, or by males for fighting during mating season. Both females and males can have horns, depending on the particular breed. Probably the most famous horned cattle are Texas Longhorns, though the Watusi actually have the largest horns--up to 8 feet from point to point and 8 inches in diameter in some cases. The rings on a horn can be used to determine the age of the cow.
The core of a horn is made of bone. The bone grows from the skull through to the tip of the horn. The bone has blood vessels running throughout as well as nerves. The bone is a living part of the cow as with any other bone. With proper nutrition and good health, the bone will continue to grow throughout the life of the cow, although growth slows as the cow ages.
The outer shell of a horn is made of keratin. Keratin is a tough, non-mineralized, structural protein. It is found most commonly in fingernails but is also a building block in claws, hair, feathers and hooves.
A thin layer of tissue connects the outer keratin shell of the horn to the bone. This tissue is also a living organism of the cow. Upon death of the cow, this tissue dies. Eventually, the loss of the tissue will cause the outer shell to slip off the underlying bone.
Horns differ greatly from antlers found on deer, elk, and other species. The anatomy of antlers is simpler--antlers are made only of bone. Antlers die upon maturity each season and are shed annually. Therefore, they are not a permanent part of the animal's anatomy as cow horns are. Unlike antlers, cow horns do not branch out, having just one point at each end.
Horns can be removed early in the cow's life, as process known simply as "dehorning." If allowed to grow, horns are sometimes blunted, as process known as "tipping." Tipping generally does not hurt the cow as only the ends of the horn are trimmed, avoiding the blood vessels and nerves within the horn much like trimming toe nails on a dog.
If a horn is broken, it should be repaired immediately. If broken low enough to break blood vessels, broken horns can cause tremendous pain to the cow. With immediate attention and casting, horns will grow back together and the cow will remain healthy.