Polled livestock arelivestock withouthorns in species which are normally horned. The term refers to bothbreeds and strains that are naturally polled throughselective breeding and also to naturally horned animals that have beendisbudded.[1] Natural polling occurs incattle,yaks,water buffalo, andgoats, and in these animals it affects both sexes equally; insheep, by contrast, both sexes may be horned, both polled, or only the females polled. The history of breeding polled livestock starts about 6000 years BC.[2]
Thearchaic termmuley ormulley[3] is sometimes used to refer to hornless livestock (especially cattle) infolk songs,folk tales, andpoetry,[4] and in the name of the polledIrish Moiled cattle breed. "Muley" derives fromIrish andScottish Gaelicmaol, andWelshmoel.
Incattle, the polledallele is geneticallydominant to that for horns. The polled trait is far more common in beef breeds than in dairy breeds.CRISPR technology is being developed to create polled versions of dairy breeds.[5]
Insheep, theallele for horns in both sexes ispartially dominant to the allele for being polled in both sexes, and both of these are dominant to that for polling in the female only.[6]
The development of true breeding polledgoats is discouraged as the polled locus is strongly associated withPolled Intersex Syndrome (PIS).[7][8][9]
Naturally polledwater buffalo also have genital defects.[10]
Naturally polled animals may havescurs – small, loose, horny growths in the skin where their horns would be. In cattle, this trait has been traced to a separategene (on a differentchromosome) from that responsible for polling. However, the presence of the allele for scurs in cattle can only be seen in a polled animal, because horns replace the scurs in horned animals.[11] Similar scurs may also occur where disbudding of a naturally horned animal has been incomplete.
Polled livestock are preferred by manyfarmers for a variety of reasons, the foremost being that horns can pose a physical danger to humans, other livestock and equipment. Horns may also interfere with equipment used with livestock (such as acattle crush), or they may become damaged during handling.
In other circumstances, horned animals may be preferred, for example, to help the animal defend itself againstpredators, to allow the attachment of headyokes to draughtoxen, to provide a hand-hold on smaller animals such as sheep, or for aesthetic reasons – in some breeds the retention of horns is required forshowing.
In the US no show requires horns to be left on. Most shows require at a minimum blunting of the horns to a minimum diameter of1⁄2 inch. Boer Goat Shows allow disbudded goats.[12] Dairy breeds of goats are required to be naturally hornless or disbudded.[13] 4H and FFA show goats must be hornless or blunted so as not to be sharp and dangerous.[14][15]
Polled strains have been developed of many cattle breeds which were originally horned. This has usually been done by crossing with naturally polled breeds, most commonlyAngus andGalloway cattle. For example, polledJersey cattle originated inOhio sometime prior to 1895. Two strains were developed, the first to appear being founded by crosses of registered Jersey bulls (the standard) on common muley (hornless) cows. These were graded up by the continued use of purebred Jersey sires, selection being made of the polled offspring of each generation, the horned progeny being discarded. Thus originated what was later known as the single-standard strain. As in the case of the Polled Shorthorns and Polled Herefords, the development of the single-standard strain was soon followed by the appearance of a double-standard strain, founded by a few naturally hornlesssports that were discovered in registered herds of horned Jersey cattle. These standards were bred among themselves or crossed with registered horned Jerseys, followed by selection for the polled head, and the strain was developed in this way.[16]