Polled livestock

Polled livestock are livestock without horns, of species that normally have them. The term refers both to breeds or strains which are naturally polled through selective breeding and also to naturally horned animals which have been dehorned. Natural polling occurs in cattle, yaks, water buffalo and goats, and in these it affects both sexes equally; however in sheep both sexes may be horned, both polled, or only the females polled.

Terminology
The archaic term  is sometimes used to refer to hornless livestock (especially cattle) in folk songs, tales and poetry, and in the name of the polled Irish Moiled cattle breed. "Muley" derives from Irish and Scottish Gaelic maol, and Welsh moel.

Genetics
In cattle, the polled allele is genetically dominant to that for horns.

In sheep, the allele for polling in both sexes is dominant to that for being horned in both sexes, and both these are dominant to that for polling in the female only.

The development of wholly polled strains in goats is deterred by the fact that in goats the polling gene is linked to hermaphrodism; naturally polled water buffalo also have genital defects.

Scurs
Naturally polled animals occasionally have scurs—small, loose horny growths in the skin where their horns would be. In cattle this trait has been traced to a separate gene (on a different chromosome) from that responsible for polling. However the presence of the allele for scurs can only be seen in a polled animal, because horns replace the scurs in horned animals. Similar scurs may also occur where dehorning of a naturally horned animal has been incomplete.

Reasons for polling
Polled livestock are preferred by many farmers 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 a cattle crush), or they may become damaged during handling.

In other circumstances horned animals may be preferred, for example to help the animal defend itself against predators, to allow the attachment of head yokes to draught oxen, to provide a hand-hold on smaller animals such as sheep, or for aesthetic reasons – for example in some breeds the retention of horns is required for showing.

Development of polled strains
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 commonly Angus and Galloway cattle. For example, polled Jersey cattle originated in Ohio sometime prior to 1895. Two strains were developed, the first to appear being founded by crosses of registered Jersey bulls on common mulley (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 hornless sports which were discovered in registered herds of horned Jersey cattle. These 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.