Cleanliness of food is important, as that which is dirty may cause indigestion, colic, serious disease, or even death. Sand, earth, or gravel, when taken in with the food, is generally retained in the large intestines, accumulating until there is sometimes a very great quantity. Horses fed on dusty food, such as the sweepings from grain-mills, or with that which contains husks of oats, often have large calculi, or what are called "dust balls," form in the intestines, which very frequently cause rupture, obstruction, or strangulation of these. Even a small pebble, small nail, or scrap of metal, gaining access to the intestines with the food, may form the nucleus for one of these accumulations, stones, or "concretions."
Hay or grain may be unclean from bad preservation or gathering, or even when growing. The most frequent cause of uncleanliness in this respect is the presence of moulds of various kinds, which not only diminish the value of the food in nutrition, but may even prove injurious or poisonous to the horses consuming it. The most common sources of damage to growing plants or grain are the parasitic fungi, commonly known as "Bunt," "Rust," "Mildew," "Smut," and "Ergot."
"Bunt" grows on grasses, straws, and grains; but it chiefly attacks the head of wheat, which it destroys, replacing the flour by a dark powder with a fish-like smell. When it affects the stalks and leaves, these become pale, dry, and shrivelled up.
"Rust" appears as a reddish-yellow powder on the grasses, and on the stalk, leaves, and flowering heads of plants, and it more or less destroys them.
"Mildew" shows itself as brown or black spots that are really parasitic spores, which penetrate the plant, and change the part into a black powder.
"Smut" grows in large black clusters, somewhat like soot, but solid, on grasses and grain-plants, causing them to look sickly, bleached, and eventually killing them.
"Ergot" grows on grasses and grain-plants, in the form of a cock's spur or horn-shaped body, dark purple in colour, and unpleasant in odour.
Mouldiness is usually produced by faulty preservation or storing, and is due to the presence of various kinds of microscopical fungi, which, damp or wet having removed many of the nutritive elements of the hay or grain, completely destroy forage if allowed to grow on it unchecked. It appears as a fine dust, or black, bluish, or brown patches, according to the kind of mould. Mouldy hay, if dry, breaks readily, and when shaken gives off what appears to be dust, but which is really the spores of the fungus, and which are very irritating to the eyes, nostrils, and throat.
Forage damaged by these moulds is not only less nutritious than clean forage, but it is more or less indigestible and injurious, causing loss of condition, colic, constipation, diarrhoea, inflammation of the intestines, diabetes, skin disease, paralysis, and sometimes abortion in breeding animals. Mouldy oats have been known to kill horses fed on them for only a short time.
Insects, very minute in size, also damage forage, but not at all to the same extent as the parasitic fungi.
Care is often necessary in changing from one kind of food to another kind. A change from dry to green food, if sudden, is very likely to cause diarrhoea; and even imperfectly dried, or new hay, will often do this, especially with hard-worked horses, or those required to go at a fast pace. A sudden change from oats to barley, from a poor to a rich diet, or from an easily digested to a dry, indigestible food, is to be guarded against. Changing from oats to wheat is especially to be carefully done, or serious damage will ensue; indeed, wheat for horses is at all times a dangerous diet.