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An Introduction to Protein Supplements for HorsesBy Kentucky Equine Research Staff · October 27, 2004

Feed manufacturers in the United States typically use a select group of ingredients to fulfill protein requirements of horse feeds. Soybean meal, linseed meal, corn gluten, canola meal, and cottonseed meal are the most common protein supplements used in textured and pelleted feeds. While all of these ingredients are suitable components of a horse feed, there are some differences in the quality of protein they deliver.

Quality of protein is determined by amino acid composition. A single amino acid, lysine, is of particular importance to managers of young horses. Research has shown that horses fed diets deficient in lysine will grow more slowly than horses fed diets high in lysine, even if the percentage of crude protein in diets is identical. The protein supplement used in feeds often depends on the ingredients available to the feed manufacturer in its vicinity. In areas of the United States where cotton production is widespread such as California and Arizona, feed manufacturers may be inclined to use cottonseed meal as a source of protein in feeds.

By and large, soybean meal is the most widely used protein supplement included in horse feeds in the United States. Once oil is extracted from soybeans, the remaining flakes are cooked and ground into a meal. A typical analysis of soybean meal reveals about 48% crude protein. Ground soybean hulls, which are low in protein and high in fiber, are added to dilute the meal and decrease the crude protein content to approximately 44%. The popularity of soybean meal stems largely from the fact that it contains an amino acid profile complementary to the protein requirements of most horses, particularly growing stock. Most notably, soybean meal contains abundant quantities of lysine.

Interestingly, soybean meal is not especially palatable to horses. Because of this, it is often coupled with vitamins, minerals, and other additives (yeast, for example) in a pellet and mixed with cereal grains and molasses in textured feeds.

Linseed meal is a derivative of flaxseed processing. Linseed meal may contain up to 40% crude protein, but 33-35% is a more typical analysis. Although it is palatable to horses, linseed meal, like cottonseed meal and corn gluten, is low in lysine and inappropriate as a protein source for young animals unless the ration contains other sources of lysine.

Linseed oil was once used extensively in the fitting of horses for sales and shows. However, its use has decreased as the popularity of rice bran and vegetable oils such as corn and soy oil has skyrocketed. Linseed oil should not be confused with linseed meal. The oil contains little protein.

Corn gluten meal is produced during the manufacture of cornstarch and corn syrup. Crude protein content is approximately 47%, though the percentage can be substantially higher. Corn gluten meal contains about one quarter the lysine that soybean meal contains. Thus, it is unacceptable as the sole protein source for growing horses.

Occasionally the bran and germ of the corn is included in gluten meal, producing gluten feed. Gluten feed contains double the energy but one-half as much protein as gluten meal.

The primary concern of using corn gluten meal or corn gluten feed as an ingredient is the possible variation in energy-rich nutrients. Because corn gluten contains varying amounts of digestible energy, consistency of the product from batch to batch may significantly affect the digestible energy content of the entire feed. Therefore, vigilant monitoring of corn gluten is necessary by manufacturers to ensure consistent final products. From a nutritional standpoint, corn gluten is high in phosphorus and low in calcium. Hence, in the formulation process, high-calcium ingredients must be added to balance the calcium to phosphorus ratio.

Canola meal is a high-protein by-product of oil removal from canola. The meal contains approximately 35-44% crude protein and comes closest to meeting the nutritional profile of soybean meal. Though it has slightly less lysine than soybean meal, canola meal does include sufficient quantities to meet the requirements of growing horses.

Canola meal is often confused with rapeseed meal. Historically, rapeseed meal has been avoided in feed manufacturing because it contains glucosinolate, a bitter compound that has been linked conclusively to goiter in cattle and hogs. Varieties of rapeseed that contain significantly less glucosinolate have been developed recently. When these varieties were fed, incidence of goiter decreased substantially and palatability increased. These improved varieties are called canola meal and are more appropriate for inclusion in horse feeds than older rapeseed meal.

Canola meal, which is virtually glucosinolate-free, may have a place in horse feed formulation. No differences were found in growth or general health during two studies that compared the use of canola meal and soybean meal in weanling and yearling horses. Therefore, according to growth rate, lysine content, and digestibility, canola meal appears to be an acceptable protein source for all horses. Another oilseed by-product used to provide protein in horse feeds is cottonseed meal, which is derived from cottonseed processing. Its use, however, is notably less extensive than that of soybean meal and corn gluten. If the hulls are removed from seeds, the meal may contain 36-48% crude protein.

Though cottonseed meal is more palatable to horses than soybean meal, the quality of protein derived from it is inferior. Cottonseed meal contains inadequate lysine to promote optimal growth in young horses.

In addition to suboptimal protein quality, cottonseed meal contains gossypol, a potentially toxic compound. Gossypol toxicity is rarely a problem in horses and other livestock unless animals are overfed cottonseed meal. Therefore, cottonseed meal should be fed only when the batch does not exceed safe gossypol levels.

A number of feedstuffs can be incorporated into horse feeds to satisfy protein requirements. Though soybean meal is used preponderantly in the horse feed industry, other sources can deliver adequate protein, especially to mature horses.