Proteins are the building blocks of tissue and are composed of units called amino acids. Although there are 22 different amino acids that are needed for protein synthesis, several can be made by the tissues of the body. There are 10 that must be supplied to the horse – arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Lysine is the one most often lacking in growing horse rations.
The protein needs of the horse vary according to use and age. Table 2 gives the minimum level needed for various types of horses. From the table it can be readily seen that protein needs are greatest for foals and for lactating mares. Work does not increase protein needs very much. For example, if we were feeding 15 lb. of a ration that is 10% protein to a mature horse and if we begin riding the horse hard, we would maintain a 10% protein level, but increase the total amount of feed used to maintain the horse in good flesh. This would also increase the total amount of protein given and be sufficient to meet extra protein needs caused by work.
Because of the high cost of protein, it is economically wasteful to feed more than is needed. The body breaks down excess protein to carbohydrates and urea. The carbohydrates are used for energy or changed to fat and stored, and the urea is passed in the urine.
It was previously mentioned that urea is a feed supplement fed by cattle feeders that a cow can use to make protein in the rumen. Horses cannot use urea as a protein source, and feeds with their protein level based on urea will not reflect an accurate protein level for the horse.
In addition to cost, another possible detriment of excess protein is that it produces more body heat in breaking the proteins into energy as compared to using carbohydrates or fats for energy. This could be a problem for the endurance trail horse that has to keep as cool as possible as he works long and hard. Current research has shown that feeding a diet containing 10% added fat reduces the daily heat load by 5%, which may benefit hard working horses, particularly in hot weather. Therefore, high protein rations are not recommended for these horses.
Occasionally horses will develop hives, commonly called protein bumps, on the skin. These can be an allergic reaction of the body to foreign proteins in the feed but are more likely are the result of skin contact with some chemical in the bedding. Insect bites also cause these hives. Generally, the hives are short lived and no serious problem occurs.
High protein rations have also been blamed for the diseases such as epiphysitis and contracted tendons. These problems are associated with fast-growing foals, as these foals are typically on high-protein high-energy rations that allow the foal to grow quickly. However, it is the imbalance or deficiency of other factors such as minerals that are the real culprits, not the protein.
High-protein rations have been reported to cause kidney damage in horses. This is an untrue old wives tale brought about because a horse on a high-protein ration urinated more, and the smell of the urine was stronger due to the excess urea being passed.
These are basically the high-protein parts of grains that have had the oil removed for use in other industries.
Soybean Oil Meal
This oil
meal has the highest biological value and has a 44% protein content on an as-fed
basis. The quality (biological value) of a protein supplement is based on a
comparison of the amino acids that make up soybean protein to the amino acids
required by the horse to make up his proteins. Not only do they need to have
the same amino acids, but they should be present in the same relative percentages.
Soybean oil meal is especially high in lysine, which is commonly low in most
grains. Soybean oil meal is also usually the cheapest source of protein available
for horse feeds.
Soybeans should not be fed to horses in their raw form. In the raw form they contain an inhibitor of protein digestion in the horse. Raw soybeans are especially detrimental in foal rations.
Linseed Oil Meal
Linseed oil meal
used to be in great demand for inclusion in feed rations for horses. This meal
is high in sulfur-containing amino acids, which were thought to aid the quality
of the hair coat. Actually, the quality of the haircoat came more from the oil
in the meal. However, modern processing removes most of the oil, thereby decreasing
its value for horse feeding. Linseed oil meal is about 35% protein as fed, costs
more, and has a lower biological value than soybean oil meal. Linseed has been
recommended in rations of horses that suffer from tying-up syndrome, because
it is a good source of selenium, which helps maintain normal muscle function.
Cottonseed Oil Meal
Cottonseed
oil meal contains about 39% protein on an as-fed basis and is second to soybean
oil meal in quality. If it is available and cost effective, it may be used for
horses. Cottonseed oil meal contains a substance called gossypol that interferes
with digestion and is particularly undesirable for feeding foals. Adult horses
can tolerate the gossypol. Research in other species has shown cottonseed oil
meal to decrease sperm production in males, and this effect is thought to occur
in stallions, too.
Other Sources
Other protein sources
such as sunflower and canola meals can be used in horse feeds. Also, brewers
grains, distillers grains, and gluten are commonly used in horse feeds as protein
sources.
Animal protein supplements such as meat scraps or blood meal are not acceptable in most horse rations in large amounts. If much is added to the ration, horses will refuse to eat the feed. In addition, animal by-products are an excellent media for bacterial growth that could produce toxins, causing digestive problems.