Several readers of the Soybean Meal INFOcenter website over the years have requested information on the use of soybean meal in horse diets. There is not many references on the topic. An informative website was found that discussed the importance of formulating equine feeds with quality proteins that contain the correct amounts and rations of essential amino acids. The amino acid lysine, followed by threonine and methionine, are the key amino acids that are most limiting in equine rations. Soybean meal contains more lysine and threonine compared to other common protein sources (canola, cottonseed and linseed meals).
The website discusses how protein quality is also determined by its digestibility, specifically in the small intestine (prececal) of the horse where intact amino acids can be readily absorbed. The prececal digestibility of protein varies with protein source and protein concentration in the diet. In addition, numerous factors including other ingredients in the diet, protein amount consumed in a single meal and transit time through the tract will affect the prececal digestibility of amino acids in the equine digestive tract.
Protein quality of a feed ingredient can also be affected by processing. Soybean meal is generally available in two forms – 44% protein meal and 48% protein dehulled meal. Dehulled soybean meal is produced by removing the hulls and extracting the oil from soybeans and then the meal is cooked at specific temperatures for a specific amount of time to provide optimal nutritional value. The higher protein dehulled soybean meal is lower in fiber and better utilized by younger horses. Undercooking and overcooking of soybean meal can affect nutrient quality. Raw soybeans should not be fed to horses as they will interfere with protein digestion.