There are 3 main processing methods for raw soybeans that are used in animal feeds:
1) Solvent extracted soymeal. This is the most common form of soybean. The soybean is crushed and separated, and the outer hull (which is high in fiber) from the inner germ and the oil extracted with solvents. This provides the most complete removal of oil from the soybean itself, leaving the soybean meal with approximately 2-3% fat. Some of the outer hull is then added back in to standardize the protein content to 44-48% crude protein.
2) Roasted soybeans. This is pretty self-explanatory; the whole soybeans are roasted, which helps to destroy the anti-nutritional factors in the soybeans and also makes them very palatable. These beans will contain approximately 38% protein and 16-17% fat.
3) Extruded soybean meal/mechanically extracted soymeal. Instead of using solvents to extract the fat from whole soybeans, they use mechanical extrusion to remove the fat. This is far less efficient and thus, the remaining soymeal has much more fat than solvent extracted soymeal (6-15%) depending on how efficient the mechanical extraction is. Mechanical extraction may leave more of the anti-nutritional factors intact, but they should not present an issue when fed in small amounts.
I certainly have no issues feeding soybean meal/roasted soybeans to horses. The higher fat content in the extracted or whole roasted soy can certainly put a shine on a horse. Of all the plant protein sources readily available, they also have one of the best amino acid profiles, no question.
In terms of how much to feed, it largely depends on the quality of hay being fed, specifically the protein content. Almost all classes of mature horses only need 10-12% crude protein for the total diet (the exception being late gestation and lactating mares). Horses exercising hard do require more total protein per day than a horse at maintenance, but this extra protein is acquired by a higher rate of feed intake, so the percent protein in the diet remains the same. How to figure out how much to feed:
Your average horse (weighing 450-500 kg) will consume approximately 10-12 kg of feed per day. If the diet should be 12% protein, that means they consume (.12*11=1.32) 1.3 kg of protein per day. If the hay is 10% crude protein, an additional 200-300 grams of protein will be needed. The extruded soy is around 38% protein, therefore, need to feed (0.200/0.38=0.53 kg). If the hay is over 13% crude protein, then technically you don’t need any of the soy, but could feed 100-200 grams to get the fat and a bit of extra high quality protein. If the protein is too high in the diet, you just end up with a smelly barn (excess ammonia being excreted).
Raw or unprocessed soybeans should be avoided due to enzymes and anti-nutritional factors present.