Soybean hulls are a by-product of the extraction of oil from soybean seeds (Glycine max (L.) Merr.). After entering the oil mill, soybeans are screened to remove broken and damaged beans, and foreign material (Extension, 2008). The beans are then cracked, and their hulls, which mainly consist of the outer coats, are removed (see figure above). Hulls are fibrous materials with no place in human food, but are very valuable for ruminants (Ipharraguerre et al., 2003). Soybean hulls are often reintroduced in the final oil meal in order to reduce its protein content, resulting in soybean meal types with a maximum protein + fat guarantee of 44 to 48%. However, this end use decreases when the demand for high protein soybean meal increases. Soybean hulls are thus available and very valuable feeds for on-farm feeding of cattle (Extension, 2008).
Soybean hulls are light, flaky, and bulky. They require special consideration when handling: closed feeders are necessary when soybean hulls are fed outside, since the wind tends to blow the hulls away. During transportation, closed and covered trailers are also required (Extension, 2008). Pelleting hulls is a way to reduce bulkiness and reduce transportation costs, even though many manufacturers prefer unpelleted hulls to prepare compound feed (Blasi et al., 2000).Distribution
Soybean hulls are available wherever soybeans are produced. It is estimated that soybean hulls represent about 5% of soybean weight (Blasi et al., 2000). According to world production of soybean, which was 308 million tons in 2014, it can be estimated that the production of soybean hulls was about 15 million tons in that year (FAO, 2016). Only a part of the total production is used directly as feed, since hulls are reincorporated in soybean meal to comply with the intended protein content (Extension, 2008).Processes
Soybean hulls have to be heat-treated and milled to reduce their bulkiness and lower their urease activity (Extension, 2008). After cracking of soybeans, the hulls first pass through a sieve which separates fines and meats from the true hulls. The hulls are then toasted in order to destroy the urease enzyme (see Potential constraints on the "Nutritional aspects" tab) (Blasi et al., 2000). After heat treatment soybean hulls are referred to as soybean mill run, soybean flakes, or soybran flakes (Boyles, 1999). Since soybean hulls have a very low density, they can be milled and pelleted to lower bulkiness. However, many feed manufacturers prefer using unpelleted soybean hulls to make their own pellet mixtures (Blasi et al., 2000).