Antinutritional factors
Soybean seeds contain antinutritional factors, notably trypsin inhibitors, hemagglutinins, lectins and saponins (Rackis et al., 1986). Soybean hulls, as part of the seed, also contain antinutritional factors:
Trypsin inhibitors: raw soybeans and soybean hulls contain trypsin inhibitors that bind with trypsin in the small intestine, and form an inactive complex that prevents trypsin from degrading feed proteins (van Eys et al., 2004). Trypsin inhibitors also induce pancreatic enlargement, increase trypsin secretion and, therefore, lower N retention, growth and feed conversion (van Eys et al., 2004; Rackis et al., 1986). Due to these antinutritional factors, raw soybean hulls are not recommended for feeding. Ruminants are not as sensitive as other livestock species because trypsin inhibitors are deactivated and degraded in the rumen (Hoffmann et al., 2003). However, trypsin inhibitors are heat labile and largely destroyed by it (Rackis et al., 1986), so heat treatment is required before feeding soybean products to animals, particularly to monogastrics.
Lectins are also heat-labile. They are found in soybean seeds, but no information could be found about their presence in soybean hulls.
Saponins are glycosides that reduce the uptake of other nutrients.
Tannin content of soybean hulls is higher than that of whole soybeans (2.31 vs. 1.52 mg catechin equivalent/g) (Egounlety et al., 2003).
Others antinutritional factors: soybean may contain antivitamins, urease and plant sterols that may interfere with the absorption of carotenoids and vitamin E (Brown et al., 2008).
Urease activity
Like soybean seeds, soybean hulls may contain the urease enzyme. If soybean hulls are fed in association with feeds containing urea, urease may cause the rapid breakdown of urea into ammonia, which accumulates in the rumen. Rumen microflora may be unable to handle large amounts of NH3, which are carried by the systemic circulation and may cause sudden death in cases of acute toxicity (Newkirk, 2010; Decker, 1996). In other cases, it may lead to excessive NH3 excretion into the environment, and poor N utilization (Kertz, 2010). When soybean hulls are mixed with other feeds, urease is generally destroyed (Extension, 2008).
Risk of choking
When hungry animals eat soybean hulls too quickly, there is some risk of choking. However, when this occurs animals mostly relieve the obstruction without help. A way of limiting the risk of choking is to mix soybean hulls with other feed, top-dress with other feeds, or feed the soybean hulls after other elements of the diet (Schoenian, 2015).
Copper toxicity
The copper content of soybean hulls is about 17 ppm. In sheep, it is thus likely to raise the copper content of the ration above the maximum tolerable level (15 ppm) that sheep can tolerate when they are offered diets containing “normal” levels of molybdenum and sulphur. Where the risk of copper toxicity occurs, it may be advised to provide sufficient molybdenum in the diet to prevent problems (Schoenian, 2015).
Mycotoxins
When the weather is both warm and humid at the time of soybean harvest, fungi development may result in the production of mycotoxins. Among them, zearalenone in particular is concentrated in the hulls (Jacobsen et al., 1995; Valenta et al., 2002).
Genetically-modified soybeans
The potential health issues of genetically-modified soybeans and other GM foods is a matter of considerable debate. While most studies have failed to show deleterious side-effects to the use of GM soybean (EFSA GMO Panel, 2008), these varieties remain controversial and subject to legal authorisation in some countries.