Variability
Soybean meal is a very consistent product and one of the least variable protein sources for animal nutrition (Smith, 1986). However, genetics, growing conditions, storage conditions and processes cause variations in its composition and nutritional quality. Because soybean meal can be included in large amounts in animal diets, small changes in quality might translate into important changes in animal performance, therefore, it is necessary to monitor its quality very closely (van Eys et al., 2004).
Antinutritional Factors
Soybean seeds contain antinutritional factors. Soybean meal usually undergoes several heat treatments that destroy heat-labile antinutritional factors (particularly trypsin inhibitors and lectins) but it is necessary to assess whether the meal was adequately processed. Inclusion of soybean meal in broilers (21 day-old) diets containing low levels of TIA - trypsin inhibitor activity (1,8 mg/g vs. 4.8 mg/g) resulted in higher dietary digestibility coefficients for DM, N, energy, and aminoacids (AA) (Dourado et al., 2011).
Both under- and over-processing of soybean meal has been shown to depress average daily gains in broilers (Perilla et al., 1997). In pigs, performance of swine fed soybean meal will be depressed if it has not been adequately processed to inactivate the anti-quality factors (Grala et al., 1998). Underheating, that may result in the incomplete destruction of antinutritional factors, is verified by the urease test, which determines residual urease activity and is an indirect indicator of active trypsin inhibitors. Overheating causes Maillard reactions that decrease the concentration and availability of heat-sensitive amino acids, particularly lysine (van Eys et al., 2004). Overheating also suppresses phytate degradation in the rumen and leads to lower availability of dietary phosphorus (Konishi et al., 1999). Several methods assess overheating, including KOH protein solubility, Protein Dispersibility Index (PDI) and Nitrogen Solubility Index (NSI). Soybean meals adequately heat-processed should have PDI values between 15 and 30%, KOH solubilities between 70 and 85% and a urease index of 0.3 pH unit change or below. Residual antitrypsic activity can also be directly measured by reference methods, but the procedure is less adapted to routine quality control (van Eys et al., 2004).
Goitrogens and oestrogens
Soybean meal may contain goitrogenic substances. Soybean meal is goitrogenic for monogastrics and it has been shown to be responsible for the goitrous calves born to cows receiving soybean meal as the major source of supplementary protein (Hemken et al., 1971). Soybean meal contains 1 g/kg of genistein, which has oestrogenic properties (McDonald et al., 2002).
Non-Starch Polysaccharides (NSP)
Some carbohydrate components in the feed may interfere with digestion. For instance, soybean meal may contain a substantial level of α-galactosaccharide which has been associated with reduced digestibility of soybean meal-based diets (Araba et al., 1994). The addition of 40 g/kg NSP to a commercial broiler diet decreased weight gain, feed efficiency and apparent metabolizable energy (AME) by 28.6, 27.0 and 21.2%, respectively (Choct et al., 1995). The antinutritive effects of NSP in poultry and pigs might be due to their physicochemical properties. In particular, soluble viscous NSP depress the digestibilities of protein, starch and fat (Smits et al., 1996). NSP content of soybean meal is approximately 61 and 103 g/kg (dry matter basis) for soluble NSP and insoluble NSP, respectively (Bach Knudsen, 1997). NSP increase microbial activity (fermentations) and may cause intestinal disorder. A comparison between conventional soybean meal and low-oligosaccharide soybean meals have shown an average difference of 7% in total metabolizable energy (TME), and up to 9.8% when the comparison was done with the lowest-oligosaccharide containing soybean meals, in roosters (Parsons et al., 2000).
Enzyme addition (xylanase, protease and amylase) in poultry and pig diets could be a good way to limit NSP issues (Dourado et al., 2011). Birds cannot degrade α-1:6 galactoside, thus the addition of galactosidase could alleviate this problem (Leeson et al., 2005; Zanella et al., 1999). However, in a treatment with α-galactosidase optimized for oligosaccharide degradation, raffinose and stachyose were effectively reduced by 69 and 54%, respectively, but the diets containing enzyme-treated soybean meal failed to improve growth performance (Graham et al., 2002).
Phytates and mineral availability
Though soybean meal has a relatively high content in phosphorus, much of it is present in the form of phosphorus-phytate, a poorly digestible complex for monogastric animals. Most of phosphorus is thus excreted in manure, which raises growing concern about the effects of phosphorus upon the eutrophication of surface waters (Waldroup et al., 2008). Phytates also link to zinc whose availability is then low in soybean meal (Blair, 2007). Pigs fed on soybean meal should receive 50 mg/kg zinc, whereas the recommendation is 18 mg/kg for pigs fed casein (animal protein) as the source of protein in the diet (NRC, 1998 cited by Blair, 2007).
GM soybean meal
The potential health issues of genetically-modified soybean and other GM foods have been the matter of considerable debate. While most studies have failed to show deleterious side-effects to GM soybean use (EFSA GMO Panel, 2008), these varieties remain controversial, and are subject to legal authorisation in some countries. In the EU, for example, only 15 soybean varieties are allowed to be used as feeds (GMO Register, 2016).