Plant protein sources provide most of the protein in swine diets. Soybean meal is the leading protein source for swine due to its superior quality and amino acid profile. Soybean meal is generally the base to which alternative plant protein sources are compared.
Soybeans are the most widely used protein in the world and is the primary protein source in most swine diets. Soybean products used in swine diets include soybean meal, full-fat soybeans, fermented soybean meal, enzyme-treated soybean meal, soy protein concentrate, and soy protein isolate.
Soybeans contain anti-nutritional factors that reduce nutrient utilization, most notably trypsin inhibitors. The trypsin inhibitors have to be inactivated by heating or toasting soybeans prior to use in swine diets. Raw soybeans are not recommended for use as such in swine diets.
Pigs have a transitory hypersensitivity reaction to soybean meal induced by allergenic proteins, namely glycinin and β-conglycinin, and indigestible carbohydrates of soybeans. Pigs experience a period of poor nutrient absorption and low growth performance following the first exposure to a diet with high amounts of soybean meal (Li et al., 1990). The effects are transitory and pigs develop tolerance after 7 to 10 days (Engle, 1994). To alleviate the effects during this period, pigs are gradually acclimated to diets with increasing amounts of soybean meal after weaning. Furthermore, soybean meal can be further processed to remove the allergenic compounds and improve the utilization of soy proteins by weanling pigs (Jones et al., 2010).
Soybean meal is the standard protein source in swine diets and is used as the reference ingredient for protein quality. The amino acid profile, balance, and digestibility in soybean meal is better than any other plant protein source used in swine diets.
Soybean meal is produced from hulled or dehulled soybeans. Dehulled soybean meal is often referred to as high-protein soybean meal and contains approximately 48% crude protein and 3% lysine content, whereas hulled soybean meal contains approximately 44% crude protein and 2.8% lysine content and is referred to as low-protein soybean meal (NRC, 2012). Standardized ileal digestibility of lysine and most amino acids is above 85 to 90% (Cervantes-Pahm and Stein, 2010).
Processing methods to extract oil from soybeans include expelling and solvent extraction. In the expelling method, oil is mechanically extracted from soybeans after an extrusion process is used to inactive trypsin inhibitors. In the solvent extraction method, oil is extracted using a solvent and then a toasting process is used to inactive trypsin inhibitors. The expelled soybean meal contains higher oil content than solvent-extracted soybean meal because mechanical extraction is less efficient in de-oiling soybeans. The oil content in dehulled, solvent-extracted soybean meal is around 1.5% (NRC, 2012).
Full-fat soybeans are produced by avoiding the oil extraction process after extrusion of soybeans. Properly processed full-fat soybeans are a good source of both protein and energy. The critical factor during extrusion is the prevention of over- or under-processing, since either reduce the nutritional value of full-fat soybeans.
Full-fat soybeans have an oil content of approximately 15% (NRC, 2012), which is a means of providing oil to the diet. However, full-fat soybeans contain less crude protein (35 to 40%) and lysine (2%) than soybean meal (NRC, 2012).
Fermented or enzyme-treated soybean meal
Further-processed soybean meal by microbial fermentation or enzymatic treatment is done to reduce the allergenic proteins and indigestible carbohydrates of soybeans (Stein et al., 2016). Microbial fermentation is usually accomplished by the inclusion of microbes to soybean meal, such as Aspergillus oryzae, Bifidobacterium lactis, Lactobacillus subtilis, among others. Enzymatic treatment is commonly performed by inclusion of proprietary enzymes and yeast to soybean meal (Stein et al., 2016).
Fermented or enzyme-treated soybean meal have greater concentration of crude protein than soybean meal, approximately 50 to 55% (Cervantes-Pahm and Stein, 2010; Jones et al., 2010). However, the standardized ileal digestibility of most amino acids and particularly lysine is lower in fermented or enzyme-treated soybean meal compared to conventional soybean meal (Cervantes-Pahm and Stein, 2010). The reduction in digestibility of amino acids is due to heat during the drying process to produce fermented or enzyme-treated soybean meal.
Soy protein concentrate and isolate
Soy protein concentrate and isolate are high protein products derived from soybeans.
Soy protein concentrate is produced from dehulled, de-oiled soybeans (or soy flakes). The concentration of protein is increased by removing most of the soluble non-protein constituents. Soy protein concentrate contains at least 65% crude protein (NRC, 2012).
Soy protein isolate is also produced from dehulled, de-oiled soybeans (or soy flakes). The process starts by removing most of the soluble non-protein constituents and then the isolation of protein is produced by precipitating the protein in solution. Soy protein isolate is the most concentrated soy protein source and contains at least 85% crude protein (NRC, 2012).
During processing of soy protein concentrate and isolate, the allergenic proteins and indigestible carbohydrates of soybeans are mostly removed (Stein et al., 2016). However, the antinutritional factor trypsin inhibitor might be present in greater quantities compared to soybean meal because processing does not necessarily involve heat-treatment (Cervantes-Pahm and Stein, 2010).
Canola meal is a by-product of oil extraction from canola seeds. Varieties were developed with reduced concentrations of the anti-nutritional factor glucosinolates and referred to as canola in Canada and United States, and double-low rapeseed or 00-rapeseed in Europe. Glucosinolates are goitrogenic compounds that affect the thyroid function and iodine metabolism, impairing feed intake and growth performance of pigs fed diets with high concentrations (Parr et al., 2015). The concentration of glucosinolates in modern varieties is generally less than 30 μmol/g and it varies in canola meal depending on the extent of degradation during toasting (Mejicanos et al., 2016).
Canola meal contains between 35 to 40% crude protein and 2% lysine content (NRC, 2012). Compared to soybean meal, canola meal contains lower crude protein and lysine content but greater concentration of methionine and cysteine. Standardized ileal digestibility of lysine and most amino acids is lower than soybean meal, approximately 70 to 75% (Cervantes-Pahm and Stein, 2010).
Recently, new varieties of high-protein canola meal were developed that contain approximately 45% crude protein (Liu et al., 2014). Although the crude protein value is closer to that of soybean meal, the amino acid digestibility in high-protein canola meal is similar to canola meal and, therefore, less than soybean meal.
The fiber content of canola meal is between 20 to 25% NDF and 3 times greater than soybean meal due to the use of hulled canola seeds (NRC, 2012). The high fiber content reduces the energy value of canola meal.
Sunflower meal is a by-product of oil extraction from sunflower seeds. Sunflower meal is free of most anti-nutritional factors.
Sunflower meal contains approximately 30% crude protein and 1% lysine content (NRC, 2012). Similar to canola meal, sunflower meal contains lower crude protein and lysine content but greater concentration of methionine and cysteine than soybean meal. Standardized ileal digestibility of lysine and most amino acids is lower than soybean meal, approximately 75 to 80% (Cervantes-Pahm and Stein, 2010).
The fiber content of sunflower meal is very high, approximately 30% NDF in dehulled sunflower meal, which is around 4 times greater than soybean meal (NRC, 2012). The inclusion of sunflower meal in swine diets is mostly limited by its high fiber content (González-Vega and Stein, 2012).
Cottonseed meal is a by-product of oil extraction from cotton seeds. The limitation to the use of cottonseed meal in swine diets is the anti-nutritional factor gossypol found in the pigment glands of cotton seeds. The free form of gossypol is toxic and not allowed over 100 ppm in complete diets for pigs (Gadelha et al., 2014). Heat processing of cotton seeds is used to inactivate gossypol, but heating allows free gossypol to bind to lysine and reduces lysine digestibility (González-Vega and Stein, 2012). New varieties of cotton seeds commonly referred to as glandless cottonseed do not contain gossypol, but unfortunately are not common (Stein et al., 2016).
Cottonseed meal contains around 40% crude protein and 1.5% lysine content (NRC, 2012). Compared to soybean meal, cottonseed meal contains lower crude protein and lower concentration of lysine and most essential amino acids. Standardized ileal digestibility of lysine and most amino acids is lower in cottonseed meal than in any other oilseed meal, approximately 60% (Cervantes-Pahm and Stein, 2010).
Field peas are predominantly produced in Canada and temperate areas where oilseeds are not grown. Field peas are pulses that can fix most of their own nitrogen and do not require substantial use of nitrogenous fertilizer for cultivation, which considerably reduce the environmental concerns (White et al., 2015). Field peas contain low concentrations of the anti-nutritional factors trypsin and chymotrypsin inhibitors, which are usually inactivated by heat processing.
Field peas contain around 22% crude protein and relatively high lysine content, around 1.5% (NRC, 2012). Compared to soybean meal, field peas have considerably lower crude protein, lysine, methionine, cysteine, and tryptophan. Standardized ileal digestibility of lysine and most amino acids is similar to that of soybean meal, around 80% (Stein et al., 2016).
Field peas have a relatively high energy value compared to other oilseed meals. This is a result of relatively low fiber (13% NDF) and high starch (43%) content in field peas (NRC, 2012), which is similar to the composition of some cereal grains.