Soybeans are a good source of protein and energy for ruminants.
Soybeans have high a ME content (16 MJ/kg DM; INRA, 2007). Soybean seeds have a high in vitro digestibility (Itavo et al., 2015; Rao et al., 2009) and in vivo digestibility (on average 88%; INRA, 2007). The protein of raw soybeans is rapidly degraded in the rumen so that the metabolisable protein content does not exceed 90 g/kg DM (Poncet et al., 2003). The degradation rate can be slowered by heat treatments such as flaking or toasting, by extrusion or by formaldehye treatment (N effective degradability: 69%, 63%, 47%, 40%, respectively; INRA, 2007), or other processes (see below). The metabolisable protein content can reach 170 g/kg DM for toasted and flaked seeds, 250 g/kg DM for extruded or formaldehyde-treated seeds (INRA, 2007). These processes indirectly protect the lipid fraction from ruminal degradation, allowing a higher proportion of the polyunsaturated fatty acids (PUFA) to be delivered postruminally.
Whole soybeans can be included at 18% of DM intake in dairy cows (Dhiman et al., 1997). Higher levels can be used (Gralak et al., 1997). When soybeans are fed in large quantities, the diet must also contain adequate amounts of vitamin A (Newkirk, 2010).
For cattle, soybeans can an excellent feed even without processing or heat conditioning (Newkirk, 2010). Raw soybeans can be fed to beef cows during mid to late gestation. Although they result in reduced weight gain compared to soybean meal/hulls supplementation, raw soybeans do not affect reproduction, calf weaning weight, forage intake and digestion (Banta et al., 2008). Comparable results are obtained with steers between raw soybeans (16.5%) and sweet lupin seeds (20%) in complete pelleted diets (Vicenti et al., 2009). In feedlot lambs, the inclusion of raw soybeans up to 21% DM of isonitrogenous, high concentrate diets, although decreasing DM intake, did not affect feed conversion, carcass yield and lamb cuts, providing satisfactory lamb performance (Urano et al., 2006).
Reducing particle size is not efficient to improve the nutritive value of soybeans, because it enhances its protein degradability (Poncet et al., 2003). In lactating cows, grinding soybean seeds reduces the DM intake, but not apparent digestibility. It decreases milk production, but does not affect fat and protein content, nor body weight changes (Pereira et al., 1998a; Pereira et al., 1998b).
Heat and pressure treatments
Roasting (dry heating) is the most commonly used treatment. It reduces N degradability and increases the intestinal digestibility of rumen undegraded protein (RUP) (Poncet et al., 2003; Nasri et al., 2008). Optimal heating conditions for soybean seeds are 145°C during 30 min (Faldet et al., 1991): roasting at this temperature increases the proportion of N escaping ruminal digestion from 25 to 50% (Tice et al., 1993). In dairy cows, roasting soybean seeds does not affect DM intake, digestibility of OM, lipids and NDF, milk production, milk fat, protein and lactose, but tends to reduce DM and CP digestibility and to increase the milk fat concentration of c9 and t11-CLA (Abdi et al., 2013). Roasted soybeans in half and quarter sizes (mean particle size of 2.9 mm) are optimal to reduce fecal losses, and lower mean particle sizes inhibit the positive effect of roasting (Dhiman et al., 1997).
Moist heating is more efficient than dry heating, so than comparable results can be obtained with lower temperatures and duration (Poncet et al., 2003). At similar temperature and duration (120°C, 1h), compared with dry heating, moist heating greatly decreases soluble crude protein and non protein N. It also increases neutral detergent insoluble protein, inducing decreased ruminal degradability of protein and increased intestinal digestibility of RUP (Samadi et al., 2011).
Extrusion decreases protein degradability and increases the in vitro digestibility of the RUP, resulting in an increase of metabolisable protein content of + 90 g/kg DM. The extrusion of soybean seeds together with maize enhances these effects (Solanas et al., 2008). Extrusion of soybean seeds improves feed efficiency in lambs fed a high forage diet. Whereas it does not affect DM intake, it improves average daily gain and concentration of PUFA concentration in muscle, by reducing both N ruminal degradability and fatty acid biohydrogenation (Petit et al., 1997). Adding extruded soybean seeds as a fat source in dairy cows does not affect DM intake. Extrusion improves milk production (+2.8 kg) with a efficiency similar to that of calcium soaps of fatty acids (Kim et al., 1991; Kim et al., 1993). Extrusion tends to decrease milk protein and casein content, even with additional protein (Kim et al., 1991), and reduces milk fat content and increases PUFA concentration in milk fat (Kim et al., 1993). For most PUFA, extrusion provides results similar as thise obtained with roasting, but is more efficient for increasing trans-11 C18:1 production in vitro (Troegeler-Meynadier et al., 2014).
Irradiation (electron beam or gamma) applied to eliminate antinutritional factors such as phytic acid and trypsin inhibitor activity (Taghinejad et al., 2009) decreases protein degradability and increases the digestibility of RUP (-26.5% and +28% , respectively, Ebrahimi-Mahmoudabad et al., 2011). Irradiation improved N retention in goat kids (from 37 to 43%) by reducing urinary N without affecting fecal N excretion, whereas intake and digestibility coefficients for various nutrients were not affected significantly (Mani et al., 2003).
Rumen protection by commercial tannic acid reduced the effective degradability of DM and N following a dose-dependent effect up to 50 g/kg DM of tannin-treated seeds (Martinez et al., 2004).
Nonenzymatic browning (Maillard reaction) can also be used to reduce protein ruminal degradation. It also indirectly protects the lipid fraction from ruminal degradation, allowing a higher proportion of the polyunsaturated to be delivered postruminally. In dairy cows, this provides beneficial increase in milk C18:2 and C18:3, and fat-corrected milk yields similar to those obtained with oil protected with Ca salts (Abel-Caines et al., 1998).
Glucose or xylose treatment: heating soybeans for 2h at 100°C with glucose or xylose (2-3 % DM) reduced the effective degradability of DM and N (-18% with 3% glucose, -28% with 3% xylose) (Sacakli et al., 2009; Sacakli et al., 2011).