Soybeans are a good source of protein and energy for ruminants.
Nutritional value
Soybean seeds have a high ME content (16 MJ/kg DM; INRA, 2007). They also have a high in vitro (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 metabolizable protein content does not exceed 90 g/kg DM (Poncet et al., 2003). The degradation rate can be lowered by heat treatments such as flaking or toasting, by extrusion, or by formaldehyde treatment, N effective degradability being 69%, 63%, 47% and 40%, respectively (INRA, 2007), or other processes (see below). The metabolizable protein content can reach 170 g/kg DM for toasted and flaked seeds, and 250 g/kg DM for extruded or formaldehyde-treated seeds (INRA, 2007). These processes indirectly protect the lipid fraction from rumen degradation, allowing a higher proportion of the polyunsaturated fatty acids (PUFA) to be delivered postruminally.
Inclusion rates
Whole soybeans can be included at 18% of DM intake for dairy cows (Dhiman et al., 1997). Higher levels can be used (Gralak et al., 1997). However, when soybeans are fed in large quantities, the diet must also contain adequate amounts of vitamin A (Newkirk, 2010).
For cattle, soybeans are 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 supplementation with soybean meal and hulls, raw soybeans do not affect reproduction, calf weaning weight, forage intake and digestion (Banta et al., 2008). Comparable results are obtained with steers receiving either raw soybeans (16.5%) or sweet lupin seeds (20%) in complete pelleted diets (Vicenti et al., 2009). In feedlot lambs, the inclusion of raw soybeans, up to 21% of DM in isonitrogenous, high concentrate diets, decreased DM intake, but did not affect feed conversion, carcass yield and lamb cuts, thus providing satisfactory lamb performance (Urano et al., 2006).
Mechanical treatments
Reducing particle size is not efficient in improving 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, or body weight change (Pereira et al., 1998a; Pereira et al., 1998b).
Heat and pressure treatments
Roasting (dry heating) is the most commonly used treatment for soybean seeds. 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 for 30 min (Faldet et al., 1991). Roasting at this temperature increases the proportion of N escaping rumen 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 yield, and concentrations of milk fat, protein and lactose. However, it 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 faecal losses, and lower mean particle sizes inhibit the positive effect of roasting (Dhiman et al., 1997).
Moist heating of soybean seeds is more efficient than dry heating, so comparable results can be obtained with lower temperatures and less time (Poncet et al., 2003). At similar temperature and duration (120°C for 1h), when compared with dry heating, moist heating greatly decreases soluble crude protein and non protein nitrogen. It also increases neutral detergent insoluble protein, inducing decreased rumen degradability of protein and increased intestinal digestibility of RUP (Samadi et al., 2011).
Extrusion of soybean seeds decreases protein degradability and increases the in vitro digestibility of the RUP, resulting in an increase of metabolizable protein content of an extra 90 g/kg DM. 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 rumen degradability and fatty acid biohydrogenation (Petit et al., 1997). Adding extruded soybean seeds as a fat source in dairy cow diets does not affect DM intake. Extrusion improved milk production (+ 2.8 kg) with an efficiency similar to that of calcium soaps of fatty acids (Kim et al., 1991; Kim et al., 1993). Extrusion tended to decrease milk protein and casein content, even with additional protein (Kim et al., 1991), and reduced milk fat content and increased PUFA concentration in the milk fat (Kim et al., 1993). For most PUFA, extrusion provides results similar as those obtained with roasting, but is more efficient for increasing trans-11 C18:1 production in vitro (Troegeler-Meynadier et al., 2014).
Irradiation of soybean seeds by gamma rays or electron beam, applied to eliminate antinutritional factors such as phytic acid and trypsin inhibitor activity, decreased protein rumen degradability and increased the digestibility of the RUP (-26.5% and +28%, respectively (Taghinejad et al., 2009; Ebrahimi-Mahmoudabad et al., 2011). Irradiation improved N retention in goat kids (from 37 to 43%) by reducing urinary N without affecting faecal N excretion, where intake and digestibility coefficients for various nutrients were not affected significantly (Mani et al., 2003).
Chemical treatments
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 soybean seeds (Martinez et al., 2004).
Nonenzymatic browning (Maillard reaction) can also be used to reduce rumen degradation of soybean protein. It also indirectly protects the lipid fraction from rumen degradation, allowing a higher proportion of the polyunsaturated fatty acids to be delivered postruminally. In dairy cows, this provided a beneficial increase of C18:2 and C18:3 in milk, with 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% of 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).
