Feeding value of whole raw soya beans as a protein supplement for beef cattle consuming low‐quality forages

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Low‐quality high‐fibre forages are abundant and diverse in nature, such as cereal straws, as well as low‐quality native, spontaneous or introduced plant species, which are frequently baled as hay or grazed as deferred forages. Wheat straw is a common available resource potentially improved by processing, chemical treatments and supplementation (Wanapat, Chenost, Munoz, & Kayouli, 1996). Other annual species such as sorghum, because of their high dry matter (DM) yield, is stockpiled for winter grazing. When grain sorghum rather than forage sorghum is deferred, grain would contribute to the energy content of an otherwise high‐fibre diet. Many hybrids of grain sorghum yield a large forage biomass. A ratio grain, total forage biomass ranging from 18.7 to 33.2%, with a total yield of 4,000–9,000 kg of DM/ha, respectively, was reported by Bolletta (2008) for grain‐sorghum hybrids.
However, the main constraints to ruminant performance on these forages are rumen distention limiting voluntary intake (Allen, 1996), the slow rate of cell wall degradation and poor N content (Van Soest, 2006). The stimulating effect of different N sources on low‐quality forages is associated with improvement in rumen fermentation, fibre degradation and utilization (McCollum & Horn, 1990; Leng, 2004). A non‐protein N source such as urea can be fed to ruminants as an economical replacement for a part of the dietary protein. Compared with protein meals of plant origin, a limited animal response should be expected when urea only is supplied to cattle consuming low‐quality forages (Chalupa, 1968; McCollum & Horn, 1990; Arelovich, Laborde, Villalba, Amela, & Torrea, 1992; Leng, 2004). Raw whole soya beans (WSB) could theoretically replace oilseed meals as protein source for ruminants. Because of the high oil content, WSB would contribute with additional energy in addition to the degradable protein provided (Ishler & Varga, 2000). The antinutritional factors such as lipases, urease, trypsin inhibitor (Ishler & Varga, 2000) and haemagglutinins (Venturelli et al., 2015) present in raw WSB are usually considered a feeding restriction for single‐stomach animals. Consequently, deactivation through roasting or extruding might not be necessary for ruminants (McNiven, Grimmelt, MacLeod, & Voldeng, 1992; Rush, 2001) except for increasing protein bypass (Venturelli et al., 2015). However, the active trypsin inhibitor appears to inhibit the proteinase activity of rumen bacteria (Brock, Forsberg, & Buchanan‐Smith, 1982), and, if undegraded in the rumen proteolysis in duodenum would be affected (Orias, Aldrich, Elizalde, Bauer, & Merchen, 2002). Providing 1.5 kg/day of whole raw SB or 1.36 of extruded SB added to low‐quality forage induced an equivalent growth rate (Albro, Weber, & DelCurto, 1993). Besides the latter report, studies including WSB and its effect on beef cattle performance receiving low‐ to medium‐quality forages are limited, and most studies were conducted with high‐quality dairy diets.
Raw soya beans that do not meet the quality standards to be trade as a commodity in the oil seeds market, then, they can be used at a lower cost than any oil seed meal as a protein supplement for ruminants. Then, the objective was to determine in two different experiments the effect of supplementing: (i) raw WSB, roasted WSB or a soya bean meal based mix on digestion and blood parameters in ruminally cannulated steers receiving wheat straw; and (ii) raw WSB contrasted with sunflower meal while grazing a deferred grain‐sorghum forage upon ruminal and blood variables as well as performance in beef cattle.
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