107 Soybean meal energy value under field conditions: Where are we at and where are we going?

Abstract Soybean meal (SBM) is the main protein source for swine diets globally. Considering its wide utilization and relevance to the swine industry, it is critical to accurately characterize its nutritional profile. The net energy (NE) of SBM has been particularly scrutinized as an increasing amount of evidence suggests it may be underestimated. For instance, the NRC (2012) reports the NE of SBM as 2,087 kcal/kg. Recent indirect calorimetry trials suggest, however, that this value could be at least 8% greater. Furthermore, large scale growth assays conducted under the rigors and challenges of field conditions indicate that the energy of SBM may be even greater. The foundation of such growth assays is to relate the energy value of a test ingredient, in this case SBM, to the energy value of a known ingredient it replaces in diet formulation, such as corn. Changes in feed and caloric efficiencies are measured and can then be used to estimate the energy content of SBM relative to corn. Multiple field trials established that when fed increasing levels of SBM, late nursery, growing, and finishing pigs respond with linear improvements in feed and caloric efficiencies, suggesting an underestimation of the energy value of SBM. In those cases, while the NRC value for SBM NE is 78% of corn NE, this value ranges from 100 to 125% under field conditions. It is hypothesized that the discrepancy between NE values derived from academic and commercial facilities could be driven by differences in health and environmental conditions and the functional molecules within SBM. These functional molecules, such as isoflavones and saponins, have been reported to exert anti-inflammatory, anti-viral, and antioxidant properties. In a challenging field environment, such molecules could bolster the immune response, consequently conserving and redirecting energy from the immune system to growth. Although the evidence is robust, it is critical to understand the particularities of such trials. The methodology utilized is clearly not the same as classic NE measurements. Thus, the term productive energy has been proposed to describe the energy value observed in growth assays. Although all studies agree that the energy of SBM should be greater than traditional publications, there is significant variability across studies which should be further investigated. Nevertheless, these findings provide important insight regarding the value of SBM, particularly for practical swine nutritionists.