Estimate of the energy value of soybean meal relative to corn based on growth performance of nursery pigs

Two experiments were conducted to determine the effects of increasing amounts of soybean meal (SBM) in swine diets and estimate the energy value of SBM. A total of 2233 pigs (PIC 337 × 1050, Hendersonville, TN) and 3796 pigs (PIC 359 × C40), initially 11.0 kg and 17.6 kg body weight (BW), were used in Exp. 1 and 2, respectively. In Exp. 1, pigs were placed in 92 pens each containing 20 to 27 pigs. In Exp. 2, pigs were placed in 84 pens each containing 37 to 43 pigs. Treatments were assigned in a randomized complete block design with BW as the blocking factor. Dietary treatments consisted of 21%, 27%, 33%, or 39% SBM in Exp. 1 and 17.5%, 22%, 26.5%, 31%, 35.5%, or 40% SBM in Exp. 2, obtained by changing the inclusion rate of feed-grade amino acids and corn grain. For Exp. 1, representative samples of corn grain, SBM, and distillers dried grains with solubles were analyzed for total AA content prior to diet formulation. For Exp. 2, diets were formulated using NRC (2012) nutrient loadings. Treatment diets were fed for 21 and 22 d (Exp. 1 and 2) and there were 23 replicates in Exp. 1 and 14 replicates in Exp. 2. Pigs were weighed and feed disappearance measured weekly to calculate average daily gain (ADG), average daily feed intake (ADFI), gain-to-feed ratio (G:F), and caloric efficiency (CE). Data were analyzed with block as a random effect and treatment as a fixed effect, and contrasts were constructed to test the linear and quadratic effects of increasing SBM. In Exp. 1, there was a tendency (linear, P = 0.092) for a decrease in ADFI as SBM increased. There was a tendency (P = 0.090) for a quadratic response for ADG, with a decrease in ADG observed with 39% SBM inclusion. Pigs fed diets with increasing SBM had a tendency (quadratic, P = 0.069) for an increase in G:F up to 33% SBM and an improvement (linear, P = 0.001; quadratic, P = 0.063) in CE with increasing SBM. Using CE to estimate the energy of SBM relative to corn, a value of 105.4% of corn energy or 2816 kcal/kg NE was determined using all data points. When removing the CE value of the 39% SBM treatment due to the quadratic tendency, SBM was estimated to have 121.1% of corn energy or 3236 kcal/kg NE. In Exp. 2, there was a decrease (linear, P = 0.001) in ADFI. Pigs fed increasing SBM had a tendency (linear, P = 0.065) for reduced ADG but an improvement (linear, P = 0.001) in G:F and CE as SBM increased. The energy value of SBM was estimated as 124.7% of corn energy or 3332 kcal/kg NE. The results suggest that feeding increasing levels of SBM improves G:F and CE. The energy value of SBM was estimated to be between 105% and 125% of corn, which is much greater than the NRC (2012) would indicate.


Background
Soybean meal (SBM) is the primary plant-protein source for swine diets in the United States. The amino acid (AA) profile of SBM is well-balanced and complements the AA profile of grains such as corn and wheat, and these AA are highly digestible for pigs [1]. The energy content of SBM has been reported [1] as 3619 kcal/kg digestible energy (DE) and 3294 kcal/kg metabolizable energy (ME), which suggests that SBM has 105% and 97% of corn grain DE and ME values, respectively. More recently, swine nutritionists have adopted the net energy (NE) system due to the higher correlation to performance relative to DE or ME systems [2]. Using the NE system, SBM contains 2087 kcal/kg, which is only 78% of the corn energy value [1]. However, recent research shows improvements in gain-to-feed ratio (G:F) of pigs fed increasing levels of SBM [3,4], which could indicate that the NRC [1] underestimates the NE of SBM.
Calorimetry trials to measure NE involve laborintensive procedures that require highly specialized equipment. A practical approach conducted under field conditions to estimate energy values has gained acceptance among swine nutritionists. Feeding increasing amounts of an ingredient and using the differences in caloric efficiency (CE) to estimate the energy content of a test ingredient relative to a known ingredient, usually corn, has been reported by others and sometimes termed productive energy [5][6][7][8][9]. As the inclusion of the test ingredient increases, CE should not change if its energy estimate is accurate. Increases or decreases in CE indicate over-or underestimation of the energy content. Therefore, the objective of this study was to determine differences in growth performance of pigs fed increasing amounts of SBM and, by using changes in CE, to estimate SBM energy value relative to corn.

Methods
The Kansas State University Institutional Animal Care and Use Committee approved the protocol used in these experiments.  [7] equation. Diets were provided in mash form. The energy value of SBM relative to corn was estimated based on CE, which was obtained by multiplying ADFI by kcal of NE per kg of diet and dividing by ADG. In order to obtain an energy estimate, the energy value of SBM was adjusted for the slope of CE to be zero. were placed in 84 pens with 37 to 43 pigs per pen. Each pen (6.9 m × 3.6 m) had fully slated floors and was equipped with a 4-hole stainless steel wet-dry feeder and a nipple waterer.

Animals and housing
Pigs were weaned at approximately 21 d of age, placed in pens based on initial body weight (BW), and fed common diets until the start of the experiments. Pens of pigs were blocked by BW (initial BW = 11.0 kg in Exp. 1 and 17.6 kg in Exp. 2, respectively) and allotted to 1 of 4 or 6 treatments in Exp. 1 and 2, respectively, in a randomized complete block design. Pens of pigs were weighed, and feed disappearance was measured weekly to determine average daily gain (ADG), average daily feed intake (ADFI), G:F, and CE. Culls and mortality were recorded daily.

Statistical analysis
Data were analyzed as a randomized complete block design with initial BW as the blocking factor. Single degree-of-freedom contrasts were constructed to test the linear and quadratic effects of increasing SBM. Block was included as a random effect and treatment as a fixed effect. Pen was considered the experimental unit. Data were analyzed using the GLIMMIX procedure of SAS 9.4 (SAS Institute Inc., Cary, NC, USA). Data distribution was assessed using visual inspection of histograms prior to statistical analysis. Results were considered significant at P ≤ 0.05 and a tendency at 0.05 < P ≤ 0.10.

Chemical analysis
The analyzed amino acid profiles of corn, DDGS, and SBM used in Exp. 1 were, in general, within the expected values (Table 1). Soybean meal and DDGS had a similar amino acid composition to NRC [1] values, whereas AA in corn were slightly lower than NRC [1] estimates, especially for Met and Leu. The chemical analysis of diets was consistent with formulated values (Tables 2 and 3).

Growth performance and energy estimate
In Exp. 1, there was a tendency (linear, P = 0.092) for a decrease in ADFI as dietary SBM increased (Table 4). Pigs fed diets with increasing SBM had a tendency (quadratic, P = 0.090) for an improvement in ADG up to 33% SBM, followed by a decrease in ADG when 39% SBM was fed. The changes in ADG and ADFI resulted in a tendency (quadratic, P = 0.069) for an improvement in G:F up to 33% SBM. There was an improvement (linear, P = 0.001; quadratic, P = 0.063) in CE with increasing SBM. There was no evidence (P > 0.10) for difference in cull and mortality rate. In Exp. 2, there was a reduction (linear, P = 0.001) in ADFI as SBM increased. Pigs fed increasing SBM had a tendency (linear, P = 0.065) for reduced ADG (Table 5). However, the differences were relatively small and did not result in differences (P ≥ 0.27) in final BW. There was an improvement (linear, P = 0.001) in G:F and CE as SBM increased. There was a reduction (linear, P = 0.050) in cull and mortality rate as SBM increased.
A considerable amount of research has been conducted to evaluate the effects of SBM on growth performance of pigs. It is well known that the addition of SBM should be restricted in the diet immediately after weaning due to a hypersensitivity reaction [13,14], but the restriction is not necessary after initial exposure. Nevertheless, feeding SBM is usually limited due to the high cost compared to diets formulated with high amounts of feed-grade AA as a replacement of intact protein sources. However, some research suggests feeding diets with higher amounts of SBM could prove beneficial, especially for health challenged pigs. Johnston et al. [15] fed 21% or 32% SBM to grow-finish pigs that were infected with porcine reproductive and respiratory syndrome (PRRS) and observed that pigs fed 32% SBM had improved ADG and G:F compared with those fed 21% SBM. Similarly, Rocha et al. [16] observed improvements in G:F of nursery pigs inoculated with PRRS virus and fed 22.5% SBM compared to 12.5% SBM. Rochell et al. [17] found that PRRS positive nursery pigs had improved ADG when fed 29% SBM compared to 17.5% SBM. Conversely, Cemin et al. [4] fed 27% or 35% SBM to PRRS negative nursery pigs and observed improvements in ADG and G:F as SBM increased. Moran et al. [3] conducted two trials evaluating increasing amounts of SBM for nursery pigs. In the first trial, pigs were PRRS negative and the authors observed a consistent improvement in G:F. However, the results were not repeated in the subsequent study when pigs originated from a PRRS positive sow farm performance.
Interestingly, Moran et al. [3] found a reduction in the percentage of pigs removed for medical treatment from 11.1% to 8.4% as SBM increased. This observation is in agreement to our finding in Exp. 2, where increasing SBM linearly reduced cull rate. The benefits of SBM on growth performance, especially for health challenged pigs, have also been hypothesized to be driven by bioactive components such as isoflavones and saponins, which have anti-inflammatory, antioxidant, and antiviral properties [18,19]. However, the available research is inconsistent regarding the effects of isoflavones on growth performance of pigs. Kuhn et al. [20] compared SBM and soy protein concentrate, an ingredient with markedly lower isoflavones relative to SBM, in a weanto-finish study and observed higher plasma isoflavones in pigs fed SBM but no evidence for differences in growth performance. Greiner et al. [21,22] evaluated increasing dietary isoflavones and observed improvements in performance of PRRS positive pigs, mostly during periods of peak viremia. Smith et al. [19] fed diets based on soy protein concentrate or enzyme-treated SBM with or without isoflavones and observed changes in activation of the adaptive immune system, although no impact on growth performance was observed. There was a tendency for a quadratic response in ADG with increasing SBM in Exp. 1, with a decrease observed in ADG of pigs fed the highest SBM inclusion. Similarly, in Exp. 2 there was a slight reduction in ADG with increasing SBM. However, the differences between treatments were relatively small and did not result in statistical differences in final BW. Feed intake was also negatively affected by high levels of SBM. The reason for the negative response of high SBM inclusion on ADFI and ADG is unclear. Although the available literature generally does not agree with this finding, as the majority of studies [3,4,[15][16][17] found no change or improvements in ADG with increasing SBM, the current experiment evaluated higher SBM levels than most of the previous research. It could be hypothesized that the high level of crude protein in the diet with 39% SBM provided excess nitrogen which needs to be metabolized and excreted by the animal [23]. The excess nitrogen represents an energy cost that may ultimately translate to decreased growth performance. Moreover, the Leu: Lys increased with increasing levels of SBM. It is well known that excessive Leu leads to reduction in feed intake [24]; however, recent research suggests that the negative impact may be counteracted by concomitant increases in Ile, Val, and Trp [25], as observed in our experimental diets.
Improvements in feed efficiency with increasing amounts of SBM seem to be more consistently reported in the literature and agree with our findings. Energy is the most expensive component of any swine diet, thus it is critical to accurately determine the energy value of feedstuffs. Direct measurement of NE is a procedure that requires highly specialized equipment. Therefore, the estimation of the energy value of a test ingredient based on CE relative to a known ingredient such as corn grain is suggested as a practical approach, and is sometimes termed productive energy [5][6][7][8][9]. Besides the practical advantage, the estimates using CE conducted under field conditions may be more predictive of growth performance than other energy values. The diets used in our experiments were formulated using the NRC [ 1 based on CE suggest that the energy value of SBM is 105.4% of corn grain energy or 2816 kcal/kg NE. It is important to note that, while CE response was significantly linear (P = 0.001), there was also a tendency (P = 0.065) for a quadratic response. Therefore, it could be hypothesized that the CE value of 39% SBM treatment should not be considered for estimating energy because slope- Another important consideration is that the responses in performance could have been driven by underestimation of the AA requirements relative to Lys. Our diets were formulated to meet or exceed the NRC [1] requirement estimates; nevertheless, if any of these estimates is not accurate, by increasing the inclusion of SBM we could have potentially corrected an AA deficiency. However, most of the AA ratios were well above that recommended by the NRC [1], thus the responses to SBM are unlikely to be driven by changes in AA ratios.

Conclusion
Nursery pigs fed diets with increasing amounts of SBM presented inconsistent responses in ADG, but G:F and CE were improved in both experiments. The results of the current study suggest that the energy value of SBM may be estimated to range between 105% and 125% of corn energy, or 2816 and 3332 kcal/kg NE, which indicates that the NRC [1] potentially underestimates the SBM NE value. This has important ramifications as it increases the value of SBM in diet formulation. However, it is unclear if the benefit of higher inclusion of SBM is entirely driven by energy or if another underlying mechanism, potentially involving intrinsic SBM components such as isoflavones, could be partially responsible for the response observed in this study.