Soybean Meal Composition Studies

A study was undertaken at North Carolina State University to identify soybean meal’s nutritional characteristics desired by swine nutritionists. Specifically, the research was designed to evaluate compositional variation in experimental and commercial soybean varieties and whether these compositional differences were large enough to be nutritionally or environmentally important to the swine nutritionists. In a study reported in 2002, the digestibility of soybean meal was found to be relatively uniform across the U.S. and Netherlands. This lack of variability was attributed to minimal genetic variation in nutritional traits in commercial soybean cultivars and to uniform meal processing conditions. This study was designed to expand these research investigations and to evaluate experimental soybean germplasm lines with unique traits.

Seventy-two soybean samples were analyzed for crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF) and in vitro dry matter (DM) digestibility. Based on the variation found, eight samples (five genetically modified lines and three commercial varieties) were selected for testing. The eight soybean sources were sent to the Texas A&M Oilseed Processing Laboratory to be processed into meal using carefully controlled uniform processing conditions. The pilot-plant solvent-extracted meals were analyzed for moisture, crude fat, crude protein, crude fiber, ash, urease activity, trypsin inhibitor, protein dispersibility, phytate phosphorus and oligosaccharides using officially accepted methods.

A Latin square designed study with eight pigs and eight diets was used for the ileal digestibility experiments. The sixteen-percent crude protein experimental diets were formulated with soybean meal as the only protein source. Soybean meal levels in the diets ranged from 31-34 percent. Feed and freeze-dried ileal digesta were analyzed for the chromic oxide marker, amino acids and dry matter.

In another experiment, the total track digestion was determined using five soybean meals, ten crossbred barrows and a 5x5 Latin square designed study. Urine and feces were collected, stored and evaluated for gross energy, nitrogen and odor characteristics. Using data from the two trials, the researchers calculated fermentation rates for CP and DM based on actual ileal digestibilities of the feed ingredients and total tract digestibilities of the complete diet.

Results indicated that the 72 soybean samples was fairly uniform; the coefficients of variation (CV) for CP, NDF and ADF were 3.6, 7.4 and 8.8%, respectively. However, the researchers were successful in selecting eight soybean samples with a higher CV for ADF (26%), but were not successful increasing the CV for CP (3.7%). The ranges of values were: CP, 48.2-53.1%; CF, 2.6-5.6%; NDF, 5.8-9.2%, ADF, 2.6-5.0%; hemicellulose, 3.1-4.7%; raffinosis, 0.15-1.22% and stachyose, 0.28-5.93%. These data suggest that important composition differences exist between different soybean germplasm lines. These differences may have an impact on the meal’s nutritional value. These results also indicate that the variations in protein levels in soybean lines are relatively small and processing does not increase the amount of variation in the meal produced.

Several correlations were found between the meal components. The CP content of the meal was negatively correlated with fiber (r= -0.70, P=0.05), raffinose (r= -0.90, P<0.01), and stachyose (r= -0.96, P<0.01). Trypsin inhibitor, urease value and protein dispersion index were all correlated (r= -0.63 to -0.83) indicating processing had a similar effect on all of these meal components.

The researchers reported differences up to 6% in ileal DM digestibilities and 8% in ileal CP digestibilities. These differences were reduced to 1.1% for total tract DM digestibility and 4% for total tract CP digestibilities by further break down of the materials in the lower digestive track. Ileal digestibilities of the eight soybean meals ranged from 79.6 to 83.2% and were highly correlated with energy digestibility (r= 0.91, P<0.05). Crude protein ileal digestibilities ranged from 80.6 to 84.6% and were not correlated with DM digestibility. It should be noted that the apparent ileal digestibilities for CP and lysine, methionine and threonine were highly correlated (r>0.96, P<0.01). Protein dispersibility index, trypsin inhibitor and urease levels had little impact on digestibilities in this study.

Stachyose was the only composition trait that had a significant impact on ideal digestion. Stachyose had a negative correlation with ileal DM (r= -0.80, P=0.02) and energy (r= -0.73, P=0.04) digestibilities. Negative correlations were also reported for hemicellulose and oligosaccharides, but the researchers attributed the effects to stachyose. These observations were similar to several poultry studies showing that soy oligosaccharides reduce energy digestibility. Stachyose was not significantly correlated with ileal CP digestibility (r=-0.28, P=0.50), suggesting that stachyose may not be a major factor in protein digestion in practical corn-soybean meal swine diets.

Small differences were observed in total tract digestibilities between the five soybean meal samples. Ranges for total tract digestibilities of DM (87.9-88.9%), energy (86.5-87.7%), and CP (84.4-87.5%) were reported. These data indicate that fermentation in the large intestine minimizes meal treatment differences.

Manure and urine samples were analyzed for odor components. The results of these analyses showed a lot of variation. No clear-cut differences between meals were reported. As expected, some soybean meal components were directly related to levels of manure odorants. More research is needed to clarify compositional changes needed in soybeans to minimize swine odors.

The researchers conclude that there is variation in the soybean genetic base that could be used in soybean breeding programs to develop varieties with greater potential nutritional and environmental benefits to the swine nutritionist. The key trait seems to be stachyose; reduced stachyose levels in meal would result in increased ileal energy digestibility of the soybean meal. No composition trait was identified in this study that explains the differences in ileal CP digestibilities important to both maximizing the nutritional value of the meal and possibly reducing ammonia and odor emissions.

Van Kempen, T.A. and co-workers. 2006. Selecting soybean meal characteristics preferred for swine nutrition. J. Animal Sci. 84(6): 1387-1395.