Nutrition

U.S. Soybean Meal: Quality and Economic Benefits

A new meta-analytical study reinforces U.S. Soy’s reputation for being a global leader in quality and nutrient density. The summary of this study is below.

Chemical Composition, Protein Quality and Nutritive Value of Commercial Soybean Meals Produced from Beans from Different Countries: A Meta-Analytical Study

Soybean meal (SBM) is the primary source of protein for the feed industry worldwide, and it is an important protein source in livestock, poultry, and aquaculture diets. Among protein sources of plant origin, soybeans have the highest crude protein content, a balanced amino acid profile, and a high level of digestibility. Soybeans have become a top choice for protein and as such, they are strategically traded around the world. Soybean meal is the standard against which other protein sources are compared. Understanding more about the chemical composition, protein quality, and nutritive value of commercial soybean meals enables us to better determine important agricultural outcomes.

A meta-analysis of eighteen different studies with 1,944 samples was performed by Ibañez et al. (2020) to quantify the relationship between country of origin of the bean and the chemical composition, protein quality, and nutritive value of the soybean meal. Soybeans from the following origins were evaluated: Argentina (ARG), Brazil (BRA), USA (USA), and India (IND).

Crude Protein. Crude protein (CP) is calculated by measuring the Nitrogen (N x 6.25) content of food. Higher levels of CP were found for BRA meals than for USA meals; CP was lowest for ARG. This is likely due to differences in seed genotype, planting area of the beans within countries, environmental conditions during the growing and harvesting seasons, and crop year. Crude protein content and amino acid (AA) profile might also be dependent on latitude as research indicates soybeans grown closer to the equator have higher CP content and less Lysine, Methionine, and Threonine per unit of CP for soybean meal.

Crude Fiber & Neutral Detergent Fiber. Both Crude Fiber (CF) and Neutral Detergent Fiber (NDF) are measurements that quantify fiber content. Fiber is defined as the indigestible but partially fermentable component of feed. Both CF and NDF were higher for BRA and IND meals than for USA and ARG meals, consistent with most published research. As a highly palatable feed, soybean meal is characterized by having a high protein content and a low CF content.

Sugars. Soybeans contain nearly as much carbohydrates as they do protein. The carbohydrates in soybean meal consist of free sugars, oligosaccharides, some starch, and non-starch polysaccharides. Of particular importance is the disaccharide sucrose. Sucrose is a highly digestible carbohydrate that when present in meal, increases the energy content and palatability of the feed. Sucrose was higher in USA and ARG soybean meals than BRA and IND soybean meals. In addition, oligosaccharides like stachyose were higher for USA and ARG than BRA and IND soybean meals while raffinose was lower. Though indigestible in non-ruminant species, oligosaccharides might be fermented in the large intestine and when fed in limited quantities, they may have prebiotic properties with benefits on health and growth.

Amino Acid Profile. Amino acids (AAs) are subunits that make up proteins. They are critical for animal function and are responsible for the growth and development of an organism from building muscles and regulating the immune system to generating hormones and neurotransmitters. There are 20 AAs that comprise the genetic code. These can be arranged in an infinite number of combinations to create functional proteins. Out of these 20 AAs, there are ten that animals cannot make. Animals can recycle some AAs to build new proteins but other AAs such as Lysine, Methionine, and Threonine are only be obtained through feed. Soybeans have ten essential and ten non-essential AAs. (Essential AAs: Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine; Non-essential AAs: Alanine, Asparagine, Aspartic Acid, Cysteine, Glutamine, Glutamic acid, Glycine, Proline, Serine, and Tyrosine). Soybean meal is cost effective in supplying essential AAs and has very good Lysine content and digestibility. Soybean meal is also ranked high Threonine digestibility. Soybean meals from USA and ARG had more Lysine, total Sulfur AA (sulfur-containing AAsàCysteine and Methionine), and Threonine per unit of protein than did BRA or IND.

Energy Content. The energy content of soybean meals depends on the content and the digestibility of the protein fraction and on the sucrose, oligosaccharide, and fiber content. Apparent Metabolizable Energy (AMEn) is used to generate energy values of soybean meals for feed formulation. AME is the gross energy of the feed consumed minus the gross energy contained in the feces, urine, and gaseous products of digestion. The prediction equation is recommended by the World Poultry Science Association (WPSA; 1989), and is used by the industry, but the equation does not take into account the digestibility of the protein fraction, or the differences in sucrose and fiber content. As such, the equation is likely to under-evaluate the real contribution to energy of the USA meals.

Utilizing the recommended prediction equations by the WPSA, the AMEn of the BRA and USA soybean meals were similar in value, reflecting the higher CP but not the lower digestibility of the protein fraction and lower sucrose content of the BRA meals compared with the USA meals. In spite of having a lower CP content, the soybean meals from the USA had similar or even higher energy content for poultry and swine than the soybean meal from BRA, suggesting higher protein digestibility.

Protein Quality Profile (% per unit of protein). Protein quality refers to the digestibility and quantity of essential AAs. There are numerous ways to assess protein quality of a soybean meal sample, one of which is through Trypsin Inhibitors (TI) content. Trypsin Inhibitors are an important group of antinutritional factors present in raw beans. Trypsin inhibitors prevent protein digestion, often leading to digestive disturbances, but in general, they are not a major issue in traditionally processed soybean meal (solvent or extruded) due to temperature, pressure, and time thresholds in processing (overcooking = low to no TI; undercooking = high TI). Procedures to measure TIs are costly so indirect methods are often utilized to measure them and include:

  • Urease Activity (UA)
    • Urease is a metalloenzyme that catalyzes the hydrolysis of urea to ammonia and CO2. High UA is undesirable in most cases (ruminant vs. poultry). In most cases, UA was below the threshold recommended for high quality meals for all SBMs, being highest for IND, but not reaching significance.
  • Protein Solubility in Potassium Hydroxide (KOH)
    • A solubility test used to evaluate heating processes. Results varied widely among origins with higher values for USA meals than ARG and BRA, and IND meals being intermediate (which might be indicative of less heat being applied to USA meals).
  • Protein Dispersibility Index (PDI)
    • This index compares the solubility of protein in water and it is an indicator of adequate heat processing of soybean meals. Results differed among origins with higher values for IND and USA meals than for South American meals.

Soybeans have excellent agronomic and nutritional qualities and soybean meal is one of the most important protein ingredients in animal diets today. With higher sucrose levels, a well-balanced amino acid profile, higher digestibility, increased metabolizable energy, and lower fiber content, quality traits are better for USA meals than those from the other origins. With growing demand for vegetable proteins, there will likely be an even greater demand for soybeans as the developing world becomes more affluent, adding overall value and demand for U.S. Soy.

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Emily Dustman
Emily Dustman

Director of Science Communications

United Soybean Board

Emily Dustman serves as director of science communications for the United Soybean Board.