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Understanding soybean quality

Article cover photo
Soybean grain quality is more than protein. Understanding and improving essential amino acids in soybeans will help the livestock industry and meat quality. (Photo by Joseph L. Murphy/Iowa Soybean Association)

By Scott Nelson, ISA On-Farm Network® Director

World markets value soybeans for its concentrations of oil and protein. Generally, soybean grain produced in southern climates such as Brazil have higher concentrations of protein compared to soybeans produced in the north central region of the United States. Thus, there is some preference in world markets for southernly produced soybeans as they are believed to have greater feed value for monogastric animals and poultry.

New research indicates that valuation of soybean grain by amino acid composition is a better indicator of soybean quality than crude protein. It is very important for soybean farmers to understand the implications of this research. Following is a synthesis of these recent studies.

Understanding the biochemistry of soybeans

To understand soybean protein, we must understand some simplified biochemistry. Amino acids are small molecules that are necessary in animal and human nutrition. Amino acids can be thought of as the building blocks of life, as they combine to form proteins that are essential in virtually all cell functions in plants and animals. In human and animal nutrition, there are 20 amino acids that are most important in the construction of necessary proteins.

In humans and animals, some amino acids are produced naturally by the body and some require external nutrition sources. Amino acids produced by the body are called nonessential amino acids: alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine and tyrosine.

Amino acids not produced naturally by the body are called essential amino acids. These include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. These essential amino acids must be acquired through diet. Unlike humans and animals, plants produce all their essential and nonessential amino acids.

Monogastric animals (e.g. pigs, chickens, humans) require essential amino acids in their diets at the right concentration or their growth will be impaired. An animal fed a supra-optimum diet of protein from nonessential amino acids will die of malnutrition.

In soybeans, five essential amino acids are lowest in concentration: lysine, cysteine, methionine, threonine and tryptophan. These amino acids are typically added to animal rations as synthetic supplements. Soybean meal that contain higher concentrations of these amino acids should be more valuable than meal with lower concentrations, as it reduces the need for synthetic supplements.

Meal or grain termed “high protein” means nothing to a feed ration as it does not specify the concentrations of essential and nonessential amino acids. To optimize animal growth and reduce excretion of nitrogen as waste, feed rations need to have a balance of amino acids for a proper diet. What is important to livestock and poultry producers is the content of the amino acids and not the level of crude protein.

Raffinose is complex sugar molecule found in soybeans and is characterized as anti-nutritional. Humans and monogastric animals lack an enzyme to break down raffinose, therefore the nutrition value is lost and expelled as gas. Contrary to raffinose, the sucrose content in soy is considered nutritional and contributes readily usable energy to the animal. Ideal rations for monogastric animals are lower in raffinose and higher in sucrose.

How do soybeans produced in the U.S. north central region compare in terms of protein, essential amino acids and feed value?

As previously mentioned, soybean grain produced in the U.S. north central region can be 1 to 2 percent lower in crude protein. However, grain protein is a poor predictor of feed value. In fact, research conducted by the University of Minnesota shows a negative correlation between protein levels in soybeans and essential amino acids. As protein levels become higher, the amount of essential amino acids decreases. Characterizations of soybean samples from across the world have shown that those produced in the north central United States have higher concentrations of essential amino acids. The table below is from a recent comparison of U.S.-produced soybeans with those grown in Brazil. Also note that U.S. soybeans are higher in sucrose and lower in anti-nutritional raffinose.

Table 1. Comparison of more than 600 soybean grain samples from 2012-2014.


Protein (%)

Total Essential Amino Acids (%)





United States










Although Brazilian soybeans are typically higher in crude protein than U.S. soybeans, the lower protein U.S. soybeans are enriched in lysine, cysteine, methionine and threonine. They can be expected to produce higher quality meal that can meet animals’ essential amino acid needs at lower dietary protein concentrations. Table 2 compares U.S. soybean meal quality versus meal from Brazil and Argentina.

Table 2. Comparisons of soybean meal samples collected in 2007.


Protein (%)

Total Essential Amino Acids (%)

Digestible Limiting Amino Acids (Swine)


Digestible Limiting Amino Acids (Poultry)


United States















Things to remember about soybean quality

  • Crude protein is not a good predictor of feed value. The amount of essential amino acids and concentrations of raffinose and sucrose are most important regarding soybean feed value.
  • While U.S. produced soybeans tend to have lower levels of crude protein, they are generally higher in essential amino acids and sucrose and lower in anti-nutritive raffinose.
  • Soybeans and soybean meal produced in the U.S. have a significant advantage over South American produced soybeans in terms of feed value.

For more information, download the presentation “Quality of Soybeans in the US: 2016” from the University of Minnesota.

Contact Scott Nelson at

For media inquiries, please contact Katie Johnson, ISA Public Relations Manager at or Aaron Putze, ISA Communications Director at

For permission to republish articles or to request high-res photos contact Aaron Putze at Iowa Soybean Association | 1255 SW Prairie Trail Pkwy | Ankeny | IA | 50023 | US

©2018 Iowa Soybean Association On-Farm Network®. All rights reserved. On-Farm Network® is a registered trademark of the Iowa Soybean Association, Ankeny, IA.Portions of some On-Farm Network trials are paid for in total or in part by the soybean checkoff.