Understanding Soybean Quality

Scott Nelson

Scott Nelson

Iowa Soybean Association

World markets value soybeans for their 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 southerly 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 contains higher concentrations of these amino acids should be more valuable than meal with lower concentrations, as it reduces the need for synthetic supplements.