The nature of the soybean plant limits its ability to produce high quality protein, Iowa State University researchers have discovered.
Soybean protein is deficient in the sulfur-based amino acids methionine and cysteine.
Soybean meal must be fortified with synthetic forms of those amino acids to provide the proper nutrition when preparing feed for swine and poultry. Agronomist Richard Shibles and his research team delved into soybean's sulfur utilization to identify its limiting factors in a two-year study funded by the Iowa Soybean Promotion Board. Sulfur in the soybean plant is not moved to the seeds as efficiently as nitrogen during seed-fill. The sulfur harvest index, or the recycling of vegetative sulfur for seed use, was consistently 15 to 20 percent less than that of nitrogen, even in conditions of sulfur deficiency. Providing sulfur as methionine in hydroponic studies enhances protein quality, but not enough to provide the nutritional requirements for animal feed.