6/22/98

Contacts:
Richard Shibles, Agronomy, (515) 294-7232
Ed Adcock, Agriculture Information, (515) 294-2314

FEED VALUE OF SOY PROTEIN LIMITED BY SOYBEAN PLANT

AMES -- 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.

The research found:

• Iowa soils possess more than adequate sulfur for soybean yields of 60 bushels an acre and current protein quality. About 45 pounds an acre of sulfur are needed for "critical protein quality" and Ida soils, among the lowest sulfur content soils in the state, contain about 55 pounds of available sulfur an acre.
• 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.
• The capacity for sulfur assimilation, or the synthesis of methionine from sulfate, continues late into the seed-fill stage and sulfur uptake on a whole-plant level continues late into seed-filling.
• Soybean seeds appear to be the main sites of methionine synthesis for seed protein. Most of the sulfur taken up during seed-filling quickly finds its way to the developing seed. This suggests that seed tissue may be the predominant site of sulfur metabolism and should be the focus of future work on increasing the amount of methionine synthesis.

"It is a plant problem that has to be approached in a team fashion," said Shibles.

He said the research suggests a two-pronged approach: Through molecular biology, increase the expression of high quality protein and increase the capacity of the soybean plant to use sulfur and make more of the amino acids.

The goal of increasing the content of methionine and cysteine 1 percent, from .72 percent to 1.72 percent, was identified in an Iowa State economic analysis as one way to increase the value of soybeans to producers and end-users. The analysis estimated that increasing those amino acids in soybean meal would justify a 68-cent-a-bushel premium from poultry producers. European livestock producers could be another potential market for high methionine soybeans because they use protein-poor dried cassava in feed rations.

Soybean growers could see an increase of up to 2.5 cents a bushel by increasing methionine and cysteine, according to the analysis sponsored by the ISPB and the American Soybean Association.

Solving soybean's protein problem would require a long-term commitment, Shibles said. He estimated it might take eight to 10 years of basic research to improve soybean's protein before plant breeding could begin.

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