Selecting for yield and pest resistance, plant breeders have been able to improve the yield potential of soybeans by about 1/3 bu/year. But little is known about the physiology of yield improvement.

Using computer-model research plots, Midwestern university scientists have been looking for ways to improve yields. In 1997 and 1998, scientists planted both older varieties and newer ones in Illinois, Iowa and Wisconsin, and then compared the crop growth measurements to computer simulations using the Cropgro soybean model.

Through the analysis, scientists determined that increasing the leaf area or vegetative growth patterns did not increase yields. Improved seed yield resulted from generally earlier pod and seed addition by two to four days, 10-20% faster pod addition, greater determinacy, two to five days longer duration of seed growth from beginning seed to physiological maturity and 1-7% higher leaf photosynthesis.

The crop modeling indicates there's still room for genetic yield improvement. The scientists are concerned, however, that they may hit a yield ceiling because most of the observed yield improvements have come from shifting dry matter allocation to seeds rather than from increasing the total amount of dry matter.

(K.J. Boote, University of Florida; W.D. Batchelor, Iowa State University; K. Boedhram, Iowa State University; E.D. Nafziger, University of Illinois; E.S. Oplinger, University of Wisconsin; O. Myers, Southern Illinois University)