Physiological Characteristics of High Yield Soybeans.

Mr. Kip Cullers has reported soybean [Glycine max (L.) Merr.] yields in the Missouri Soybean Association’s yield contest of 9339 kg ha^-1 (2006), 10,388 kg ha^-1 (2007) and 10,791 kg ha^-1 (2010), all of which more than tripled the 2010 U.S. average yield of 2788 kg ha^-1.  In 2011 and 2012, measurements were collected from four plots in each of four or five varieties within Mr. Cullers’ contest field. Key physiological measurements included radiation use efficiency (RUE), nitrogen (N) accumulation rate, the fraction of N derived from N₂ fixation via a non-nodulating reference soybean inter-planted within the field, the dry matter allocation coefficient (DMAC), yield, harvest index (HI), seeds m^-2, and average seed mass. In 2011, extreme heat in June, July and August occurred during reproductive development and likely suppressed yields, which ranged from 7145 to 5296 kg ha^-1. The HI, seed number, and average seed mass ranged from 0.498 to 0.428 kg kg^-1, 3421 to 4428 seeds m^-2, and 15.27 to 17.9 g 100 seed^-1, respectively. The DMAC values were abnormally small (0.0088 to 0.0111) resulting in slow seed fill rates and long seed fill durations. In 2012, the N accumulation rate with a full canopy ranged from 0.83 to 0.90 g N m^-2 d^-1, which was greater than previous reports. In 2011, it was determined that Mr. Cullers’ soybean crop received 7.1 to 16.0% of the N from N₂ fixation. The RUE was also high, ranging from 2.06 to 1.80 g MJ^-1 of photosynthetically active radiation (PAR) in 2011 and 2.4 to 1.7 g MJ^-1 of PAR in 2012. The abnormally high RUE and N accumulation rates document how such a large canopy and N pool is achieved. This coupled with the low DMAC and long seed fill duration provides the physiological foundation for these extraordinary yields.