Genetic Gain X Management Interactions in Soybean: II Plant Density Effects.

Plant breeders have increased genetic yield potential of soybean during the past century.  The plant characteristics which soybean breeders have selected to contribute the most to yield gains are largely not understood.  One hypothesis is plants from recently released cultivars can withstand increased plant population (inter-plant competition) more effectively than earlier released cultivars.  Soybean cultivars released over the last 80 years were evaluated at high and low seeding rates in research trials conducted in Illinois, Indiana, Minnesota and Wisconsin.  In Illinois and Indiana.  More specifically, we examined 59 maturity group II cultivars released between 1928 and 2008 in WI and MN, and we examined 57 maturity group III cultivars released between 1923 and 2007 in IL and IN.  These cultivars were compared between seeding rates of 445,000 seeds ha^-1 (high rate) and 148,000 seeds ha^-1 (low rate).  Yield components from plants in one meter of row were separated between main stems and branch stems.  Plants were measured for final height, main stem nodes, pods, and seeds were counted, and total biomass and seeds were weighed.  Final plant stands averaged 311,000 and 94,000 plants ha^-1 for the high and low seeding rates, respectively.  Seed yield was higher for the high seeding rate versus low seeding rate throughout all cultivar release years and had a greater difference in newer cultivars.  The low seeding rate had greater branching ability for newer cultivars while high seeding rate branching decreased slightly.  This suggests that newer cultivars under lower population are able to compensate yield by putting more seed on branches.