Optimal Soybean Plant Populations in Indiana.

A large number of decisions are made by the farmer to optimize yield and profitability in soybean [Glycine max (L.) Merr] production including variety selection and seeding rates. Seed companies and crop professionals provide target seed rates for the respective varieties based on growth habit (e.g., short or tall, bush or slender), soil types (e.g., sand, loam, or clay), and field layers (topography, organic matter, yield history). Precision agriculture technology (software to hardware) has allowed the development of prescription maps to vary the seed rates throughout any field with or without agronomically just cause. Our first objective was to determine optimal plant population of soybean across Indiana using farmer and field-scale experiments. These experiments started in 2010 and continued until 2014. Our most recent objective was to determine optimal plant population for different growth habit or plant types of soybean. In 2015 and 2016, two varieties were factored across seeding rates in field-scale trials located in NW, NE, and SE Indiana. Plot dimensions were 9.15 m wide, and minimum of 100 m long. Commercial size grain drill (19-cm row) was used at each site. Planted varieties ranged from maturity group 2.7 to 3.9 to be appropriate for the site. Target seeding rate ranged from 125,000 to 675,000 seeds ha^-1 with 5 or 6 increments based on field size. The range of maximum yields was 2.75 to 5.04 Mg ha^-1 across site-years. Relative yield levels were 88 to 97% across site-years regardless of the wide range of seed rates. The 95% yield potential was achieved at 284000, 269200, and 232200 seeds ha^-1 at NW, NE, and SE Indiana sites, respectively. Target seeding rates will change based on equipment (grain drill vs. narrow row planter), but target plant populations to optimize yield and profitability will be the same (~250000 plants ha^-1). This multi-year and multi sites study showed similar relative yield response in wide range of seeding rates, regardless of varieties’ maturity group.