Planting Date and Maturity Group Analysis for the Upper Midwest to Maximize Soybean Seed Yield.

Soybean [Glycine max (L.) Merr.] planting date trends have steadily shifted earlier within the northern Corn Belt due to the higher yield potential associated with earlier planting. However, some believe that while producers are planting earlier and experiencing a longer growing season, they have not adequately adjusted their soybean maturity group’s (MG) to maximize solar radiation interception.  Coincident with earlier planting dates is the increased risk of a spring killing frost, drowned-out fields, and early season insects and seedling diseases that may result in sub-optimal stands and the need for replanting some years.  Proper replanting methods (fill-in) and optimal final plant stands (>247,000 plants ha^-1) have been determined but again, the proper MG to use in replant or essentially late planting scenarios to maximize yield and avoid fall frost damage is unclear. Therefore, this studies objective was to determine the optimal MG across various planting dates, which would maximize yield while reducing the risk of fall frost damage. Trials were conducted in 2014 and 2015 at four location of different latitudes spanning from southern to northern WI, totaling 8 site-years. Fourteen varieties (MG 00.5 – 2.5) were seeded at five planting dates targeted for May 1^st, May 20^th, May 31^st, June 10^th, and June 20^th.  Planting in early May maximized yield across all locations and years which then declined by 24.1 kg ha^-1 d^-1 on average when planting was delayed.  The yield decline from delayed planting was similar between years at Hancock (30.6 kg ha^-1 d^-1) and Spooner (15.4 kg ha^-1 d^-1) but differed at Arlington. Seven of ten planting dates within Arlington and Hancock showed significant yield increases for the longest MG planted, while this was not the case at Spooner.  In conclusion, early planting is a proven management practice to increase yield and is potentially more specific to the location, than year; while planting the longest possible MG is a cost effect method to more often increase yields with early and delayed planting up to June 10^th except at the northern most latitude in this study.