Soybean Maturity Group Response to Planting Date in Iowa.

In the past, farmers have selected the longest season soybean [Glycine max (Merr.)] maturity group (MG) that would reach maturity before the first killing freeze. However, observation of farmer practices today suggests that shorter season MGs are planted at locations where longer season maturity groups could safely be planted. The objective was to determine the optimal MG for locations in northern, central, and southern Iowa when planted at three dates. Experiments were conducted during 2007 and 2008. Averaged across two years at the northern location MG II soybean produced the greatest yield at the early May planting date but produced similar yields to MG I at mid-May and early June planting dates. At the central and southern locations, MG III cultivars produced 8% greater yield compared with MG II cultivars when planted in the middle of May. When planting was delayed into June MGs III and II produced a similar yield. Planting MGs III or II at the central or southern Iowa locations in the middle of May resulted in 19% greater yield than MG I. Common to all locations, MG 0 cultivars did not yield competitively to the longer season MGs and as planting was delayed the yield difference among MGs decreased. Greatest yield potential can be achieved by selecting long MG II and short MG III cultivars for the southern two-thirds of Iowa while the northern third should select early MG II cultivars, regardless of planting date. This experiment will be conducted again in 2009.