Soybean Seed Yield and Yield Components as affected by Planting Date and Maturity Group for Oklahoma Production Alexandre S. Barreiro and Chad B. Godsey Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK. Soybean (Glycine max) is normally planted between early April and early May using maturity groups (MG) III and IV, or from May through June using MG IV and V in Oklahoma. Planting date (PD) and MG can greatly impact yield components and consequently seed yield. Our objectives were to determine the effects of PD and MG on yields in an effort to assist Oklahoma soybean producers in making better decisions in choosing the correct MG for a specific planting period. Five MGs ranging between 3.8 and 5.6 were sown at five PDs between early April and July in Stillwater, OK, in 2009. Split Plot Design was used as the experimental design, with 3 replications, PD as the main plot and MG cultivar as the sub plot. Soybean yield components were assessed and correlated to seed yield. Planting in April and early May, MG IV cultivars produced higher yields compared to the MG 3.8 cultivar, mainly due to a lower number of reproductive nodes per m2. No difference was found in seed yield among the MG IV and V cultivars planted between late May and June. However, when averaged across MG, yield decreased significantly with later planting dates after late May, losing an average of 160 kg ha-1 per week. The decrease in yield can be attributed to a lower number of reproductive nodes per m2 and a lower number of pods per m2. For the 2010 growing season, five MGs were sown at six PDs in Stillwater, OK, and four of these MGs were sown in the same PDs in Lahoma, OK. For Stillwater, late MG IV and MG V cultivars produced higher yields compared to MG 3.8 and early MG IV cultivars when planted from April to late May. For Lahoma study, interaction between MG and PD was observed for MGs 4.4, 4.8 and 5.6, which presented greater yields when planted in May. MG 5.2 had no interaction among these PDs. For both locations, no difference in yield was observed among MGs, however yield decreased significantly with later planting dates after middle June PDs, losing an average of 235 kg ha-1 per week in Stillwater and 190 kg ha-1 in Lahoma. Number of reproductive nodes per m2 was again the yield component which most affected seed yield. Choosing the correct MG according to the PD, a wide planting period can be used for soybean production in Oklahoma.