Optimizing Soybean Seeding Rate for Yield Across Early to Late Sowing Dates in a Humid Subtropical Environment

Soybean grain yield across extended planting calendars depends on sowing date, cultivar maturity group, and plant population. We evaluated BRS 1061 IPRO (MG 6.1) and DM 66i68 IPRO (MG 6.6) sown from September to January in 2021/2022 and 2022/2023 in Londrina, Brazil, at 200,000, 280,000, 360,000, and 440,000 viable seeds ha−1 in a split-plot randomised complete block design. Grain yield, final stand, and yield components were analysed using mixed-model ANOVA, mean comparisons, regressions, correlations, principal component analysis, clustering, and descriptive permutation-based predictor ranking. Sowing date produced the largest performance gradient: grain yield ranged from 314 to 5771 kg ha−1, and January sowings were consistently low yielding. Final stands ranged from approximately 1.6 × 105 to 4.2 × 105 plants ha−1. Grain yield was strongly associated with grains m−2 (r = 0.92), while PC1 and PC2 explained 71.3% of multivariate variation. Higher seeding rates did not provide a uniform advantage; intermediate rates were beneficial only in specific sowing-date × cultivar contexts, whereas 440,000 seeds ha−1 sometimes reduced early-sowing yield. Seeding-rate decisions should, therefore, be cultivar- and date-specific, and January sowing is not recommended in this environment.