Effect of Field Weathering on the Viability and Vigor of Soybean Seed

The quality of soybean [Glycine max (L.) Merr.] seed at harvest is dependent on the field production environment during development, maturation, and storage on the plant. These investigations were conducted to evaluate the effects of field weathering after physiological maturity (maximum dry seed weight) on seed viability and vigor. Five production environments were compared for two cultivars (‘Cutler 71’ - 1973, ‘Kent’ and Cutler 71 - 1974 and 1975). Seed was hand-harvested at physiological maturity, harvest maturity (first time the seed dried to less than 14% moisture content), and at regular intervals for up to 3 months after harvest maturity. All seed were evaluated for viability (standard germination test) and vigor (accelerated-aging germination test). Physiological maturity occurred at a seed moisture content of approximately 55% for both cultivars. The time interval from physiological maturity to harvest maturity (desiccation period) ranged from 10 to 20 days and was closely related (R² = 0.88) to the mean open pan evaporation during the period. Seed viability and vigor were highest at physiological maturity and remained high (>80% germination) until harvest maturity for all but one production environment. Viability was maintained at these high levels for 1 to 2 months following harvest maturity for four of five production environments. However, seed vigor declined rapidly, reaching levels that were significantly (α = 0.05) less than those at harvest maturity within 4 to 39 days after harvest maturity. In four of the production environments, seed vigor reached a level of less than 50% germination within 1 month of harvest maturity. The time required for seed vigor to significantly decline following harvest maturity was closely related to mean air temperature (R² = 0.90), mean minimum relative humidity (R² = 0.94), and precipitation per day (R² = 0.75). The wide range in seed vigor observed shortly after harvest maturity emphasizes the importance of timely harvest of seed fields and evaluation of seed viability and vigor soon after harvest.