Evaluating Pre- and Post-Harvest Effects on Soybean Carbohydrate Content.

Mechanisms that influence soybean seed carbohydrates such sucrose, which is readily digested by monogastric animals, are not well understood. It is likely that seed sucrose levels are affected by conditions at the very last stages of seed filling, prior to physiological maturity. However, it is also possible that postharvest storage conditions could affect this respiration-susceptible constituent. A 3-yr (2013-2015) experiment was conducted at the University of Minnesota in St. Paul, MN. Our objectives were to investigate how seed carbohydrates, especially sucrose, are affected by maturity, environmental conditions during seed fill, and postharvest storage conditions. The experimental design was a randomized complete block design with four replications and a split-split-split plot structure, where planting date (May, June, July) was the main plot, soybean variety (early, mid, late season) was the subplot, and sample date (at seed growth stages R6.5, R7.0, and R7.5) was the sub-subplot. Seed yield and seed size was determined on each sample, which subsequently were split into four postharvest treatments: i) freezing; ii) air-dry immediately at 60°C; or iii) at 25°C; and iv) accelerated aging treatment (35°C, 30% RH). Seed quality analysis included analytical chemical analysis of proximates and sugar profile (sucrose, raffinose, and stachyose). The maturity group of tested soybeans did not have a direct impact on the effect of temperature on seed quality, except that short-season soybeans tended to mature early under warmer conditions. It appears that by the time the seeds reach maturity, sucrose and oil concentrations are “set”, and postharvest treatments do not affect those concentrations, except when harvest occurred at a normal time, slow air-drying at 25°C of R6 seed showed loss of sucrose and an increase in oil. These preliminary results indicate seed-filling temperatures affect sucrose concentration, with warmer seed-filling temperatures producing lower sucrose seed.