Chemical composition and physical damage of soybeans [Glycine max (L.) Merill] as affected by cultivar, harvest date, and field weathering

Changes in the composition of soybeans left in the field after maturity were made to provide information that would permit management decisions by the growers with respect to timely harvesting for optimum price and end-use value. Specifically, the investigation was conducted to identify the effects of field exposure of soybeans after maturity on the chemical composition and physical damage, especially as related to atmospheric changes.

The soybean cultivars 'TN4-86' (maturity group IV), 'Essex' (maturity group V) and 'Leflore' (maturity group VI) were investigated across sixteen weeks of harvest following physiological maturity. When the beans were left unharvested in the field after harvest maturity, the effects of field weathering resulted in increased bean moisture. Repeated exposure of the mature beans to alternate wetting and drying after full natural desiccation caused the seed coat to develop cracks, wrinkles and splits. The damaged seed coat enhanced moisture entry into the beans as a result of elevated and uniform relative humidity, rainfall and above-freezing temperatures. The most striking effect of field weathering was evidenced by the apparent increase in percentage values of protein and oil due to reduction in soluble carbohydrates accompanied by a reduction in dry matter. However, the protein, oil and ash did not change. The percentage increased because, as dry matter reduced with field exposure, more beans were required to provide the same sample weight as compared to the undamaged sample before weathering. Another significant effect was the increase of the free fatty acid component of the total oil which lowered the end-use value.

The comparison of cultivars showed that TN4-86 and Essex possessed higher percentage of protein, oil, free fatty acids and total damaged beans than Leflore. Free fatty acid content of TN4-86 and Essex increased substantially more than that of Leflore due to extended field exposure under high bean moisture conditions.