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Sudip Poudel, Graduate Research Assistant

Dr. Anuj Chiluwal, Assistant Professor of Agronomy, Kentucky State University

Do you know why farmers say, “August and September make a bean”? This old adage reflects a critical truth.

Among the different growth stages of soybean, the R5 stage to R7 stage is the seed-filling period, which occurs mostly from August to the end of September. These are considered critical stages when pods and seeds develop, which determine the final yield. During these stages, soybean plants direct most of their energy and nutrients into seed development, and any stresses at this critical time whether from drought, disease, or nutrient shortage can significantly reduce yields.

This stage marks the completion of pod development and the beginning of seed formation. When the seeds reach approximately sim3 mm (1/8 inch) in length, the plant enters the R5 stage, also known as the beginning of the seed-filling period. From this point, seed filling accelerates rapidly, while root growth begins to slow. As seeds develop, the plant shifts its energy and resources (photoassimilates) toward seed development, resulting in increased dry matter accumulation in the seeds.

This stage is also sensitive to diseases. In warm and moist environments, the chance of disease infestation increases. For example, Soybean rust (causes defoliation during seed filling, leading to yield loss), Leaf spot (reduces the photosynthetic area, directly affecting seed development), Cercospora Leaf Blight (causes purple pigmentation on the upper leaf surface and the seed coat, reducing seed quality), etc. can increase during this stage. Similarly, white mold and sudden death syndrome (SDS) begin to show up at this stage, causing premature wilting and death of upper plant parts. Hence, this stage is an ideal time to assess the severity of these diseases.

During the R5 to R6 stages, soybean plants undergo intense physiological activity to fill seeds, which then slows down as the seeds reach full maturity. This active seed filling phase lasts for about 2 to 3 weeks. Several important processes undergoes during this period. There is rapid dry matter accumulation; once seeds begin forming at R5, the plant's dry weight accumulation transfers largely to the seeds. In the weeks following R5 stage until around the 3^{rd} or 4^{th} week, nitrogen uptake by the roots and existing nitrogen in vegetative tissue begin rapid remobilization to the seeds.

Essentially, the plant shifts entirely from building structure to seed filling during the R5 to R6 stages. During this stage (R6), i.e., by around 3rd week after the R5 stage, the soybean plant reaches its maximum height, node number, and leaf area.

Canopy and Root growth: The canopy reaches its full size, maximizing light interception for photosynthesis. Vegetative growth slows to a stop on indeterminate varieties; the last new flowers and pods at the top of the plant appear by the R5 stage. Root growth also changes. Primary and lateral roots grow vigorously up to the R5 stage, but after the R5 stage, the shallow roots begin to senescence while deeper roots continue growing until roughly R6.5, i.e., R5+3^{rd} to 4^{th} week. By about R6.5, root growth is completed.

What changes can be seen in soybean pods and seeds as they mature?

▶ Pods and seeds gradually change color and moisture content.

▶ At R6 (full seed), the pods are green, and the seeds moisture is ~70-75%.

▶ By R6.5, seeds have accumulated most of their dry weight, and moisture drops to sim60-65%.

▶ By R7 (beginning maturity), pods turn yellow-brown and seeds have <50% moisture (shows physiological maturity). At this stage, seeds have lost much of their moisture, and pods turn brown.

▶ After R7 stage to R8 stage, seeds continue to dry down (to~13% for harvest), seed dry weight reaches its maximum.

At the R8 stage, marked by dry pods and mature color, indicates full maturity. At this point, no additional dry weight is added.

▶ After the R8 stage, with the pods and stems fully dry, the plant is ready for harvest.

Indeed, this farmer's old adage provides wisdom on how to be careful in seed filling months, August and September, which coincide with the mid-seed filling phase, during which the final seed size is determined. As the plant reaches peak physiological and structural development, different practices that enhance the assimilate supply – maintaining green leaf area, avoiding defoliation, and supplementing nutrients – can extend seed filling, improve seed composition, and maximize soybean yield potential. Farmers can make timely, yield increasing decision by understanding these dynamics.

Remember, what you do in August and September shapes the soybean harvest in October/November.