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Late-season weather impacts corn and soybean growth and development

By Kyle Poling, Pioneer Field Agronomist, Ada, Ohio

Physiological maturity of a soybean seed occurs when the seed has completely lost all green color and turns yellow. At this point grain moisture is still over 50%, but a harvestable moisture of near 13% can be reached in as little as two weeks under good drying conditions. In order to time harvest perfectly, it is necessary to monitor soybean drying very closely. At full maturity (R8), 95% of pods have reached their mature pod color. At the R8 growth stage, only five to 10 good drying days are needed before harvest. Begin checking grain moisture before all the leaves have dropped off all the plants as various stresses can cause soybeans to retain some leaves. It is not uncommon to see a few green leaves and stems on some plants after the pods are fully ripe and the soybeans are dry enough for harvest.

Soybeans should be harvested the first time they reach 13% to 14% moisture. Moisture above 13% will incur a price discount, but moisture below 13% results in less weight at the elevator. What is the cost in lost yield from shrink when soybean moisture levels get down to 10% or less? Delivering soybeans at 12% moisture is a 1.14% yield loss; at 10% moisture there is a 3.3% yield reduction. If a field of soybeans yields 60 bushels per acre at 13% moisture, but if harvest moisture is at 10% moisture than the amount of bushels sold will be reduced by 2.3 bushels per acre from just the moisture shrink. Additionally, when harvest is delayed, a number of potential losses may occur, including increased tendency to shatter.

Several factors affect the rate at which crops develop — photoperiod, heat, moisture, and fertility. Heat and photoperiod are the two primary factors influencing soybean maturity. Soybeans are considered short-day plants, meaning that physiological development is accelerated as daylength shortens. However, the rate of maturity is sped up by hotter temperatures and slowed down by cooler weather. Soybeans can compensate for stresses/shortcomings that occur during early to mid-reproduction provided ample sunlight, adequate temperatures, and soil moisture is available. Favorable late-season temperatures (not too hot) and rainfall during late stages of development (R5 to R6) can create larger seed weight by extending the seed fill duration.

For corn, the reproductive stages last for approximately 65 days. Nearly half of this time during reproduction is spent in the dent stage (R5). At the beginning of R5, the kernel has accumulated about 45% of its total dry weight. Kernels will begin to “dent” as the soft dough at the top of the kernel begins to be convert into a solid starch. A visible “milk line” on the kernel marks the progression of the solid starch formation as it moves from the kernel cap towards the kernel tip.

Stress during the R5 growth stage can reduce the time for additional starch accumulation thereby reducing yield potential. At the beginning of dent stage approximately 60% of yield potential has been reached. At growth stage R5.5 (50% kernel milk) nearly 90% of a corn plant’s yield is final. Visual signs of stress that are speeding crop maturation and negatively impacting yield include leaf firing, ears prematurely tipping over, or plant top die-back from anthracnose stalk rot. Physiological maturity, also known as black layer (R6), is complete when an abscission layer forms at the base of the kernel eliminating further dry matter accumulation.

Kernel drying following black layer is entirely due to evaporative moisture loss. In standing corn, kernel moisture loss requires approximately 30 GDUs to remove 1% of grain moisture between 30 and 25% moisture. As corn continues to dry down, approximately 45 GDUs are needed to remove each moisture point between 25% to 20% moisture. When weather conditions are warm and dry, corn may lose 1% of grain moisture per day, but during periods of cool and/or wet weather moisture loss may be minimal. Hybrid characteristics such as husk leaf coverage, husk leaf senescence, ear angle, and kernel pericarp thickness can affect drydown. Corn that matures earlier will dry down faster due to more favorable drying conditions early in the harvest season. Later maturing corn has fewer warm days to aid in drying thus will dry down at a slower rate.


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