The death of a soybean plant follows a pre-determined path. You first notice a few yellow leaves on plants. Then several more yellow leaves follow with the leaves rapidly falling off the plant. At this time if you walk through a soybean field if you touch the petiole still attached to the stem it will break off the plant with just a light touch. Stems go next.
The plant is simply digesting all available starch and minerals remaining in tissue in order to maximize grain fill. When we hear complaints of green stem in beans at harvest this typically is associated with higher grain yield as the soil was able to mineralize more nutrition late season and provide beans a more peaceful end.
Later this summer, soybeans in their early reproductive stages will similarly march toward
self-destruction, believing the short-term goal of filling seeds is aligned with your broader production targets. Soybeans will sacrifice roots and abandon nodules at late V5 and V6, cutting off nitrogen supply. Leaves, unable to maintain protein, will lose photosynthetic capabilities and, in turn, be cannibalized to fill beans. Yields will retreat from what they could have been if they had better command of the plant’s resources.
Plant development must include a balance of resource allocations. Some resources should be used for immediate needs, while others maintained for future plant needs.
In the soybean plant at R3, a balancing act should occur with sugar. The plant has an immediate need to develop pods and nourish developing seeds. At the same time, the plant must invest in roots and nodules for nutrient uptake and nitrogen fixation to produce new leaves that make sugar to fill future pods and seeds.
However, too often at R3, a soybean plant fills the first seeds and invests in the future. It doesn’t maintain roots. It doesn’t make new leaves. It raids nutrients from existing leaves to move them to seeds. Early-setting seeds, seize all the sugar they can get, then release a barrage of hormones that force pods to abort.
A soybean plant exemplifies this poor strategy for two reasons. As a legume that requires far more nitrogen than corn for grain fill, it’s decision to prematurely stop support for the roots and nodules that supply this nitrogen has a dramatic effect on crop yield and quality. Secondly, soybean yield is not determined early, like corn. Late-season behavior continues to affect seed number and yield.
What can be done about it? As growers we may better command these processes with soybean finisher products that improve crop growth and seed production. Unlike many other yield-improving practices, these new technologies are deployed later in the season, instead of being crammed in with other early-season applications.
Our most fundamental tactic is to ensure soybeans have adequate nutrition. Potassium, manganese and boron are critical in maintaining leaf tissue and the adequate movement of sugars throughout the plants. Micronutrients are best fed through the leaf; in dry soils, foliar potassium is important.
Supplemental nitrogen can be used and may increase the amount of nitrogen as protein harvested with the crop. Beans use about 6# of N for each bushel produced. However, mid to late season N applications have been highly inconsistent providing return on investment.
Plant growth regulator gibberellin will help facilitate sugar movement to roots, while auxin and salicylic acid (aspirin) will suppress production of ethylene – a gas that triggers plant stress responses, including leaf senescence (death). Properly timed foliar fungicides have also demonstrated ability to reduce ethylene senescence as well.
In the future, the solution to soybeans that mature too soon may be to “apply two aspirins and call me when your bin is full.”
Progressive Crop Technology offers a late-season nutrient product, PCT Soybean Finisher to consider.