You have dreamed about it for years, but you may have given up seeing 100 bushel soybean yields in your farming career. But maybe you can actually achieve it with a plan. Sure genetics help, but what about all of the agronomic factors that you may be able to control?
Agronomic issues are important to soybeans, such as timing of canopy, density of the stand, and retention of flowers. Those are some of the issues addressed by University of Missouri soybean specialist Bill Wiebold, who is familiar with record soybean yields produced by MO farmer Kip Cullers. In a two part session about 30 minutes each, Wiebold explores those concepts you can create on your farm to join the 100 bushel club.
In the first program, Wiebold says yield is determined by several factors. During the first 70% of the life of the soybean plant the number of nodes on stems will be determined. The middle 30% will help determine the number of nodes on branches. During that same time the number of flowers and successful pollination will be determined, then later will come flower and pod abortion. The number of seeds per pod is determined in a 10% segment, and the last 30% of the life time will determine seed size. Wiebold says the vegetative and reproductive stages of soybeans are complex and many factors are happening at the same time, unlike corn which proceeds from one stage to another.
Most of the yield is produced in the upper to middle third of the canopy, but light is captured in the upper part of the canopy. Light drives photosynthesis and that drives yield. Yields at the top of the canopy are driven by the light that is absorbed in that area. Sugars are produced by photosynthesis and feed the pods in that area, not pods that are in nodes further down the stem. The focus of researchers is to increase the number of nodes on the plant and the number of pods that are in the node. Wiebold says the number of nodes on stems does not respond to the density of the stand, but the number of branch nodes decreases as the density of the stand increases. He says a denser stand will cause plants to grow taller, but not increase nodes and pods. A stand that is less dense will have plants that have more branching and more nodes on the branches.
Each node will produce up to 3 racemes and each will have 2 to 12 flowers. All flowers are fertilized. 65% to 85% of them will be aborted, with only a few left on the plant to produced yield. The greatest amount of pod loss will be in the bottom part of the canopy. At a node, once pods begin to grow, the other flowers and pods begin to abort, and the pod abortion will increase if the plant is under stress. Older pods will produce hormones that influence pod abortion and will steal nutrients from the younger pods. The third youngest pod will have a 40% abortion rate, and the youngest will almost always abort. More understanding is needed about the loss of the pods to be able to keep them and have them contributed to yield.
In the second program Wiebold discusses actions that can be taken to reach the 100 bushel mark. However, he says it will take a lot of effort in time and land ability, and it must be worth that investment. One of the first decisions is selection of a variety that is already high yield. The second is to ensure that the greatest amount of light is available to the soybean plant at the right time, and that requires an early planting date. Since the greatest amount of light will be available about June 21st, an early planting date will move the pod filling time period earlier when more light is available. Grain fill is sensitive to light capture.
Wiebold says the greatest yields occur when the planting date is about April 24 for Central Missouri. When the planting date moves to May 22 the yield will decline by 4%, and then decrease rapidly as the planting date moves toward June 20, with a 28% loss from the potential. Another change you can make that increases light capture is row spacing. Wiebold says beans in 7.5 in rows will capture more light earlier in the growing season. Any light that reaches the soil will be lost to the photosynthetic process. This concept will probably increase in importance the further north you farm. The concept is also important for cases where there is late planting or poor growth for the crop.
For beans planted on May 15, a 15 in. row had a 1% decrease in yield from a 30 in. row. On June 24, the 15 in. row had a 3.5% yield advantage to 30 in. row soybeans, which may mean double cropped beans may yield better in 15 in. rows. On a July 20 planting date, the yields were only in the 20 bu. range, but the 15 in. rows had a 17% better yield than the 30 in. rows. That data from 2009 was based on a 150,000 seed per acre planting rate. In 2010, the 15 in. rows out yielded 30 in. rows by 17% for May 28 planting, by 23% for a June 29 planting, and by 35% for a July 16 planting.
Another factor that is determined by sunlight is seeding rate. Wiebold says the seeding rate must be held constant throughout the field, although some areas in the field will have lesser emergence because of soil issues. He says some farmers may be quite surprised with the variation in emergence throughout a given field. Yield is lost when the density is insufficient, but yield is not lost when the density is too high. However, there will be a lesser return to seed at the higher planting rates which do not provide a yield response. Emergence rate improves with later planting dates, because early planting may provide temperature or moisture stress on the seed. So Wiebold says that makes it more important for early planted beans to be protected with a seed treatment.
Another consideration is the need to protect the leaves in the upper one half of the canopy from insects and disease, which requires constant scouting for such pests. That will help protection of the leaves, which are needed as solar collectors. The soil itself must be treated as if it were alive, because Wiebold says 40% of the soybean biomass is in the soil and depends on it, which means issues like compaction should be avoided and corrected.
Summary:Soybean yields can reach beyond 100 bushels per acre, but take careful management. Those management issues primarily include capturing as much light as possible at the times blooms and pod are in jeopardy of abortion and at the time of seed fill for the retained pods. Other factors include seeding rate to ensure that lesser fertile areas of a field will not be penalized, as well as row spacing to ensure as much light as possible reaches leaves and not the soil. Our thanks go to the Plant Management Network for making the programs available.
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