Agronomy Talk

agronomy talk: SOYBEAN PLANTING DATE AND POPULATION

Published on Tuesday, March 10, 2020

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WHAT HAVE WE OBSERVED IN PFR?

As farming becomes more complex and time becomes more precious, farmers are searching for ways to increase revenue and manage input costs on every acre. This leads us to ask, what soybean management practices can save you time, make the most sense agronomically, and make you money? At our six PFR sites, we have observed that planting soybeans during that same optimal window for corn is ideal. In the areas where we do not currently have a PFR facility, we are relying on university data as we build our cooperator database.

Planting soybeans earlier can lead to earlier soybean flowering and additional node development (table above right). Earlier planting leads to more plant development earlier in the summer. This means achieving more yield potential with reproductive stages of growth beginning at a less stressful period. 

WHY EARLY?

Planting soybeans earlier in the season also results in earlier canopy coverage. This earlier canopy coverage leads to cooler soil temperatures throughout the season. On a given day in July, a 7 to 14°F difference in soil temperature could be noted when comparing the earliest and latest planting dates in Ohio. This temperature difference was taken within the canopy cover and further demonstrated how critical early canopy coverage was in maintaining soil temperature.

With nodules performing optimally at a temperature of approximately 72°F (Lynch and Smith, McGill Univ.), cooler soil temperatures lead to an agronomic advantage. More effi cient nodules can supply a greater amount of nitrogen (N). This is critical because each bushel of soybeans requires 4 to 5 lb. of N. The graph to the right demonstrates how closely yield (grey) and N content (green) align based on planting date. Earlier planting leads to greater N content in the tissue and greater yields. 

At earlier planting dates, farmers may be tempted to increase populations. However, data has shown that this is unnecessary. Yield potential is boosted by increasing the number of nodes per plant, pods per node, soybeans per pod, or the size of the soybean. Soybeans will naturally branch and grow toward sunlight if there is room to do so. Allowing soybeans to branch provides more room for pod formation on the plant (image below). By increasing populations, the plant's ability to branch is limited. Later planting and lighter soils will likely respond to higher planting populations.

While we don't recommend seeding rates as low as 100,000, we do believe seeding rates that exceed 150,000 are likely not needed when planting early and/or on more highly productive soils. Our date also indicates that lower stands planted early can produce high yields and may not need to be replanted depending on other factors such as uniformity of the stand and weed pressure in the field.

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Author: Jim Schwartz

Categories: Agronomy, Agronomy Talk

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