No. 5 Article 11/April 23, 2004

Corn Planting and Soybean Seeding Rate

The news this week is that an astounding 40% of the corn in Illinois was planted by April 18. Given that April 19 (Monday) was a good planting day over virtually the entire state and that many people report being finished with planting corn, it is likely that more than 50% was planted before rain on Tuesday, April 20 slowed progress in some (but not all) areas.

One of the recurring questions is whether or not to rework fields before planting if rainfall prevents planting after a field was tilled and ready to plant. We don't have current research on this, but we expect that the answer is difficult to know with certainty, even if research results were available. If the soil was in good shape and the rainfall was such that the surface structure of the soil is more or less unchanged, then tilling again before planting is probably not necessary. One of the main problems with this is that many people want to plant after the surface has dried but before the soil beneath the surface is dry enough to plant into. The result can be a lot of soil compaction, and even poor seed-soil contact if the soil doesn't "flow" well enough back into the planting furrow. In fact, the impulse to till soils before planting is often to aid in surface drying, but even in such cases, driving on soil that is too wet can damage the soil structure and may disguise seed placement problems more than prevent them. Whether you till or not, be sure to plant only when you can do a good job of seed placement, without excessive sidewall smearing and with good seed coverage. Tilled soils often dry more slowly because tillage breaks capillarity, making it harder to move water to the surface for evaporation.

Soybean planting has gotten under way in some areas, with 1% listed as having been planted by April 18. If current weather patterns hold, we should be able to finish up by mid-May in most areas. This is an improvement over the late planting of recent years; with the recent fall in soybean prices, the market seems to see this as a positive for production. As I indicated last week, our recent data show no dropoff in soybean yields until planting is delayed past mid-May. We also saw that later-maturing varieties were not favored by earlier planting in most cases. Given the photoperiod response in soybean, planting early does not translate directly into earlier maturity at the end of the season, but it does contribute; planting 10 days later delays maturity by about 3 days or so, though this varies some by actual planting date and by maturity of the variety.

The growing conditions of 2003 resulted in variable, but often small, soybean seed size for the 2004 growing season. While newer planters may allow better calibration of seeding rate by seed number rather than weight per acre, setting exact seed drop rates is still a challenge. The percentage of emergence and plant establishment also varies considerably with different planting conditions, crop residue amounts, soil temperature, and seed quality. Using 80% plant establishment for row units and 70% for drill units has worked for some people. Our research usually shows higher establishment percentages than this, so it may be helpful to view these as "safe" numbers, appropriate when emergence conditions are not very good. I would suggest that individual producers start to assess this a little closer for individual planters, and perhaps to raise these percentages if plant counts versus dropped seed numbers justify that.

Though seed number per pound listed on bags tends to be somewhat approximate, it should be accurate enough to establish dropped seed number per acre; simply multiply seeds per pound times pounds of seed used per acre. Take plant counts after emergence (waiting until about the 2-leaf stage to allow for loss of small, emerging plants); then divide plants per acre by number of seeds dropped per acre, and multiply by 100 to give emergence percentage. It is helpful to record general emergence conditions and days to emergence, as things that might influence emergence percentage. If you find that plant counts are higher than you really need, even under less than ideal emergence conditions, you might consider "dialing back" seeding rates in the future, especially when planting conditions are good. In fact, it can be useful to adjust dropped seed numbers based on current planting conditions. For example, if the soil temperature is above 60 and soil moisture is ideal, it may be appropriate to use 90%, as the expected establishment percentage, and to drop seeding rate accordingly.

How many soybean plants do we need per acre? This is an area where research findings often seem to make little sense in production fields, but that's mostly due to differences in approach to risk. That is, the researcher may have an inadequate concept of establishment risk and so might feel free to say that research shows that 100,000 plants per acre is adequate for maximum yields, then wonder why the producer still plants 180,000. Part of the problem is also that the 100,000 plants that the researcher establishes are usually distributed uniformly, often in small plots, while the producer has to plant whole, and often variable, fields. In addition, low plant stands usually look bad early in the season, only to expand out with extra space and yield as much as higher stands. That's especially true of drilled soybean plants, where looking down the row early in the season gives the "feel" of inadequate stands compared to looking down rows with higher plant counts per foot of row. We also have a tendency to worry about low plant counts and more weed problems, though glyphosate use has (or should have) diminished this concern.

Regardless of planting method or what the early stand looks like, it is clear that stand counts are often higher than they need to be. Given the fact that yield usually does not decrease until soybean plant stands exceed 250,000 or more plants per acre, especially in row widths less than 20 inches, it is doubtful that most stands are high enough to actually reduce yields. The unneeded plants do represent extra seed cost, however, and can increase lodging in some cases. In general, aiming for 150,000 established plants is a "generous" goal and one that should provide adequate stands even if emergence suffers, regardless of row spacing. That means dropping 150,000 divided by expected emergence percentage, including an adjustment for germination percentage. So, if we expect 85% establishment and 92% germination, we need to drop 150,000 divided by (0.85 times 0.92) = 192,000 seeds. Data show that 100,000 or even fewer plants will often maximize yields, so those who consider themselves "lucky" can drop seeding rates. On the other hand, dropping more than 225,000 seeds per acre is probably never justified; if seed quality or planting conditions are that poor, then planting ought to be delayed or different seed located.

For a handy soybean seed drop calculator, go to the "Soybean Seed Drop" decision aid at in the online Illinois Agronomy Handbook. This calculator asks the user to consider some of what we've talked about here, and also provides cost and number of units needed to plant individual fields.--Emerson D. Nafziger

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