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Soybean Planting Considerations

May 1, 2003

Although southern Illinois is wet in places and parts of the rest of the state are getting wetter this week, corn planting is nearly completed in some places, and thoughts turn to soybean planting. Here are some considerations as we start planting soybean in Illinois:

Results from recent planting date studies clearly show that soybean yields are usually decreased by planting very early (before April 20 or so) but that soybean yields do not drop as fast as corn yields when planting is delayed into mid- or late May. Yield expectations hardly change at all with planting date over the first 3 weeks of May in the central and northern parts of the state, and during the whole month of May in the southern part of the state. Of course, we want to get planting completed within a reasonable time, but it probably does not pay to plant into wet or cold soils in the first week or even the first 2 weeks of May. If it's dry enough to plant, though, there is little reason to wait for soil temperature to rise if it's May and the forecast is for average or warmer temperatures to come.

The conventional wisdom is that soybean plants should have 6 weeks of good growing weather from emergence to first flower. First flower usually occurs sometime during the first 3 weeks of July, depending on location, variety, and temperatures. Planting by May 20 or 25 in northern Illinois and by early June in southern Illinois usually provides enough growth for good yield potential. Later-planted soybeans, such as double-cropped, develop faster in the warmer weather, but they often do not form the full canopy and vegetative growth (height and node number) needed for top yields unless flowering and vegetative growth are extended by favorable weather. Late-planted soybean should be planted in narrow rows and at higher populations to help compensate.

It helps to spread risk by planting some later or earlier varieties, but careful choice of yield and defensive traits within a maturity group is probably a better way to buffer against weather effects than going much outside the adapted maturity group for your area. In theory, later-maturing varieties should use more of the season and hence yield more. Some people also feel that early-maturing varieties can "beat the weather odds" and avoid late-season problems to yield more. Neither of these approaches is strongly supported by our research results; midseason, adapted, top-performing varieties are usually the best choice. Unless reason exists to expect unusual conditions, we would not suggest using varieties more than 0.2 or 0.3 maturity groups on either side of midseason, adapted maturities for a given area. For example, if adapted soybeans for an area average about MG 3.0, most varieties used should be between 2.8 and 3.3.

It is still dry in parts of northern Illinois, and if the current weather pattern holds, some parts of the state could be dry for soybean planting once corn planting is completed. Some producers will thus face a common dilemma: Do I plant at normal planting depth (1.5 to 2 inches) even if the soil there is too dry for soybean germination and emergence, or should I plant 3 or more inches deep, where there seems to be enough moisture for germination? This question is more common when we have warm, windy conditions that rapidly dry soils and where fields were fall-tilled and need to be worked in the spring; each trip over the field turns up more-moist soil and allows faster drying. We haven't had much weather like this so far, and if that trend continues for a few weeks, surface soil moisture might be adequate. In some areas, soil moisture may be inadequate to depths greater than 3 inches, and it will take rainfall to get the crop to emerge regardless of how deep we plant. In both of these cases, we should plant at normal depth. In general, soil moisture patterns are not uniform throughout a field, so some soybeans planted deep will often emerge in parts of the field and not in other parts. Heavy rainfall after deep planting can also deprive seeds of the oxygen they need to germinate and emerge, can provide more time and better conditions for disease attack before emergence, and can increase the risk of soil crusting. Except in sandy soils, I tend to think that deep planting carries more risk than reward in Illinois.

Row spacing is still an issue, with continued movement to 15-inch rows, either by converting from drills to planters or by adding splitters to wide-row planters (30 inches or wider) to cut row spacing in half. Unless planting is late or soils are drought prone, research results tell us not to expect yields to differ much with row spacing, from drilled (7 to 10 inches) up to 15- or 20-inch rows. With the proper variety and good growing conditions, 30-inch rows will sometimes yield as much as narrower rows. But on average, we would expect yields to drop as row width increases above 20 inches. This yield loss will vary widely over years, but it will probably average about a bushel of yield for each 4- or 5-inch increase in row width, with some acceleration in yield loss as row width rises above 30 inches. None of this means that producers who use 30- or even 36-inch rows are making a mistake; the size and condition of equipment and the numbers of acres available to pay for narrow-row equipment may support a decision to stick with wider rows. Some producers also prefer to cultivate, which gets difficult or impossible as row spacing is narrowed. Most weed control systems work well regardless of row spacing, so the need to cultivate for weed control is less of an issue today. But good yields are possible regardless of row spacing if the crop is managed well.

Seeding rate questions also arise, especially with increased seed costs. Although these figures vary considerably, we should probably expect about 85% of high-germination (90% or above) soybean seed to emerge to form plants when planted with properly adjusted row units, and 75 to 80% if drilled under good conditions using a conventional drill. Some newer drills are designed to operate more like planting units, and although the planting mechanisms on air seeders vary by brand and age, most place seed with metering and depth control somewhere between drills and planter units. Soybean seed is relatively fragile as seed goes, though, so wide swings in emergence are common. Most studies have shown that 120,000 to 150,000 plants per acre are adequate for top yield, with optimal population in wider rows typically less than those in narrower rows. The "plateau" for plant number is fairly wide: We don't expect yields to drop until plant numbers go above 200,000 to 250,000, and even then losses are often due to more disease pressure rather than to agronomic factors. But high plant populations waste seed, and they can lead to faster water loss, which can hurt yield prospects in dry years. Using the preceeding numbers, drilling 200,000 seeds per acre or planting 150,000 with row units is a reasonable seeding rate for most producers, though many will fine-tune these numbers based on equipment, conditions, and experience. For a handy seeding-rate calculator, check out the soybean chapter on the Illinois Agronomy Handbook Web site at D. Nafziger

Author: Emerson Nafziger

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