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April brings planting time considerations
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April brings planting time considerations

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Editor’s note: The following was written by Emerson Nafziger with the University of Illinois Department of Crop Sciences for the university’s farmdoc Crop Central website March 23.

A cold and wet March in many areas gave us time to think about issues related to planting and early-season management.

Should we start planting soybeans or corn first?

The forecast is for warmer and a little drier conditions moving into April. If that holds, it makes little difference which crop gets planted first — the decision can simply be based on which fields are ready first.

If it stays warm without heavy rainfall after planting, both crops will get off to a good start and benefit equally. Prolonged cool temperatures following very early planting of both corn and soybeans can limit yield potential, although this is rare. If the forecast changes to cooler and wetter, neither crop will have much advantage: Corn emerges a little better than soybeans under such conditions, but this is offset by the need to have a higher percentage of corn seeds emerge.

Planting date responses measured as percentage of maximum yield are similar for both crops, and as we have seen in recent years, even late-planted crops can yield well if the growing season is favorable.

Even though planting early helps crops get off to a good start if the weather cooperates, planting date has less influence on yields than the weather during the warm months of the growing season.

Does automated depth or down-pressure adjustment improve stands and yields?

For fields that have produced excellent stands and high yields without these features, probably not. In Illinois, it is rare that loam-textured or heavier soils are dry enough at planting to limit germination of corn or soybean seed planted at normal depths (1.25 to 2 inches).

Planting deep enough to place seeds into sensed moisture — a feature of some systems — sounds promising, but the advantage of doing this compared to uniform depth placement is not clear, especially if the topsoil has moisture, which is often the case. Most planting depth studies show that planting 3 inches deep lowers yield, and if an automated system can and does place seed that deep, it could cause harm.

Down-pressure adjustment capability assists in getting proper planting depth, and also in controlling how much “row compaction” there is. It may have value for no-till planting into dry soils, but perhaps less value for planting into tilled seedbeds.

The most important planting objectives include getting seeds placed at proper depth, reasonable spacing uniformity, and good seed-soil contact to help water move into the seed for germination. Most modern, well-maintained planters do this very well in most soils, without add-ons.

Have optimum seeding rates changed?

Corn plant populations have not continued to increase as much as many of us once thought they would. In large part that’s because hybrids have added more yield per plant, without needing more plants.

In studies, 34,000 to 38,000 plants per acre often maximizes yield. Planting 42,000 to 45,000 seeds usually does not lower yields compared to planting 36,000 to 40,000, but the added cost of seed is not covered by the average increase in yield.

Variable-rate seeding, with more plants in more productive areas of a field, makes sense, but it needs to be done without lowering the population in any part of the field to less than uniform planting. Returns from VR seeding will normally be modest in fields with normal uniformity.

Soybean seeding rates have changed in recent decades, to lower rather than higher rates. That has decreased the number of fields planted at 175,000 to 200,000 seeds, which had helped assure enough plants when seed emergence was less than it is now, and with less-capable planting equipment. Aiming for final stands of 110,000 to 120,000, using seeding rates adjusted for conditions and equipment, is a sound approach.

It is difficult to find data to support variable-rate seeding for soybeans, and we do not find much relationship, either positive or negative, between yield and optimum seeding rate.

Rather than productivity level, it might make sense to vary soybean seeding rate based on field conditions, with the goal on getting adequate stands in all parts of the field.

What’s the best way to manage cover crops?

Letting cereal rye cover crop grow to take up more N before termination conflicts with getting soybeans planted early. It may work to let the cereal rye grow for a while after planting soybeans green, but that can be tricky — cereal rye roots actively take up water and nutrients, and while they can help dry out wet soils, they can also take up water and possibly N that soybean plants may need for early growth.

Despite claims that corn can also be planted green into cereal rye at or before the time the rye is terminated, doing this can result in reduced early growth of corn plants and lower yields. Part of this effect is from competition for N, and applying N near the row with the planter can help alleviate this, but may not eliminate it.

It also helps to have corn seeds in a zone without active rye roots, either by killing the rye some weeks before planting, or by strip-till to move the roots out of the planting strip.

The safest practice is to kill rye far enough ahead of planting so there is no green tissue by the time corn is planted.

How should N be applied?

Starter fertilizer with N, N + P, or N + S can help assure access to nutrients that corn plants need as they start to grow. This is helpful especially if planting is into cool soil, where N mineralization is slow.

Any of the different ways to apply N with the planter can work.

Placement into the seed furrow directly with no more than 4 to 5 gallons of UAN solution is normally safe. More can be applied if it’s away from the seed, but applying 15 to 30 lbs. N (5 to 10 gallons of 28-0-0) with the planter should be enough.

It is unknown if starter fertilizer helps soybean plants get off to a better start, but it probably does not do so in most soils in Illinois. Some large yield increases for broadcasting stabilized urea on the surface at planting in light-textured soils have been seen, and in these cases a visible response to N was seen. Anyone with soybeans planted in such soil (loam or sandy loam) might want to spread some urea (at about 2 lbs. of urea per 1,000 square feet) in a small area after planting to see if it produces a visible response.

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