Weather challenge compact soil and emerging corn

Editor’s note: The following was written by Emerson Nafziger, with the University of Illinois Department of Crop Sciences, for the university’s crop development Bulletin.

With less than half the corn crop and less than a fourth of the soybean crop planted by June 1 this year, farmers are facing difficult decisions.

Q: Should we open soils by tilling them when they’re wet so the surface dries faster?

A: The capillary rise of water from deeper layers to the surface is disrupted by surface tillage. That means slower water movement to the surface and faster drying of surface soil. That also means that deeper layers stay wetter.

Although it may make soils look drier on the surface, tillage of already-tilled or deep soils is unlikely to be beneficial.

Q: How do I decide whether to take prevented planting or to plant the crop even if it’s late?

A: A recent paper from Iowa State University reported yield losses of about

  • 25% or 55 bushels for corn planted June 10
  • 40% or 88 bushels for corn planted June 20
  • 61% or 133 bushels for corn planted June 30

Therefore, planting corn on the last insurable date of June 25 would be expected to produce about half the normal yield. There is some anecdotal evidence that yields could exceed those numbers, but for now that’s the best guess we have.

Yield declines should be less steep for soybeans than for corn when planting later in June. Soybean yields may reach 50% of early-planted yields by early July, at least in south-central and southern Illinois. Yields of soybeans planted in late June or later in central and northern Illinois are strongly affected by growing-season weather and so are expected to vary widely. Soybeans planted after mid- or late June in northern Illinois may not mature before frost.

Q: Should I change corn hybrids or soybean varieties to those with earlier maturity?

A: Maybe. According to predictions based on growing-degree days left in the season after a given planting date, some hybrids might need to be exchanged for shorter-maturing varieties. But planting a hybrid late usually lessens its growing-degree-days requirement.

Because soybean flowering responds to photoperiod, varieties across the range of commonly used maturities will flower at about the same time if planted in the second half of June. Therefore there is little need to change soybean varieties. But north of Interstate 80 in Illinois, producers should plant varieties no later than MG 2.8 or 2.9 if planting is delayed past mid-June.

If July temperatures are normal, that might prove to be unnecessary. But cool weather in July — especially at night — could delay flowering to late July. That could substantially delay maturity and push seed filling into less favorable conditions.

Q: What do we do about nitrogen fertilizer for corn?

A: That’s one of the really difficult challenges with such a wet May following a wet fall — with limited nitrogen application and limited ability to apply nitrogen this spring.

For fields that have been very wet, it’s likely some of the nitrogen applied this past fall or in early spring has moved down in the soil. Some may have left the field in tile drainage water. If soils dry in June, that movement will slow. As plants begin to grow and take up nitrogen, some nitrogen may move back to the roots from deeper in the soil.

We should wait until we see crop color and growth before deciding to add additional nitrogen, if the full amount has already been applied.

If June is as wet as May, producers may want to go back with another 30 to 50 pounds nitrogen in the two or three weeks before tasseling — especially if growth and yield potential are good. The crop itself is a better indicator of its nitrogen status than soil or tissue samples. Having the crop take on a dark-green color in mid-vegetative stages tends to indicate that it doesn’t need more nitrogen.