Heavy rains and winds struck some sections of Iowa the past several days and nights, creating problems for corn and soybean crops. Brian Lang, Iowa State University Extension field agronomist in northeast Iowa, and Virgil Schmitt, ISU Extension agronomist in southeast Iowa, offer the following information and recommendations. Mark Licht, ISU's central Iowa agronomist, also offers observations as to what's happening in his part of the state.
What is the stand loss for corn to justify replanting?
A current corn stand (as of May 20) would need to drop below 20,000 plants per acre (assuming uniform and healthy plants) to justify replanting, says ISU's Brian Lang. Numerous gaps of up to 4 to 6 feet can reduce yields by an additional 5% to 6%. And this assumes the replant could be accomplished this week. If the replant opportunity was delayed another 10 days, then a current stand of 15,000 plants per acre at that time would be considered a keeper.
A corn replant table and checklist is available here.
How long can corn, soybean and forage crops be under water and still survive?
With the recent rains and the forecast for more to come this week, farmers who have flooded fields or parts of fields are asking: How long can crops be under water and survive?
Emerged corn and soybeans can normally only survive complete submersion for two to three days—assuming an 80 degree F daytime high for air temperature. Most forages can survive for one or two days. "I have seen survival for considerably longer periods of emersion," says Virgil Schmitt, ISU agronomist in southeast Iowa. "The cooler the air temperature, the longer the plants can survive."
Plants not totally submerged will survive considerably longer, he adds. By the time the water has receded and the field dries out, it will be easy to see whether the crop has survived or not. Flooding can lead to greater disease problems on all crops, so be sure to dig a few seedlings up and determine if the roots have the normal white color.~~~PAGE_BREAK_HERE~~~
A light rain after the water recedes or drains away might be beneficial to wash off any mud on the plants.
Soybeans are less capable of emerging from greater depths in the soil than corn
Some low lying fields have had new deposits of soil on top of planted ground, notes Lang. Corn and beans that had emerged and are buried by this soil will be lost. Corn and soybeans that had not yet emerged can still come up from greater depths, especially corn. The limit from which corn can emerge is depths of 4 to 5 inches, says Schmitt. Soybeans, because the plant must push the seed up to the surface, is less capable of emerging from greater depths. A University of Illinois study of relative emergence of soybeans planted at different depths found:
Planting depth Relative emergence (percent of the best)
0.75 inch 85.5%
1.0 inch 100%
1.5 inches 99.5%
2.5 inches 55.2%
It's getting late; when should you switch to an earlier maturity corn hybrid?
In Iowa, ISU agronomists recommend staying with planting full season corn hybrids through May 20-25. This requires some common sense as to how you define full season, says Lang. Some farmers in the most northern tiers of counties in Iowa might plant 110-day corn if they can plant it early enough. But they may drop back to a 105-day "full season" corn by mid-May, and a shorter season corn after May 25. Those with a corn silage option could stay with a "full season" hybrid longer than those planning on a grain harvest.
In general the date to switch to earlier maturities for corn hybrids is later in southern Iowa. Virgil Schmitt, in southeast Iowa, says "A general rule is that if planting is delayed until May 25, after that date you should select a hybrid that matures five days earlier than an adapted full-season hybrid for the area. If planting is delayed another seven days, select a hybrid that matures another five days earlier than the previous one."~~~PAGE_BREAK_HERE~~~
When should you switch to an earlier soybean variety if you are planting late? "We have a lot of time on this one," says ISU's Lang. "It is May 20 now. You should stay with a full season soybean variety through mid-June."
How is the early planted corn doing this spring? Watch for emergence problems
"Most of the early planted corn appears to be emerging well. But I am hearing scattered reports of stands that are less than hoped for," says Schmitt in southeast Iowa. "Keep an eye on your corn stands for signs of seedling disease. Pythium seedling rots are promoted by cool, wet soils."
In central Iowa, ISU's Mark Licht observes "Corn planted before the snow and rain (prior to May 1) looks to be emerging pretty good. In fields I have assessed, corn stands seem to be running about 10% lower than expected. However, things are looking pretty good considering what those early planted seeds went through. Also, I'm reminding farmers to keep checking their fields once a week. Keep an eye out for cutworms for possible damage to emerging and young corn seedlings."
Another observation Licht has made over the last two weeks is the amount of soil erosion that has occurred following the hard rains that have hit various areas of Iowa. For the most part, central Iowa is fairly flat. But that does not mean soil erosion is not a problem, he points out.
Soil erosion is another consideration; check your fields now to see where more soil conservation efforts are needed
"Farmers need to go out and assess where water flowed across their fields," says Licht. "Neither of the two most recent big rains we've had were amounts uncommon for the year. Consider repairing grass waterways, and constructing new grass waterways where needed in fields. Also, it helps to place buffer strips where needed, and to plant cover crops in targeted areas. And last but not least, it helps to reduce the tillage intensity. A combination of soil conservation practices will reduce the amount of soil that is eroding and leaving your fields. Soil leaving your fields is yield potential leaving your fields."