ROCKVILLE, Md. (DTN) -- In central Illinois, one farmer just finished reseeding wet holes in some cornfields and is eying other fields anxiously. In southwestern Indiana, another is walking fields and counting corn stands, with up to a third of acres facing replant.
Midwest farmers face difficult decisions in the coming weeks. Widespread flooding and cold snaps have left many growers with sub-par corn and soybean fields, and the planting clock is ticking.
The decision to replant is a nuanced one, however. Take the time to review the many factors in play, from the perspective of two crop experts.
Soybeans: At 100,000 plants per acre, a soybean field's yield potential is at 100%, said University of Wisconsin Small Grains Specialist Shawn Conley.
What about stands between 50,000 and 100,000 plants per acre? "Don't touch it," Conley told DTN. "Our data shows basically that replanting (at those populations) is a breakeven possibility at best."
Instead, for stands above 50,000 plants per acre, invest in an "in-season residual herbicide" along with your first post-emergence herbicide pass, Conley said. That will help keep weeds at bay while the canopy takes longer to close.
Soybean populations below 50,000 are good candidates for replanting, Conley said.
Corn: The good news?
"Today's hybrids are more tolerant to populations on both ends of spectrum," Purdue University corn agronomist Bob Nielsen told DTN. "They will tolerate higher populations better, and by the same token, they won't fall off the cliff if they're planted too thin."
But how thin is too thin? "Our data from most trials shows that stands in the mid-20,000s can still yield more than 95% of the optimum yield potential," Nielsen said.
There is a catch: That thin stand has to be extremely healthy. Which brings us to...
Soybeans: Unless a plant is completely missing, it can be hard to know which plants to count as productive and which to leave out.
Conley recommends a simple rule of thumb for soybeans: "Count a plant out if there is death or severe damage below the cotyledon," he said. "With damage above the cotyledon, the plant will regrow with minimal yield loss."
Corn: Many young corn plants are facing disease and compromised root systems right now. "A lot of fields are going on four weeks now in the ground, and with that kind of time frame, most fungicide seed treatments have broken down by now," Nielsen said. "They're now vulnerable if diseases take off."
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The best way to be sure of their survival is waiting another week to evaluate them -- but that is a luxury many farmers don't have, Nielsen said.
Dig up questionable plants and evaluate their roots and mesocotyl. Look for firm, white, living plant tissue, said Iowa State University plant pathologist Alison Robertson. Count out sickly, slow-growing plants, which will become a liability down the road, she said.
"With corn, a diseased seedling will always be compromised even though it may survive," she said. "Then it becomes a weed."
Soybeans: For help calculating how yield potential changes for soybeans as May slips into June, see this article from Conley: http://bit.ly/….
Corn: Experts most often turn to a chart on planting date, stand population and yield potential from University of Illinois crop scientist Emerson Nafziger. See it here: http://bit.ly/….
But don't base your replant decision entirely on planting date charts, Nielsen cautioned.
"Sometimes a full corn replant at the end of May into better conditions and enduring less stress can yield more than early planted stands that suffered all sorts of stress in the first four weeks of life," he said. "So I struggle with using planting date charts, which can be a little too black and white."
PRODUCTS IN THE FIELD
Replanting soybeans into soybeans or corn into corn is unlikely to pose many challenges.
But if you're switching to soybeans from corn, check the labels of any corn herbicides applied to the field. Some require long plant-back restrictions for a following soybean crop.
Other herbicide issues may arise when growers seek to kill a soybean or cornfield before replanting. See this article from the University of Illinois weed scientist Aaron Hager on common herbicide pitfalls during replanting: http://bit.ly/….
See another article from University of Missouri weed scientist Kevin Bradley on how best to use herbicides to zero out a compromised cornfield: http://bit.ly/….
Soybeans planted in a previous cornfield fully loaded with nitrogen are likely to face overgrowth issues, added Conley.
"The beans will take up that free nitrogen and put it into vegetative growth and get big and lanky," he warned. "You run into a higher risk of lodging and also a higher risk of diseases in the North, like white mold."
Soybeans: So you've made the decision to replant. How many seeds do you need?
For soybeans, Conley recommends pushing seeding rates higher during replant, toward 180,000 to 200,000 seeds per acre.
"We're effectively trying to get more nodes per unit, and we won't get that with same seeding rate," he explained.
A higher seeding rate also forces plant growth upward, which pushes that bottom node up higher, so the newer plants will be easy to harvest, he added.
Corn: Less is more when it comes to corn replanting, Nielsen said. "In late May, seeds are generally going into warmer soils, so the percent of germination is better and your final stand will likely be very close to the seeding rate," he explained.
He recommends reducing your original seeding rate 1,000 to 2,000 seeds per acre to account for this increased plant survival during replant.
Soybeans: Given that most soybean replanting will occur when the original stand is in the V2 stage or earlier, switching to an earlier-maturity soybean isn't usually necessary, Conley said.
"If you're patching in within a three-week window of the initial planting date, stick with the same maturity group," he recommended. "If you're outside that, don't go any further than a half-maturity group earlier than the original variety."
Corn: Likewise, Nielsen urged restraint when considering earlier-maturing corn hybrids for replant.
"If you switch to a maturity group that is unusually early for your area, they will tend to be more northern hybrids and not have as good a disease package for your area," he warned. "They can be magnets for foliar diseases early on."
He recommends picking hybrids that are no more than a few days faster maturing than your normal hybrids. For example, an Indiana grower who routinely grows 108- to 113-day hybrids shouldn't go any earlier than 102- to 106-day hybrids.
Emily Unglesbee can be reached at Emily.firstname.lastname@example.org
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