June 1, 2008
Heavy rains that fell on already wet soil May 29 and 30 created "instant ponds" in a number of central Iowa fields.
"We are at the end of May and we have serious issues with corn and soybean crops in Iowa and other Midwest states caused by continuing wet conditions," says Palle Pedersen, Iowa State University Extension soybean agronomist.
Some beans that were emerging from the soil were suddenly submerged under water on May 30. In other fields, soybeans haven’t been planted yet and it’ll take awhile for fields to dry out before they can be planted. The result will be a lot of replanted and late-planted soybeans this year.
Plant an earlier maturing bean?
Normally, in central Iowa planting would be finished by May 10 to 15. If a farmer is planting beans late—say sometime during the last 10 days of May or during the first half of June--what should the farmer do differently?
"Today is May 30, so we are still in May and that’s positive," answers Pedersen. "It’s going to take at least a week before some of these fields are going to be dry enough to plant. That is, if we don’t get more rain."
So when should you switch to an earlier maturity soybean?
"You can plant the same maturity group soybeans you normally plant—the full season maturity beans for your area," he says. "You can continue to plant them through the third week of June without any problem. So you don’t have to switch to a shorter season variety yet."
"I don’t have a problem with planting a full season variety for your location until about June 20 in central and north Iowa. For southern Iowa you can wait until July 1 before switching to an earlier maturing variety."
What about plant population?
When you plant in June should you change the seeding rate? Some farmers believe you need to plant more seeds per acre when planting late, to get a quicker soybean canopy to help with weed control. A fast developing soybean canopy will help shade out weeds. But Pedersen doesn’t advise planting more seeds per acre than is normally recommended.
"You don’t need to increase your seeding rate," he says. "You may have done that in the old days when planting late, but it’s not necessary today. We have glyphosate tolerant soybean varieties and very good postemergence weed management programs you can use. So you can plant at the same, normally-recommended seeding rate."
"If you have access to a drill or a split-row planter (15 inch) I would highly recommend you either drill the beans in narrow rows or plant them in narrow rows—just to get the quick canopy closure," adds Pedersen.
The challenge is to get the stand established well and do a good job in preparing the seedbed. No matter what kind of planting system you use, make sure you get good seed-to-soil contact.
Evaluate your soybean stands
"I will not be surprised if we see more phytophthora root rot disease this year," says Pedersen. "Damping-off pathogens on soybean seedlings will likely also be more of a problem."
What about beans that are already planted—either emerged or still sitting in the ground? What if those beans are under water today due to ponding in fields? What kind of stand will you get out of that? Should you replant?
A lot of time what happens when corn and soybean plants are covered by water too long is a build-up of carbon dioxide take place, and that’s toxic. The corn or soybean plants will die from that. "We can have beans underwater for as long as 3 to 5 days without any problem," says Pedersen. "When the water recedes, the beans can grow and recover."
Need to be cool to survive
However, beans under water survive best if the weather conditions are cool, he explains. If the weather gets hot—say it gets into the 80 to 85-degree Fahrenheit range or above in the afternoon, the plants will try to grow out of their predicament and likely won’t survive.
Another problem you have when water is covering the plants is all the silt that sits on the leaves. You’ll need another rain to wash the plants off so they can produce chlorophyll and keep growing. "There are a lot of negatives involved with this water and wet soil issue," he notes.
Bean stands hit by hail damage
Some soybeans in Iowa have suffered hail damage the past few days. Should you replant those beans? Don’t get in a rush to replant, he advises. "Wait three to five days before you take a stand count and then make a decision on whether or not to replant," says Pedersen. "Be sure to do a good job when you make your stand count."
He says you want a uniform stand of around 73,000 viable plants per acre. That’s enough—a minimum stand to give you 90% of the yield potential you could get by replanting today. "We want 73,000 plants per acre out there," he adds. "That’s approximately a little bit more than 2 plants per foot of row in a 15 inch row width, and a little bit more than 4 plants per foot of row in a 30 inch row width."
Do good job with weed control
That’s a good number to have; if you have more than that you are in good shape. If you have less than that, then it is probably worthwhile for you to replant.
"If you can’t get into the field for the next week to 10 days it may be best to just accept the fact that you have to plant later than you’d like this year. And do a good job on your weed control to make sure the soybeans have little or no competition out there," he says.
For more information on how to evaluate soybean stands and make replant decisions, go to www.soybeanmanagement.info
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