Weeds, Hail, Postemergence Herbicides Are Timely Topics

Weeds, Hail, Postemergence Herbicides Are Timely Topics

It's been a tough week for crops in some parts of Iowa; here's some timely advice from ISU agronomists.

Heavy rains, pounding hail and high winds this past week struck a number of fields in southwest Iowa. Fields in some other areas of the state had heavy rain, too. Here's a management update from Clarke McGrath, Iowa State University Extension field agronomist based at Harlan in western Iowa. He and ISU Extension colleague Aaron Saeugling, a field agronomist who covers southwest Iowa, offer the following observations, information and recommendations.

"It has been quite a tough week for growers, dealers and anyone else beat up by the storms," says McGrath, who writes a monthly column for Wallaces Farmer magazine. "Hopefully the weather will lighten up on us."

CROP INSURANCE QUESTIONS: In corn or beans that are a total loss due to hail or flooding from heavy rain, ISU agronomists are encouraging farmers to plant something for ground cover. This is a discussion to have with your crop insurance company; see what they will allow if replanting a cash crop isn't an option.

Clarke's and Aaron's observations and recommendations
Hail damage and replanting: "
In talking with insurance folks, it sounds like there is a lot to be decided between the adjusters and the growers in the next week or so," says McGrath. "A friend of mine who works as a claims manager for a major crop insurance company was working with me last night on projected replant dates and options for corn; after he goes up the chain of command with it he will know more, but preliminary thoughts indicate that if farmers want to replant corn it will likely need to be done by the middle of next week or the crop insurance folks may not support that decision. Or that is this company's thinking so far; still a ways to go in evaluating what corn (and soybean) stand counts will actually look like."

Corn takes around 82 GDU's to shoot a new leaf until the plant gets up around V10 growth stage, and it would sure be helpful to give it a chance to grow a leaf and see what new growth looks like when it comes; that can be helpful when evaluating long-term damage to the growing point in cases where it looks like the growing point is healthy but there has been significant bruising around the growing point.


"So far since the night of the June 3 storm we are around 85 to 95 GDU's depending on location here in western Iowa yet in many fields we haven't quite got enough "new" leaf tissue to see what we have in terms of corn plant recovery from the hail. Hopefully in the next few days we will know more," says McGrath.

What about corn plants that have the leaves "tied up"?
To answer questions about the plants that are "tied up"… it depends, says McGrath. To quote Dr. Roger Elmore: "Some plants may be 'tied' or 'crippled,' for example, the leaves fail to expand in a normal manner from the whorl. Since it cannot be determined until much later whether these crippled plants will develop normally, they should be classified as non-living if replanting the stand is being considered." That is good advice.

There has been much debate about which replant chart to use, the one in the ISU Corn Field Guide, or one of the other ones out there, notes McGrath. "In discussions, study and experience, the info I shared that comes from University of Missouri's northern region, combined with our ISU field guide information, is a good compromise. The bottom line is that as we move deeper into the charts, further out into the June column, the amount and consistency of the yield data goes down dramatically. We've discussed this in meetings with growers and agronomists and their responses to our questions illustrate this very well."

What kind of yield can you get from corn replanted this late?
When farmers are asked about their experiences with June corn plantings, the results tend to be yields either in the ranges of 75% to 90% of normal, or 0% to 25% of normal. Point being, says McGrath, "we don't often see the 'average' yield from the chart; rather the chart percentages that far into the planting season are a product of a wide range of extremes; 'boom or bust' is more likely with June corn plantings than the chart percentage. We tend to use the charts more to gauge and discuss the odds of boom vs. bust with these later plantings. Hopefully that explanation helps. There have been a lot of questions and discussions this past week about 'the charts' as I've talked to growers and agronomists who are trying to decide whether or not to replant."


Beans are recovering in many cases, but are tricky to evaluate

Soybeans are bouncing back in many cases, but it is a tricky situation to evaluate. "Many of them without the equivalent of a cotyledon or a leaf left after the storm are trying hard to come back; we really won't know on those for another three to five days and even by then it is a little bit of a guess. We don't really know a lot until they start regrowth in earnest with multiple trifoliates -- that's been my experience," he says.

The bruised plants are also very hard to evaluate for long-term yield, and any evaluation of their survival and long-term prognosis for contributing to yield is very subjective, says McGrath. "Many of the bruised plants I am seeing, in my opinion and in discussions with ISU colleague Aaron Saeugling, we seem to be on the same page, we will not count the bruised plants towards a surviving stand estimate."

Soybeans are a much more forgiving crop than corn
Is there any good news? If there is any "good" news in this, it is that soybeans are much more forgiving of many things than corn, says McGrath.

* Stands of as low as 50,000 to 60,000 healthy soybean plants that are decently spaced on average can produce around 80% of normal yield

* In southern Iowa, "mid-June" soybean plantings average 80% plus of full season yield

* So the decision to replant or keep a weather-damaged soybean stand has a lot more room for interpretation and is more forgiving either way in most cases. "Where I like to see with beans anything remotely close to borderline replanted, a differing opinion is also generally going to work out just as well over time," says McGrath.

A good discussion to have with your insurance company
In corn or soybean fields that are a total loss, the ISU agronomists are hoping that growers will be encouraged to plant something for ground cover. Aaron Saeugling astutely pointed out that if these growers aren't allowed to put something out there, fields could go 12-plus months with little ground cover; ground cover provides protection from erosion and a foundation for building soil structure and health. This is a great discussion to have with your insurance company, to see when and what they will allow if replanting a cash crop isn't an option.


"As has been discussed in the many meetings and field events, probably the final word on most of the decisions will come down to the discussions between growers and their insurance companies," says McGrath. "Best of luck to everyone and let us know how we can assist."

This may be good for a laugh, watch Clarke walking in mud
After the storms hit last week in southwest Iowa, "we could use some laughs at someone else's expense," says McGrath.

He got a visit from a TV news crew last week. They shot a story on palmer amaranth, a new invasive weed in Iowa. It was the ABC TV station out of Sioux City. Pretty basic TV news ag story, but the reporter wanted one of the "big weeds" that was out in the middle of the swamp. "Nobody else was volunteering so off I went," says McGrath. "I was never thinking that my swamp journey would be part of the TV news story. Lots of friends have had things to say about the few seconds in the middle of the story showing me walking in mud -- "are you related to Schreck, or Mongo from Blazing Saddles, or did you take walking lessons from a sasquatch?"… Other clever things like that. So yea, go ahead, pile on."

Speaking of that new tough-to-control weed, Palmer Amaranth
In soybean herbicide plots near Modale (a big thanks to BASF for helping us establish these plots), this weed is blowing through some of the herbicide chemistry with ease, while some of the other herbicides are doing a pretty decent job of controlling Palmer.

"A bigger picture piece of information to take from this situation is that our residual herbicides in both corn and soybean fields are running out of gas," notes McGrath. "Attending hail field days and other field visits have taught me that either weeds are impervious to hail, or they emerged and now have grown to 2 inch to 5 inch heights in the short week since the June 3 storm. Either way, this is not what we wanted to see. Much of the corn was already treated with a post herbicide; the hailed fields will really need to be watched closely for new flushes of weeds."


McGrath adds, "Several things are working against us; lots of precipitation and the extended time it will take for the crop canopy to come back after being damaged by hail and wind. If we have to treat some of the corn a third time with herbicide (and in places we will this year), we will need to accurately assess the growth stage so that we don't further injure it with off label herbicide application. Many post corn herbicides are limited by growth stage and/or crop height. The growth stages of V5, V6 and V7 for corn are common limitations on a lot of herbicide labels. So be sure what stage your corn is in before you make that third trip with a postemergence herbicide spray (or the second trip if you haven't sprayed postemergence yet).

Stage your corn when applying postemergence weed control
"The good news is that we still have a pretty solid selection of herbicides that can be used on the later vegetative growth stages of corn," says McGrath. "So this will be a great discussion to have with your local ag chemical dealer."

Information on staging hail damaged corn (how to determine the correct growth stage) is here.

"On the soybean side of things, not a lot of acres had been post sprayed yet," says McGrath. "Your plans to spray a postemergence herbicide for soybeans may have to be altered slightly, but not to the extent of as it will with the corn. If possible, in fields that you know or suspect will have some small seeded broadleaf pressure (say, waterhemp or close relatives such as pigweed) you should take a strong look at dropping in some residual herbicides in the postemergence herbicide trip, particularly on any soybean stands that were thinned out or defoliated in the storms. Those thinner stands will allow weeds to thrive."

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