Quite a few fields across Iowa this spring are being planted to corn before the nitrogen and/or pre-emergence herbicides are applied. While UAN (urea-ammonium nitrate or 28% or 32% liquid nitrogen) and many pre-emergence herbicide products can be applied to emerged corn, using UAN as a herbicide carrier enhances the foliar activity of the herbicides and may result in leaf damage to corn.

In fact, ISU Extension weed management specialist Bob Hartzler and ISU Extension soil fertility specialist John Sawyer warn: "The combination of herbicides with UAN greatly enhances the foliar activity of these products and poses a real threat of killing all emerged tissue contacted by the spray."

TOUGH QUESTIONS: ISU Extension agronomists are getting a lot of questions about using preemergence herbicides with liquid nitrogen as a carrier. That mix can be applied to emerged corn but it can result in damage to corn plants. Check herbicide labels for restrictions on use of UAN as a carrier. Combination of herbicides with UAN greatly enhances foliar activity of these products and poses a real threat of killing all emerged tissue contacted by the spray.

Clarke McGrath can vouch for that, "as some of our customers will recall from a few of our attempts in past years to do this. If you aren't real careful, things can go south in a hurry." McGrath, who writes the "Corn-Soybean Insight" column each month in Wallaces Farmer magazine, shares the following observations from his experience in the fertilizer/ag chemical retail business and from knowledge he's gained in recent years as an ISU Extension field agronomist, trouble-shooting farmers' fields.

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Very important: check herbicide labels for restrictions on use of UAN as a carrier
It's very important to read herbicide labels and pay attention to what it says about using UAN as a carrier for the herbicide. McGrath says he has had a wide range of results when applying 80 pounds or more of UAN per acre mixed with herbicides on emerged corn from the spike stage to around V5 growth stage.

* If the corn was growing well and had endured relatively little stress, it tended to recover from the severe leaf burn caused by UAN and herbicides—if the weather remained favorable.

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* If the weather turned stressful, in particular if it was cold and wet, he could see some corn stand loss and some yield loss at harvest time.

* If corn had struggled early in the season, the additional stress of foliar burn and trying to metabolize herbicides was sometimes too much "and we had significant stand loss and/or plant stunting," says McGrath. "It was simply too risky to tankmix high rates of UAN and herbicides on corn that is under stress--we learned to seek other options."

But what if you apply the UAN/herbicide mixture when corn is small enough—if the corn's growing point is still underground?
Sometimes farmers try to rationalize the use of UAN mixed with a herbicide if the corn is only at the VE to V1 growth stage, since the growing point within the corn plant is still underground. While corn can recover quickly from the loss of the shoot at this early growth stage, the herbicide may influence the plant's ability to recover and therefore result in uneven plant size and ultimately yield loss.

McGrath cites an example. "We once tried to burn down some out of control grassy weeds in corn that was at the V1 growth stage. It was conventional corn, so the use of glyphosate wasn't an option. We used Gramoxone, a foliar burndown herbicide with no translocation to the growing point, in a tankmix with our residual herbicide and a relatively low rate of UAN, just 60 lbs. per acre."

In theory this mixture should have been just like a light frost--while the corn leaves could be burned off, the corn plants should bounce back and be fine. But unfortunately, that's not what happened.

Been there, done that and it turned out to be an costly lesson
"The weather changed on us, it got cool and wet," says McGrath. "We lost around one-third of the stand of corn, and another one-third of the corn was pretty well set back a couple leaf stages for the year. It wasn't the Gramoxone's fault, it was more likely the weather and the overload from the residual herbicide we used. It wasn't a pretty picture, and we learned an expensive lesson."

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ISU's John Sawyer and Bob Hartzler are advising farmers this spring that UAN alone can be applied to emerged corn, and the risk of injury to the corn is dependent upon the UAN application rate, corn stage and weather conditions. Conservative suggestions are to limit post applications of UAN to 90 lbs. of N per acre when corn is at the V3 to V4 stage and to 60 lbs. of N per acre at the V7 stage. Applications beyond the V7 stage are not recommended, and the risk of injury increases during hot, dry conditions.

"To be safe," says McGrath, "if corn is emerged and showing much leaf tissue at all, separating the herbicide and the UAN—making separate applications of each—while it's not the easiest or most efficient option—it is a lot less likely to hurt your corn."

Another question: What about using UAN as a carrier in burndown treatments?
Weeds are getting pretty big in some of these fields, notes McGrath. Farmers are asking questions about putting some additional N on--so what does that mean for the burndown herbicide performance?

McGrath says if you are using glyphosate herbicide, then using UAN (28% or 32% liquid nitrogen) as a carrier, it can be challenging. "We found that typically the more UAN we used, the more erratic our weed control became, particularly on any weed that glyphosate struggles with controlling," he says. "On the other hand, the more gallons of UAN we applied with Gramoxone as the herbicide we used, the better it seemed to work."

Here are a few key thoughts from McGrath about using these two primary burndown options (glyphosate and Gramoxone) and UAN. "We found that the following tactics helped with UAN/glyphosate issues." McGrath lists them as:

* Use total carrier of 10 to 15 gallons per acre--as we went over 15, control dropped off on the really tough weeds, and as we headed over 20 gallons per acre with UAN, we typically went to using Gramoxone as the herbicide.

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* Bumping glyphosate application rates up by 8 to 16 ounces over whatever was going to be the "plan" helped to a degree but wasn't the total solution.

* Using flat fan nozzles helped a lot (like an XR), much better than using turbo tees or AI's if we had tough weeds in the field. Of course, then wind became more of an issue.

* Adding 2,4-D helped immensely on control of the broadleaf weeds, if we could get it in the program without violating the label.

* Spray anything that is "borderline"—that is big weeds or if you are using higher rates of UAN--in the peak of the day. That means get it sprayed by 4 or 5 p.m. so that there is plenty of daylight left for the weed control process to have better success.

* Even with loaded glyphosate--when we get tough to control weeds, we add 1 quart of non-ionic surfactant or NIS per 100 gallon (if using non-loaded glyphosate, use whatever the label says, usually 2 to 4 quart of NIS per 100 gallons).

* Once in a while farmers or applicators will skip the AMS or ammonium sulfate additive when using UAN thinking that they are interchangeable--don't let them make that mistake.

* UAN contains only around 20% to 30% water (UAN 28% Nitrogen is around 30%, UAN 32% Nitrogen is around 20%). So if there is room to add water and still stay in that 10 to 15 gallons per acre total volume range, that helps weed control consistency when using glyphosate.

A few key points about using Gramoxone herbicide:
* Be sure to use the right additives (surfactants, etc.) with Gramoxone. As with most burndown herbicides, the use of strong rates of crop oils, methylated seed oils and/or nonionic surfactants are critical

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* While UAN can impede glyphosate activity, it seems to enhance Gramoxone activity, making it a great choice for corn burndown treatments when you are applying UAN.

* Tank mixing triazines (atrazine for corn, metribuzin or Sencor in soybeans) increases the speed and efficacy of Gramoxone in burning down the weeds.

* Use flat fan nozzles with Gramoxone and try to get your carrier up to 15 gallons per acre or so (the label goes down to 10 gallons per acre, my experience was that 15 gallons per acre was more consistent on bigger weeds)

* Gramoxone isn't nearly as temperature sensitive as glyphosate, and that's an important observation given the weather we are having this spring. Of course, it was 102 degrees F in western Iowa on May 14. But I see a few days out in the weather forecast that we are to have some lows below 50 degrees again.

* The last point is probably the most important, and I have to give credit to a friend from Syngenta for reminding me of this--"There is no weed resistance issue with Gramoxone, so using it breaks the glyphosate cycle."

Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.