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May 30 | USDA Weekly Crop Progress & Conditions Report



Post-Emergence Herbicides in Corn

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It is time for farmers to control weeds in their corn fields. However, the cool, wet start to the growing season makes it doubly important to read and follow herbicide labels.

The post-emergence herbicide labels they’ll be following usually allow applications to be made at various growth stages says University of Illinois Extension Weed Scientist Aaron Hager. He says it is really important to read the label, making sure to get the height, or the stage, maybe both, of the crop correct.

This is because most all of the products for corn have a growth stage listed on the label beyond which applications, at least broadcast applications, should not be made. It is usually either plant height - measured at the highest arch of the uppermost leaf at least 50% out of the whorl - or a leaf number. Hager says if both are listed, then it is important to use the more restrictive of the two, For example, because of some of the weather conditions we’ve had across a large part of the state this year we may have corn plants which are older than their height would suggest. Using the leaf collar method is typically a better way to stage the development of the corn plant. If you can do both the height and the counting, the leaf collar method is the better method to determine the stage of the corn plant."

Using the leaf collar method is typically a better way to stage the development of the corn plant. - Aaron Hager, University of Illinois

Corn plants under stress conditions may be more prone to injury from post-emergence herbicides. On that note, Hager says farmers should be sure to consult the product label when selecting spray additives. Many labels suggest changing from one type of additive to another when the corn crop is stressed. Also, trying to save a trip across the field by applying a post-emergence corn herbicide with liquid nitrogen as the carrier is not advisable. The U of I weed scientist says while applying high rates of UAN by itself can cause corn injury, adding a post-emergence herbicide can make it worse.

The Last Post & Red Poppies

Adjusting Nitrogen for this Corn Crop

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Despite the wet weather many think may be causing nitrogen fertilizer to get away from corn plants, it is still far too early to make that decision.

While it seems likely some nitrogen fertilizer has moved out of the upper soil as a result of rainfall this year University of Illinois Agronomist Emerson Nafziger says if soils dry out, the torrential rains stop, the sun shines, and the weather gets warmer things should be all good, “The crop is going to tell us this. If by the middle of June some of the crop has really greened up nicely and some has not, then we might need to think about those that haven’t and determine if enough nitrogen is missing to cause this to take place. My suspicion is we will not see very much of that at all. If we are warm and dry and with sunshine for a week, I think the crop is going to look good in almost every field.”

My suspicion is we will not see very much of that at all. If we are warm and dry and with sunshine for a week, I think the crop is going to look good in almost every field. - Emerson Nafziger

One indication the topsoil hasn’t been stripped clean of nitrogen is the good recovery of green leaf color. Nafziger says, as soils dry out, root systems start to expand and the color will change. He explains the corn crop at this point looks like it does not because of lack of N, but due to cool temperatures and abundant rainfall. While it is premature to revise nitrogen management based on what has happened so far, Nafziger cautions it cannot be ruled out, “I would be very reluctant now to make a decision that we need to go put more nitrogen on, especially if we’ve already put the full amount on. If we still need to side-dress and we add 10, or 15, or 20 pounds I don’t have a problem with that. But I think it is premature to decide so much of the nitrogen is gone that we put out there that we need to go back and plan to put more on at this point.”

The good news is there is still time to make such decisions. The corn crop takes up barely one pound of N per acre for every inch of growth it makes up to about knee-high.

Nitrogen deficiency develops over time, and Nafziger says it is almost always more related to current soil moisture than to the amount nitrogen in the soil. So, if fields aren’t extra wet or extra dry over the next month, this season could still turn out to be much more typical than many now expect.