MFA technical brief
By Lyndon Brush and Andy Schmidt

Using a planned two-pass herbicide program in corn

A successful weed control program is designed to control weeds in a given field in a timely manner so they do not compete with the growing crop. Knowing the field history is very important on deciding which herbicide program is best. Weed species and populations can be different from field to field, and weeds differ on germination period and competitive effect on the crop. Multiply the competitive effect by weed density potential, and weeds can dramatically affect crop yield. You need to select herbicide programs that are designed to be flexible so you can change with the changing weed spectrum, field diversity and environmental conditions.

Dr. Kevin Bradley, University of Missouri extension weed scientist, has recently shown in 42 research trials with 434 treatment observations from 2002 through 2007 that a two-pass program consisting of a pre followed by a post herbicide treatment resulted in the highest corn yields 24 out of 42 trials. A one-pass post treatment with a residual treatment resulted in the highest corn yields 11 out of 42 trials. While a pre-only one-pass program resulted in the highest corn yields 2 out of 42 trials.

Over the years, the cleanest corn fields tend to be ones with a two-pass weed program. Former University of Missouri extension weed scientist Dr. Bill Johnson’s work showed that over 12 site years of Missouri data and 2,190 observations that the best overall weed control in July was from a planned two-pass program consisting of a pre followed by a timely post treatment.

There wasn’t a year that a pre program provided better overall weed control in July than a two-pass system. There was only one year when a one-pass post treatment equaled the two-pass system. So not only does the two-pass system result in the cleanest fields from a weed control aspect, it often translates into the higher yields.

Generally, a planned two-pass program will consist of a preemergent herbicide application that contains two herbicides, one for grass control and one for broadleaf weeds. The post herbicide treatment will be designed to give control of any other weeds that the pre missed, was weak on, or, because of environmental reasons, failed sufficient control. Selecting the herbicides used in your two-pass program should be done carefully to accommodate target weeds. There are many herbicides to select from; use a 2008 MFA Agronomy Guide to help you select the product that matches up to your weed spectrum.

Glyphosate-tolerant
One way to start implementing a planned two-pass corn program is by planting Roundup Ready corn. Over the last several years there has been grower acceptance of Roundup Ready. Some of this reasoning is a result of populations of very tough-to-kill weeds along with the flexibility of using a Roundup Ready. Growers like the convenience of a corn plant they can spray glyphosate on to control weeds. The latest genetics such as rootworm resistance and corn borer protection traits are also stacked with glyphosate-resistance varieties. These traits give producers the latest technology on the market place for pest protection. For some growers, technology serves as insurance—ready to use if a post treatment is needed.

Roundup Ready corn is an excellent tool to be used where it fits into your corn herbicide program. Keep in mind this technology also brings additional management. The main issue with Roundup Ready corn is simple: continued use of repeat applications of glyphosate will create weed resistance. The only way to minimize weed resistance is to use herbicides with different modes of actions. Once again, the 2008 MFA Agronomy Guide can be used to select products with different modes of actions. Missouri has confirmed cases of glyphosate-resistant waterhemp, common ragweed and marestail. The list will get bigger. Palmer amaranth has been confirmed in Arkansas and Tennessee. Giant ragweed is in Kansas. Once resistance develops—if changes are not made—the problem spreads over more acres. Glyphosate is an excellent herbicide tool, but it needs to be managed carefully.

Proper timing
Another problem we’ve seen with the Roundup Ready approach is waiting too long to control competing weeds or not using a pre-emergent herbicide. Early season weed competition is very damaging to corn. Weeds can take up valuable nitrogen, water and light early in the growing season, a deficit from which the corn plant can’t rebound. A clean field in the fall may not always give you the maximum yield if you did not spray on time earlier in the growing season. The use of a preemergent and timely glyphosate application is critical to the success of the program.

Another consideration when using RR corn in a planned two-pass program is what rate of a preemergent product is needed. Design the pre program around the weed spectrum for the field. Using a preemergent product at acid equivalent to full labeled rate and then followed up with a timely post treatment with glyphosate based off the weed height should help give maximum weed control in the corn crop. Keep in mind you want the preemergent product to control the early weeds. If you cut the rate too much, you may find that you’re not adequately controlling the weeds you want with the pre product and then have to come back sooner with your planned post treatment.

When using glyphosate as your post treatment, also be careful to use the correct rate based off the weed height in the field. Tank mixing other post products with glyphosate maybe beneficial, but it will be weed specific. Do not try to skimp on the labeled rate of the post product. The most expensive herbicide treatment is the one that did not adequately control the weeds you were targeting.

The final issue to consider when planting Roundup Ready corn is that when you rotate to Roundup Ready soybeans, the volunteer corn growing the next season is probably Roundup Ready corn. Take appropriate actions to eliminate that weed (the corn) in a timely fashion. If you fail to realize that on the first spray, it’s harder to control by the time you recognize what is happening.

In summary, you should know the weed spectrum for the field. A planned two-pass program is the best herbicide treatment. Apply a preemergent product based on weed spectrum and then apply a timely postemergent treatment based on weed spectrum present and height.

Lyndon Brush is a staff agronomist for MFA.

Andy Schmidt is a crop protection product manager for MFA.

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