Do you want better weed control in your cornfields this spring? Then you may want to consider a two-pass program. By applying a pre-emergent herbicide followed by a postemergent herbicide, you will keep weeds to a minimum.

Spring is fast approaching, so weed control in corn is a timely subject. A successful weed control program is designed to control the weeds you have in a field in a timely manner so they do not compete with the crop you are growing.

Sometimes it seems weed control in corn often centers solely on the pre-emergent herbicides to provide complete, season-long weed control. Put a pre-emergent herbicide product down and do not come back until harvesttime. If the pre-emergent herbicide fails to control all weeds, use a postemergent herbicide only as a rescue treatment.

Many times these rescue treatments may not provide adequate weed control if the herbicide is not applied in a timely manner. It would be great if pre-emergent herbicides provided season-long weed control, but many times weeds are in the field at harvesttime.

The question now becomes did these weeds affect yield? Sometimes late-season weeds in a crop may have little or no impact on yield but may interfere with harvest and add more weeds to the seed bank. However, a corn crop cannot tolerate early season weed problems. Early germinating weeds will compete with the corn for sunlight, nutrients and moisture. If left alone, weeds can dramatically affect yield while producing weed seed.

A better way
Is there a better way to maximize weed control in corn? In some fields, the pre-emergent herbicide may provide successful weed control. In many fields, successful weed control can only be accomplished with a planned two-pass herbicide program.Put a pre-emergent product down and then follow with a planned postemergent herbicide.

The postemergent treatment should be done in a timely fashion to maximize weed control. This means you need to identify the weeds that are present, take note of weed size and identify corn stage growth so the correct herbicide can be applied.

This is not a new concept by any means. It used to be a standard practice to put a pre-emergent herbicide down and then do a planned cultivation to control weeds. This was and still is a two-pass weed control program that can work successfully. However, with limited cultivation use and new technology that makes applying postemergent herbicides easier and with more postemergent herbicide products available, two-pass corn programs can be very successful.

Survey results
Dr. Bill Johnson, University of Missouri Extension weed scientist, conducted a survey of agricultural chemical retailers and crop consultants in 1998 and again in 2000 to examine herbicide-use patterns and the potential impact of herbicide-resistant corn in Missouri.

One of the questions his survey asked was "What have been the three most common herbicide programs in the last 10 years?" Johnson broke these responses down based on application timing. In 1998, the response of a pre or pre-plant incorporated herbicide without planned post tactics was 74 percent. In 2000, the response was 67 percent. This decrease in relying on total pre-emergent herbicide coincided exactly with the responses of a pre followed by a planned post treatment in 1998 at 25 percent while 2000 was at 32 percent.

In 1998 and 2000, total corn postemergent treatments were at 1 percent. Based on these results, Johnson concluded dealers and producers are more willing to use a two-pass herbicide program for weed control in corn.

Producers dealing with tough grasses like Johnson grass and shattercane have relied on a planned two-pass program in corn for years. Most of the time a pre-emergent herbicide is applied to control other grasses and broadleaf weeds and is followed by a timed postemergent herbicide product that has good-to-excellent activity on the Johnson grass and shattercane.

This two-pass herbicide program approach has been the only way to raise a corn crop under these conditions. However, because of the overall weed spectrum in southeast Missouri, the planned two-pass program is the best herbicide program according to Dr. Andy Kendig, University of Missouri Extension weed scientist at the Delta Center.

Effects on weeds
What are the results of a two-pass herbicide program on other weeds? Johnson examined the combined results of percent control of seven weeds: giant foxtail, common ragweed, common cocklebur, common sunflower, common waterhemp, velvetleaf and ivyleaf morning glory.

He used five years of research data from Missouri to compare pre-emergent herbicide programs and postemergent herbicide programs to pre-emergent followed by planned postemergent herbicide programs. The results (Figure 1) show there is an advantage with a two-pass herbicide program when compared to a single pre-emergent herbicide application.

While there are many different species of weeds, three broadleaf weeds that consistently show problems with just a pre-emergent herbicide program alone are common cocklebur, common sunflower and common waterhemp.

What did Johnson's five years of Missouri corn data look like for these three weeds when evaluating pre-emergent programs and total postemergent programs to pre-emergent followed by planned postemergent programs?

The results (Figure 2, Figure 3 and Figure 4) all show an advantage for controlling these three weeds when using a planned two-pass herbicide program compared to a single pre-emergent herbicide application. This trend also occurred for other weeds in the study, such as giant foxtail, common ragweed, velvetleaf and ivyleaf morning glory.

Selecting herbicides
There are many herbicides to choose from when looking to put together a two-pass program. With more than 25 pre-emergent herbicides and more than 35 postemergent herbicides available, it can make for some hard decisions.

Select ones that will control the weeds you are targeting. A grass and broadleaf product down at planting or close to planting will eliminate early weed competition.

Then target your postemergent herbicide for the problem weeds. If you know a field has constant problems with a particular weed, make sure your pre-emergent product will provide some activity and then select a postemergent herbicide that will give excellent activity.

Future trends
Will the trend continue to increase for planned two-pass corn herbicide programs? Research data confirm that a planned two-pass weed control program is providing more consistent weed control than just a pre-emergent treatment alone. Dealers and producers during the last two years have shown more interest in planned two-pass programs because of better weed control.

With corn planting occurring earlier, this forces a pre-emergent herbicide to control the early and later germinating weeds. But less atrazine is being applied than in the past, resulting in a decrease of pre-emergent performance. And in some areas, atrazine cannot be used at all because of environmental issues.

More postemergent herbicide products are available, new herbicide-resistant corn hybrids are being developed and newer equipment that makes applying postemergent corn herbicides easier is available. These factors point to more planned two-pass programs.

Consistent control
Some fields will definitely require different management programs. For consistent weed control in corn, take a good, hard look at planned two-pass herbicide programs.

Applying the herbicide at planting or close to planting followed by a timely postemergent herbicide treatment based on the weeds present, the weed size and the corn stage of development should provide good weed control. It reduces weed seed production, which reduces herbicide expenses.