Soybean does not handle early season weed competition

Encyclopedia Article

Palle Pedersen, Soybean Extension Agronomist
Originally published in the Iowa Soybean Review, Spring 2006

Most soybeans should have been planted by now, but that does not mean we can start to relax. Soybeans should be scouted right now for bean leaf beetles. As soon as the beans are emerging, you will need to scout your fields to be sure that you do not have populations above the threshold.  With the mild winter we recently had, we may see bean leaf beetle numbers higher than we have seen over the last couple of years.

The next important step we will be facing is weed management. A lot of journal articles have been written on soybeans and weed competition. Most of them state that soybean is not as sensitive to early season weed competition as corn. I do not agree. Maybe that was the case when we were satisfied with 40 bushels per acre. However, if you want to consistently achieve higher yields, then you must manage weeds early.

Controlling weeds early is extremely important for successful soybean production. Weeds compete directly with soybean for nutrients, moisture and light, thus reducing yield. They can harbor insects and diseases, and can also interfere with harvest, causing extra wear on harvest equipment. Soybean yield losses resulting from weed interference and the cost of weed control does not constitute some of the highest costs involved in soybean production anymore. Yes, that is true. For example, we spend much more per acre today on seeds than we spend on herbicides - just like corn. I know that was not the case five years ago.

In Iowa, the use of transgenic soybean varieties has increased substantially during the last several years. Glyphosate-resistant soybean is now planted on more than 90 percent of the soybean acres in Iowa. Reasons for the rapid popularity of this technology include its ability to offer an alternative and new wide spectrum weed control option in the corn-soybean rotation, provide a high level of efficacy and excellent crop safety. It can also be a cost effective weed control program offering flexibility in application timing, and conventional herbicides can also be used on glyphosate-resistant soybean. This system has also been accepted so rapidly because our production practices have changed greatly. Iowa farms are getting larger and larger and many have another job besides farming. We are therefore placing greater - if not total - reliance on one herbicide since it is easier, rather than encompassing multiple herbicides that we previously did.

One of the advantages of glyphosate-resistant soybeans is glyphosate's ability to control larger weeds that we previously, prior to glyphosate-resistant crops, could not control. This is a tremendous advantage for “hobby farmers” and large farmers that would not be able to spray every acre at the right time, since nearly any size weed can be managed in this system. However, the problem is that this benefit creates the risk of delaying application beyond the time when weeds have begun to compete with crops, therefore resulting in significant yield losses. Weed extension specialist Dr. Micheal Owen is currently leading a checkoff funded research project funded through the Iowa Soybean Association that I am involved with, together with Dr. Greg Tylka, to look at pest and soil interaction with herbicide program and timing.

Preliminary data from this project for the first two years is that the first application of glyphosate needs to be early. We are working at 10 locations across Iowa every year and no preemergence herbicide have been applied to any of these locations. Our first application is at V2 and the results are very consistent. As soon as we delay the first application to V4 or later, we will lose yield.

Successful weed management is a high priority today and should be in the future. I know that everybody wants to simplify their system. Most people want to do it all in no more than two passes. You can do that if you time the applications right. Just think about how many times you went across the field managing weeds before glyphosate-resistant soybeans were introduced. Weeds are still weeds. It does not matter if you have a clean field at harvest if you have lost a lot of yield from weed competition earlier in the growing season!

For more information
Read more about the role of preemergent herbicides in the following documents published by Dr. Robert Hartzler, ISU weed extension specialist.

Role of preemergence herbicides in Roundup Ready crops. I. Protecting yields. (pdf)
Role of preemergence herbicides in Roundup Ready crops II. Reducing glyphosate selection pressure. (pdf)

Visit the ISU Weed Science web page to learn more.

Soybean Extension and Research Program