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WEED MANAGEMENT ISSUES IN ROUNDUP READY SOYBEANS
Bill Johnson & Reid Smeda

We have utilized Roundup Ready soybean technology for five growing seasons. In our discussions with growers, crop advisors, agchemical dealers and extension specialists, the following performance issues are most commonly mentioned. We will briefly address each question.

1).     What is the proper timing for postemergence application of Roundup and other glyphosates?

The first postemergence application should be targeted on 4- to 8-inch tall weeds to avoid crop yield losses due to weed interference, and reduced control of larger weeds. We have observed an increase in the number of fields treated when weeds are 10 inches tall or greater. Often, when there are performance complaints in Roundup Ready soybeans, it is due to the fact that weeds were larger than listed on the label for the rate used. Typically, an initial application of 1 quart per acre, on 4- to 8-inch weeds is recommended. Weeds will generally be 4 to 8 inches tall 3 to 5 weeks after planting. If the initial application is delayed and weeds are 8 to 18 inches tall, use 48 ounces per acre for best results or, if new flushes of weeds occur they can easily be controlled by sequential applications. Plus, Roundup may be used up to 2 quarts per acre in any single in-crop application for control of annual weeds where heavy weed densities exist.

2). Should we use tankmix partners for hard-to-control weeds such as morningglory, velvetleaf, waterhemp, and yellow nutsedge?

Our research results have not shown a consistent value in tankmixing herbicides for control of morningglory and yellow nutsedge. Velvetleaf taller than 10 inches can be controlled with the addition of Resource or Aim, but one must be careful to avoid antagonizing control of other weeds. In addition, foliar burn may be experienced with some tank mixtures. We have not seen any advantages to tankmixes in controlling waterhemp. There has been some press recently regarding a few instances of less than acceptable control of waterhemp with Roundup. It should be pointed out that the total number of cases is less than half a dozen in the entire Midwest region. This is an extremely small number of cases considering the fact that Roundup is used on over 45 million acres. However, due to the importance of waterhemp throughout much of the Midwest, performance issues with this weed tend to garner a lot of attention. We have investigated one field in Missouri. Our research has shown that single applications of Roundup at 1 quart per acre or more controlled more than 80% of 4- to 8-inch and 12- to 16-inch waterhemp. This was an average of 11% less control than waterhemp at a comparable site where no control issues with waterhemp have been reported. Evaluation of the less than 20% of waterhemp plants surviving glyphosate demonstrated that at 8 weeks after treatment, some treated 4- to 8-inch waterhemp plants (less than 10% of total treated plants) survived at glyphosate rates up to 2 gallons per acre. These plants were very short and may not have survived in the presence of a soybean canopy. The best control with minimal regrowth of waterhemp was obtained where Roundup at 1 quart per acre was used sequentially (one application on 4- to 8-inch or 12- to 16-inch plants, and a second application on 4-inch regrowth).

3). What is the value of a residual herbicide in the Roundup Ready soybean system?

This question is really dependent on the grower, their operation, and problem weeds present. When timely postemergence spraying is a challenge, use of soil-applied herbicides can lengthen the window of opportunity for control of problem weeds such as waterhemp. Our research has shown that the use of appropriate soil-applied herbicides (those matched with weeds present in a field), will decrease the overall variability in yields and net returns over the use of Roundup-only programs. Our research has not shown that yields and net returns are increased, but simply less variable from year to year or field to field. Use of soil-applied herbicides that have activity on waterhemp should be strongly considered to effectively manage this weed because of its ability to germinate throughout the growing season. With the price of many soil-applied herbicides becoming more reasonable, the question of the increased cost of using these products is becoming less of an issue.

4). What type of weed shifts can we expect with continued use of glyphosate-based herbicide technology?

There are two noticeable shifts in problem weeds in Missouri. One of the reasons for rapid adoption of Roundup Ready soybeans is the excellent control of common waterhemp. We have observed much, much fewer waterhemp control complaints with Roundup Ready soybeans than with previously used technology. We would, however, like to emphasize that waterhemp has not gone away. We still need to be careful that we don’t become overconfident in our ability to manage this weed. With decreased use of soil-applied herbicides and diphenyl ethers (Blazer, Cobra, Reflex/Flexstar), morningglory control is becoming a bigger issue than it was 5 years ago. This is due to the ability of this weed to emerge late in the year (after herbicide applications) and also the reduced use of residual herbicides that have activity on morningglory.