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Missouri's Delta Center: Weed tour features ugliest plots

In anticipation of our Sept. 2 field day, welcome to the ugliest plots on Delta Center Lee Farm in Portageville, Mo.

Some ask why our plots look so bad. Well, when a grower asks if a herbicide works, we need to have tested it under tough conditions to give him a good, thoroughly-tested answer. Any herbicide works well when there are few weeds. With the tough growing conditions of 2003, only the best weed control programs have clean plots.

The Delta Center Weed Science Project strives to provide growers with the latest weed control information for rice, cotton, corn, soybeans, wheat and grain sorghum. In addition, the project is responsible for providing support for vegetable and homeowner weed questions.

In 2003, we conducted 23 rice trials, 18 cotton trials, eight soybean trials, seven corn trials, five general weed control trials, four wheat trials and one grain sorghum trial. These trials made use of approximately 4,000 plots.

Currently, we are researching Palmer amaranth control and horseweed control for cotton producers; sprangletop control, drift injury and low cost programs for rice producers; and variations on Roundup Ready systems and no-till for soybean producers.

Highlights of our research include the testing of a new, experimental rice herbicide as well as the Liberty Link and Roundup Ready Flex Cotton systems. Our annual weed tours in late July offer growers opportunities to focus on specific weed questions.

Soybean weed control

Right now, soybean weed control is dominated by Roundup Ready systems. Questions continue to arise regarding the use of pre-emergence herbicides, postemergence tank mixes and generic glyphosate products. Also, there are occasional producers interested in producing conventional, non-GMO soybeans.

  • Pre-emergence herbicides

    We continue to get good weed control from the use of a broadleaf pre-emergence herbicide followed by glyphosate. However, if an activating rainfall does not occur, the pre-emergence herbicide usually fails. Our best no-till burndowns have combined a broadleaf pre-emergence herbicide tank-mixed with glyphosate. However, glyphosate should still be included for most burndown treatments for grass weeds.

  • Tank mixes for glyphosate

    Growers still ask whether they should add something to glyphosate for control of morningglory or other weeds. The primary argument against adding a second herbicide is that sequential applications with tank mixes rarely beat sequential applications of glyphosate alone. Benefits are observed with single applications, but a sequential application is far superior to any single application. Herbicides that have improved morning glory control in single applications include: Classic, FirstRate, Aim and Scepter. We recommend tank mixes if a grower has had difficulty controlling morningglory. However, we do not recommend tank mixes just because the tank-mix sales rep has suggested it.

  • Generic glyphosates

    We have tested over 15 different “name brand” glyphosate formulations and have found them all to provide similar activity. These products differ in their surfactant requirements, and in our research, we added surfactant when it was recommended. There are important guarantees and plant-back considerations the grower must be aware of when choosing a glyphosate. However, we have observed virtually no difference in weed control performance. In most cases, we have had to go to extra effort to obtain these glyphosate products. We do not recommend the use of “Internet” glyphosate because there is no individual to support the product if there is a problem. If you choose to use a “generic imitator,” there are numerous brands from reputable sources, including most distributors.

  • Conventional soybean weed control. Successful weed control in conventional soybeans requires the development of a weed control program designed to address the weeds present. In most cases this will require two herbicide applications. Additionally, timing is extra important with conventional herbicides. Many of us have lost the edge to spray exactly on time.

Rice weed control

Rice weed control used to be somewhat mechanical, with limited choices. Recently, newer herbicides have opened the door for customized and more-complicated weed control. But more important than individual herbicides is the development of a weed control program that addresses the weeds that exist in the field, economics, the soil texture and other agronomic considerations.

It is extremely important to regularly scout fields to check for weeds that are present, as well as their species and size. It is also of the utmost importance to make herbicide applications in a timely manner. Late herbicide application and poor water management cause the majority of weed control problems in rice.

Rice herbicides also have numerous label restrictions regarding crop safety and management. A herbicide can cause no injury to the crop 99 percent of the time — but if one condition is wrong, crop injury can be severe. Always carefully read and follow label restrictions to assure crop safety and good weed control performance.

For drill-seeded rice, remember:

  • Command

    Command-based programs are popular because Command provides good, convenient, low-cost grass control. A followup application of other herbicides is often needed for broadleaf and sedge weeds. Scouting is important, as the followup treatment will sometimes need to include a herbicide with grass activity.

  • Propanil

    Propanil-based programs might be termed as the “old standby” of rice weed control. Typically these programs center around a properly timed, early postemergence application of any number of propanil-based programs. Most often, the propanil is applied in a tank mixture with a residual herbicide (Prowl, Facet, Bolero, or Command). If weather is normal and the permanent flood is established on time, the residual herbicide will provide weed control until flood up. Propanil is also useful in other weed control programs (including Command-based programs).

  • Others

    Other programs exist; including delayed pre-emergence programs, programs with RiceStar and Clincher, sequential propanil, extremely early programs and Clearfield/Newpath programs for red rice control.

Cotton weed control tour

We received numerous questions in 2003 regarding post-directed/layby weed control options.

The best post-directed/layby option for cotton was Bladex, which is no longer available. The bad news is that none of the current (nor pending) options equal Bladex. But, the good news is that the many remaining options at least provide acceptable and economical weed control.

The most important aspect of a post-directed/layby program is residual weed control. Residual herbicides include Caparol (prometon), Cotoran (fluometuron), Direx (diruon), Goal, and Linex (linuron). Valor, Reflex and Suprend are also being developed for postemergence use. We annually test all of these products, side-by-side, and an important bottom line is that when they are carefully sprayed underneath cotton, where a good height differential has been developed, they all work well.

Some states requested Section 18 emergency exemptions for Valor for pigweed control. Valor has provided good pigweed control in our studies, but so have most of the other post directed/layby treatments. The EPA denied the Section 18 requests due to the large number of effective products currently available.

Traditionally, MSMA (or DSMA) was tank-mixed with these herbicides to add grass and nutsedge control. More recently, glyphosate has been used as the mix partner. Glyphosate will provide better grass control than MSMA. However, MSMA is better when nutsedge is an issue. MSMA also provides good control of volunteer Roundup Ready soybeans if the soybean plants are small and can be covered fully.

Prowl is a unique post-directed herbicide as it has no postemergence activity, but has residual activity. We have seen pigweed control benefits from Prowl mixtures.

Aim, Cobra, and Harvade have been promoted as mix partners for post-directed use. These herbicides do not have residual activity, and therefore, should not be the primary broadleaf herbicide. Aim and Cobra may increase postemergence control of pigweed. Harvade may improve sicklepod activity. However, glyphosate is already an excellent, non-residual pigweed and sicklepod herbicide. Again, the most important aspect to post-directed, layby applications is the use of a residual herbicide.

This tour stop will also highlight Roundup Ready Flex Cotton and Liberty Link Cotton. Both systems are providing good weed control. The Liberty system should be available in 2004 and the Flex system by 2006.

Editor's note: For more information, go to:

Andy Kendig is an Extension weed specialist at the University of Missouri Delta Center.

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