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Corn+Soybean Digest

Ragweed: Where There’s A Will, There’s A Way

After a long winter’s nap, many Corn Belt farmers may prefer to stay in bed than to fight the perennial battle with common ragweed. You’ve tried about everything and in every combination and with every potential timing variation. And it just won’t go away. Year after year it’s there, and the more glyphosate that is used, the harder they are to kill. Well, just like any general on the battlefield, you have to know your enemy better than you know your own troops.

Ragweed is an annual that multiplies by seed, and each plant can produce 32,000-62,000 seeds if they are allowed to grow and mature. Knowing your frustration with ragweed, the weed specialists from all of the Corn Belt universities merged to produce an electronic guide to controlling common ragweed. And they suggest knowing how they germinate, which requires the seed to go through a dormant period during cold temperatures before germinating in April and May. Warm temperatures help them germinate, but a hot spell will require the seed to go through the cold snap again. Those that germinate are near the soil surface, which allows a magic combination of light and temperature to start the germination process.

Ragweed causes the most trouble in no-till soybeans, and can compete with any crop if seeds germinate at the same time as the crop. However, a crop that is weed free for two to four weeks after emergence will suffer little yield loss from common ragweed. Researchers report that four ragweed in 30 ft. of row will reduce soybean yields up to 10%. Later-emerging ragweed may not cut crop yields much, but will contribute to the seed bank.

If you are finding increased pressure from ragweed, it is probably because of herbicide resistance or from stem-boring insects, which disrupt the action of herbicides like glyphosate. The insects may injure the plants vascular system, but not enough to kill, but certainly more than enough to neutralize many herbicide applications.

By 1996, many ragweed populations had developed resistance to both ALS inhibitors and to PPO inhibitors before glyphosate had a chance to allow the development of resistance from overuse, just like the others. The use of glyphosate slowed the ragweed resistance to ALS inhibitors, but that continued in fields of non-biotech beans. Glyphosate resistance has occurred in several Corn Belt states.

• So what can be done to control common ragweed? The most effective herbicide programs combine pre- (PRE) and postherbicide (POST) treatments, and two or more herbicide modes of action, especially in soybeans. Multiple POST treatments with the same herbicide create the opportunity for herbicide resistance.

• Control weeds that emerge prior to planting with tillage or PRE burndown herbicide applications.

• Apply pre-emergence herbicides with activity on common ragweed to reduce competition with crops, provide flexibility in the timing of POST herbicides and minimize the need for a second POST glyphosate application.

• Apply POST herbicides before ragweed plants exceed 4-6 in.

• Scout fields two weeks after the first POST application. Control escapes or plants that emerge after the initial POST application with a second POST application. Where needed, make the second POST application three to four weeks after the first, before plants grow too large.

1) Controlling emerged weeds at corn planting: Combine atrazine with 2,4-D ester, dicamba, glyphosate or paraquat. If only glyphosate is used, increase the rate where plants are more than 6 in. tall, and where resistance is suspected, combine glyphosate with other herbicides.

2) PRE (residual) corn herbicides: Moderate to high populations require combinations of PRE and POST herbicides. Include additional atrazine to improve control, but do not exceed the maximum rates.

3) POST corn herbicides: Many POST broadleaf herbicides effectively control emerged common ragweed in corn. In areas where there are known ALS-resistant populations, use an herbicide with a different mode of action, or combine ALS-inhibiting herbicides with herbicides that have a different mode of action. In a total POST program, it is essential to apply herbicides while weeds are less than 4 in.

4) Control of emerged weeds at soybean planting: Combine 2,4-D ester with either glyphosate or paraquat. Adding a product that contains chlorimuron.

5) PRE (residual) soybean herbicides: PRE applications of Authority First, Canopy, Envive, FirstRate, Gangster, Scepter, Sonic, Synchrony XP or Valor XLT can provide partial to adequate residual ragweed control. None of these herbicides is likely to provide adequate season long control of dense populations, and they will not adequately control ALS-resistant populations.

6) POST soybean herbicides: POST herbicides are most effective in fields that have been previously treated with PRE herbicides. Applying POST herbicides when common ragweed plants are less than 6 in. tall minimizes early season interference.

7) In fields with poor glyphosate performance, use an alternative or mix it with full rates of an alternative.

8) In fields with ALS resistant ragweed and non-biotech soybeans, rotate fields with ALS-resistant common ragweed to corn and wheat when possible. Control weeds in those crops to prevent further increases in the soil seed bank.

Common ragweed is a formidable foe for many farmers because of its growing resistance to herbicides, and it even develops resistance with the help of insects. Ragweed can be controlled with herbicide applications when weeds are under the 4-inch mark. Treatment applications that are recommended include a preplant burndown, postemergence applications at least twice with alternating herbicides.

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