Weed Management: Wet Year = Weed Control Challenges

What to do when Mother Nature messes up your soybean weed control program …

In spite of the most modern technology and best high-yield production practices, too much rain and/or rain at the wrong time made getting optimal weed control challenging if not impossible this year. Too little moisture can negatively impact the performance of soil applied residual herbicides by not activating them before weeds germinate and emerge. Too much moisture is a problem as well because the herbicide may be moved too deep into the soil to prevent shallow germinating seeds (like waterhemp) from growing.

An even bigger problem of wet soils and excessive or prolonged rainy periods is that they can keep you from making timely postemergence herbicide applications. It is important for ALL herbicide programs to treat weeds when they are actively growing and in the target size range for the rate and product you are applying. With some herbicides and some weeds you can increase the rate to get larger weeds, but at some point the weeds become too large to control or your odds of getting complete control start to decrease significantly.

In extreme cases there may be nothing you can do to achieve the level of weed control you hoped to have. But these situations are rare and good weed management practices can reduce the odds of this occurring and improve your final result when it does. Following are a list of good weed management practices that will help you “weather proof” your weed control program.

1. ALWAYS start clean with a good burndown or effective tillage. In some cases a fall burndown should be considered. Use of a fall burndown may not always eliminate the need for a spring burndown, but it will prevent the “jungle look” that some no-till fields have in the spring. Tillage escapes can be a major issue as well. Weeds that are damaged but not controlled by tillage will be very large and very tough to control later in the season. To prevent this a burndown prior to doing tillage may be required in some cases.

2. Use a good soil-applied residual herbicide with activity on the primary broadleaf weed problems in your field. In most cases that would be waterhemp. And in most cases these products perform better when applied just before or after planting. Consult the herbicide label and your herbicide supplier to determine the best application timing for your situation.

3. Be timely with your POST herbicide applications. In recent years growers have favored waiting until the crop canopy is developed and most weeds have emerged, which usually means you have some very large weeds and a very dense canopy that makes getting good spray coverage difficult. All herbicides perform better on smaller weeds and some contact-type herbicides can only control small weeds. The main cause of herbicide failure is waiting until weeds are too large to control.

4. If you know you are dealing with resistant weeds, using tank mixtures of products with multiple modes of action may be necessary. Be sure you know the target weed size for all the products you are applying. If you are dealing with glyphosate resistant weeds such as waterhemp there are no products currently on the market that will give consistent, complete control of waterhemp larger than 6” tall.

5. Since we are spraying smaller weeds and a less developed crop canopy that means we are treating earlier in the growing season and it is more likely that a second flush of weeds could emerge. You can deal with this by making a second POST application about 3 weeks later OR a more cost effective approach may be to apply additional residual herbicide with your POST application to lengthen the soil residual control until the crop canopy has fully developed.

6. It is important to know what species of weeds you are dealing with and what, if any, levels of weed resistance exist in your fields. Virtually every waterhemp plant has been ALS resistant for years. Glyphosate and PPO resistance are growing problems. In some cases weeds are cross resistant to both glyphosate and PPO herbicides—these are the hardest to control. If you have a lot of waterhemp that is cross resistant to both, you are relying on soil residuals and tillage to control them.

7. When dealing with resistant weeds, the concept of “overlapping residuals” becomes important. Most soil applied herbicides will give you 3 – 4 weeks of residual control. As the herbicide degrades, at some point the level in the soil drops below the amount necessary to prevent weed growth. Before this happens additional residual herbicide should be applied to extend the length of control until the crop canopy is sufficient to prevent weed growth.

Even if soil moisture conditions are not ideal, planting into a clean field, applying PRE and POST residuals, controlling weeds before they reach 6” in height and using appropriate tank mixtures will result in good weed control in the vast majority of cases—even if you have a lot of resistant weeds. However, you may learn that you need to add in a fall application or two residuals, early preplant and at planting, to get adequate levels of control. New options are coming that will give us more and better tools to fight weed resistance, but even when those options are fully approved and available we do not want to abandon the good weed management practices we have discussed here.

Lance Tarochione is a technical agronomist with Asgrow/DEKALB in west central Illinois. His work has focused on crop production, research and product development, and through his role at Monsanto he currently supports the Asgrow and DEKALB brands in seven counties in western Illinois.