It’s time to fight resistant pigweed


Using residual herbicides adds another step to your management programs. You will have to check rotation intervals, pH, organic matter, and rates to name a few variables. 
Rotation intervals must be considered when you have a crop rotation plan on your fields. If you have crop failure and choose to replant to a different crop, you must check the label. The label is the law.

The use of full-labeled rates are important to future weed control programs. I hear a lot about “set up” or “half rates” of residual herbicides that will provide short-term weed control suppression prior to the application of postemergence herbicide. Using full-labeled rates generally controls weeds longer into the season. If you are not willing to use full rates on the entire farm, experiment on a few acres first. I think you will be surprised by the difference in the length of weed control throughout the season.

Once the residual herbicide is applied, Mother Nature takes over. The more uniform the application or incorporation of your residual herbicide, the better weed control you can expect. However, if the herbicide is to be effective, it must be available for uptake by the weed. Residual herbicides must be incorporated by rain or mechanically. Consult the label and your local agronomist to check if your residual herbicide should be mechanically incorporated.

In-season soybean weed control has dramatically changed since the evolution of glyphosate-resistant pigweed (waterhemp or Palmer) on your farm. The use of PPO mode-of-action herbicides (i.e. Flexstar, Cobra, or Ultra Blazer, etc.) has dramatically increased. 

We also have Liberty Link crops available as an alternative to Roundup-Ready crops.
Glyphosate translocates throughout the plant to provide control with reduced gallons of water applied per acre. However, since contact herbicides are not translocated, it is critical to get good coverage of the weeds. Both the post-PPO and Liberty herbicides are contact herbicides, which require the use of proper tips, reduced sprayer speed, proper adjuvants, and increased water volume to achieve acceptable weed control.

Regardless of crop technology, application height is critical. Weeds should be targeted before they reach four inches tall. After this height, effective control dramatically decreases. Keep in mind that pigweed can grow as much as one inch per day. With this type of growth rate, you have only a couple of days to make a postemergence application.

In-season application of residuals will provide weed control longer into the season. I recommend applying another residual herbicide to the soil prior to the preemergence herbicide playing out. In essence, I would like to see you apply early post to bare ground. The best way to fight pigweeds is to not allow them to emerge.

The use of residuals is a must—we are running out of options for postemergence control of pigweed. The use of residual herbicides preemergence and postemergence must become the new “normal” to control pigweed.

One of the all-too-common phrases I hear is, “I thought I could get by one more year.” I hope you don’t catch yourself saying this in July.

Dr. Jason Weirich is director of agronomy for MFA Incorporated.

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