The growing season seems to be accelerating with crops benefitting from warmer temperatures than experienced in early May. That same growth spurt is true of weeds. While we all hope for a weed-free field, remember that performance of herbicides is affected by weather conditions and several additional factors.
Soil-applied herbicides can be influenced by soil pH, soil texture, organic matter content, tillage and moisture. Farmers can have some influence on a soil pH and tillage, and can make rate adjustments for soil texture and organic matter content. But what about soil moisture?
Depending upon the herbicide chemistry, soil applied herbicides require varying amounts of rainfall to attain ideal activity. A general rule often used is 0.5 to 1 inch of rainfall is needed for herbicide activation.
Adequate soil moisture is important because residual herbicide uptake occurs mostly through soil solution uptake by the root system. If pre-emergent herbicides are applied under dry conditions more of the herbicide may be bound by soil colloids — and less in the soil solution — decreasing herbicide efficacy.
I have also noted an abundance of plant material this year in several no-till fields prior to the burndown application. If the soil applied herbicide was tank-mixed with the burndown we may see decreased efficacy in that situation.
To simplify the previous statement, lack of adequate rainfall on our pre-emergent herbicides will likely decrease their efficacy. For limited Illinois acres irrigation is an option, incorporating the herbicide by soil moisture for some can improve the issue. But for many, we are dependent on the weather.
So, what’s a farmer to do? After all, most are familiar with the adage, “Everybody talks about the weather, but nobody does anything about it,” attributed to Mark Twain and Charles Dudley Warner.
But as with most management decisions in agriculture, knowledge is power. Knowing the dry weather pattern that many experienced so far — at least in northeastern Illinois — farmers should recognize the urgency in scouting planted fields and evaluating weed pressure.
I’m certain there will be several fields that will have post-herbicide application dates moved earlier to control emerged weeds that near the maximum size recommendations for control on post-emergent herbicide labels.
How can we get the most out of our herbicide applications? This list has been shared by the Take Action Pesticide Resource Management website, iwilltakeaction.com/about-take-action.
• Know the weeds you are trying to control.
• For post-emergence herbicides, scout your fields — I would argue this could be equally important on no-till burndown situations.
• Apply post-emergence herbicides at the proper time — and on weeds properly sized.
• Follow label requirements to make sure you use the proper application techniques.
• Monitor the weather.
Keep on top of weed management in 2021. In a year of high commodity prices, less weeds will equal more marketable bushels.
Russ Higgins is a University of Illinois Extension commercial agriculture educator.