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I n the effort to stay on top of weed control — and prevent buildup of resistance to herbicides — growers need to use every tool in the toolbox. Those tools include cultural methods, sanitary practices, and use of pre-emergent and postemergent herbicides in a way that combines and rotates different modes of action. Buildup of herbicide resistance can lead to the need for post-emergent applications to control escapes that normally would have been controlled by the pre-emergent products. Also, some problem weeds such as common waterhemp and Palmer amaranth emerge later in the season, missing the window of the pre-emergent soil residual life. “It’s important to understand the biology of the weeds we are trying to control, because the reproductive behavior of certain weeds has facilitated rapid selection for resistance to some of our most effective herbicide tools,” says Dennis Long, National Technical Service and Development Manager for Belchim Crop Protection. “Understanding the importance of combining different modes of action not only helps manage the resistant weed populations during the current crop event, but also reduces the chances of higher selection pressure and deposition of resistant seeds going into the weed seed bank.” In a recent webinar, “Get More Control of Resistant Weeds,” sponsored by Belchim Crop Protection, Long pointed out some of the numbers involved, saying even 95% control, which sounds excellent, isn’t enough. “Consider waterhemp, which can produce 400,000 seeds per plant when in competition with a crop and one million seeds without competition — on a ditch bank, for example,” Long says. “These seeds are tiny and easily dispersed by wind, equipment, animals, and livestock feed. If just 50 of those plants escape, each one can produce 400,000 seeds— that’s 460 seeds per square foot spread over an acre. And if those 50 plants escaped because of resistance to the herbicide you put on, now they can share that genetic trait with the entire population of waterhemp within a mile, because that’s how far the pollen can travel.” Weeds that are dioecious, meaning the male and female flowers are on separate plants, spread genetic traits even faster than selfpollinating plants, Long says. Unfortunately, common waterhemp and Palmer amaranth are both dioecious. A substantial percentage of the seeds of both weeds are viable for several years (even when buried 6 inches deep), so adding to the weed seed bank can cause challenges for future crops.

Make Good Control Even Better

Getting control of that last 5% is key, Long says. Tank mixing herbicides from different mode of action (MOA) groups established by the Global Herbicide Resistance Action Committee (GHRAC) and listed by the Weed Science Society of America (WSSA), makes it much less likely to select a resistant weed biotype. Belchim’s TOUGH® 5EC contains pyridate, a herbicide that targets a unique binding site within the PSII Group of herbicides that make up WSSA Groups 5 and 6. While there are many weeds resistant to Group 5, there is no known crossresistance to pyridate, and that is an important reason for its inclusion in the POST tank mix. Pyridate should be used POST in a tank-mix program with other herbicides effective on the target weeds such as atrazine, dicamba, glyphosate and HPPDs. Pyridate actually synergizes HPPDs such as mesotrione, tolpyralate or topramezone — even controlling HPPD-resistant weeds. Pyridate can be applied up to the eight-leaf corn stage. Results from numerous university trials show it significantly increases control of weeds such as Palmer amaranth, common waterhemp, lambsquarters, and kochia. “Pyridate is actually engineered as a pro-herbicide,” Long says. “It is lipophilic and penetrates through the waxy layer of the leaf. Once inside, the true active ingredient (pyridafol) is released to attach to the target site fast. It synergizes the HPPDs, which kill the weed by reducing its ability to detoxify itself of oxidants naturally generated during photosynthetic process. TOUGH® 5EC works by putting the weed into hyperdrive oxidant production. When you put them together, it’s a synergistic onetwo punch the weed can’t overcome,” Long says.