By Drew Lyon

Since the introduction of Roundup Ready crops in the mid- to late-1990s, postemergence herbicides have played an increasing role in weed control for many crops including corn, soybean, cotton, and wheat.

The introduction of Maverick herbicide (now sold as Outrider) in 2001 changed how wheat growers managed downy brome (a.k.a. cheatgrass) and other annual brome species in wheat. Maverick (now Outrider), and subsequently several other ALS-inhibiting herbicides including Osprey, Olympus, and PowerFlex HL, selectively control downy brome and other annual grass weeds in wheat. Growers quickly abandoned the use of preemergence herbicides, with their variable use rates dependent on soil type and their efficacy often dependent on timely rain, for the convenience and reliability of postemergence herbicides. However, our overuse of these products has resulted in widespread herbicide resistance to many postemergence herbicides.

The use of preplant and preemergence herbicides in U.S. soybean production increased from 25% to 70% from 2000 to 2015, largely in response to the increase in glyphosate-resistant weeds. In Eastern Washington, resistance to postemergence herbicides is now common in Italian ryegrass and downy brome populations. CoAXium wheat, which is resistant to quizalofop, the active ingredient in Aggressor herbicide, is a promising technology for postemergence control of downy brome, jointed goatgrass, and feral rye in wheat. However, overuse of this new technology will surely lead to resistance issues, as discussed in the Timely Topic titled, “A Word of Caution About Two New Weed Control Technologies”.

Particularly for downy brome control, where we know biotypes resistant to quizalofop have been identified in the Pacific Northwest, I strongly encourage growers to use a preemergence herbicide like Zidua or Anthem Flex with a postemergence application of Aggressor herbicide in CoAXium wheat. This approach will improve weed control and extend the length of time that these herbicides will remain effective.

Many wheat growers in the Palouse have incorporated Roundup Ready spring canola into their crop rotations to aid in the control of Italian ryegrass. This has been a helpful tool in the management of Italian ryegrass, but we know that glyphosate-resistant Italian ryegrass biotypes have been selected in orchard systems in the PNW, so relying solely on glyphosate for the control of Italian ryegrass in spring canola, as discussed in this previous Timely Topic titled, “Canola and Italian Ryegrass Control”, is a risky bet. We are currently looking at systems that incorporate a preemergence herbicide into a glyphosate/glufosinate-resistant spring canola system. The attached picture was taken just after canola emergence but before any postemergence herbicide applications were made. You can pick out every plot that did not have a preemergence herbicide applied by the green color due to the presence of Italian ryegrass.

Preemergence herbicides provide very early weed control, and it is early weed competition that often has the greatest impact on crop yield. The reduction in weed density resulting from the use of preemergence herbicides also reduces the selection pressure for resistance evolution from subsequent postemergence herbicide applications. Preemergence herbicides often have a different mechanism of action than postemergence herbicides, which, as discussed in this previous Weeders of the West blog post titled, “I Can’t Say This Enough!”, also reduces the risk for the development of herbicide resistance.

Due to the popularity of postemergence herbicides over the past two or three decades, a lot of people have forgotten or never learned how to optimize the performance of soil-applied herbicides. Several factors including soil texture, soil organic matter content, soil pH, cation exchange capacity (CEC), soil moisture, soil temperature, soil disturbance by tillage, crop residue type, abundance, and distribution all can affect herbicide efficacy. Growers need to be aware of these factors when using preemergence herbicides.

As resistance to postemergence herbicides continues to increase, growers should consider incorporating preemergence herbicides into their weed control programs. Doing so will not only improve weed control, but it will also extend the useful life of the few effective herbicides that we still have.

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