Mix it up
Successful long-term weed control depends on using all available methods, rather than just one repeatedly, scientists say.
“Mix it up,” Rick Boydston, weed scientist with the U.S. Department of Agriculture in Prosser, Washington, urged during a recent weed management workshop in Washington State. “Don’t use any method over and over until it fails. Mix it up to prevent resistance from developing.”
An orchard or vineyard typically has a mixture of weed species, but if some portion of that mix is tolerant or resistant to a weed control strategy that is used repeatedly, there will be a shift in the dominant species.
Scientists stress that chemicals should be used as part of an integrated program that might include other methods such as:
- mechanical (cultivation, flaming, mowing and mulches)
- cultural (screening irrigation water, cleaning field equipment, controlling weeds around the edges of the orchard or vineyard, and planting weed-free cover crops between rows)
- biological (releasing organisms, such as insects that inhibit growth or seed production).
Eliminating production of weed seeds is the key to successful weed management. Use cultivation, mowing, or herbicides with different modes of actions to prevent the weed from flowering and producing seed.
Prevention is the most effective and economical way to reduce the threat of glyphosate-resistant weeds. When using chemicals, combine an herbicide that has soil residual activity with glyphosate (which does not) or with another postemergence herbicide to extend the period of weed control and reduce the need for multiple applications of glyphosate, advises Ed Peachey, weed scientist with Oregon State University in Corvallis.
If resistance to glyphosate has not developed, use preemergence treatments followed by a tank mix of postemergence products. Also consider using other nonselective herbicides, such as glufosinate or paraquat with PPO inhibitors such as Aim (carfentrazone), Goal (oxyfluorfen), and Treevix (saflufenacil) for burndown control.
To delay resistance, use high glyphosate rates. Resistance to glyphosate is likely to be caused by several mechanisms in the plant and not just one genetic mutation, so it is important to use a full label rate to delay resistance. This is unlike other situations where reduced rates might be recommended in order to reduce selection pressure.
If weeds have become resistant to glyphosate, growers can continue to use glyphosate but should tank mix it with other herbicides that are effective on the resistant weeds and should target weeds when they are small and easier to control.