Delaying decisions on weed control may be a factor in losing ground in preventing herbicide resistant weeds. Relying on glyphosate has been convenient for putting off crop selection for a particular field until just before the planter rolled into it.
That reliance, however, has put extremely high selection pressure on weed species like Palmer amaranth. After many years of relying on a single chemistry for weed control, significant weed populations of resistant weeds have developed. Currently in the United States, 145 or so weeds have been identified as herbicide resistant. Globally 404 unique cases of herbicide resistant weeds have been observed.
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Growers can do several things to reduce or prevent development of herbicide resistant weeds. The first is applying residual herbicides with multiple modes of action. Unfortunately, that may force a grower into locking in a crop planting decision, but it is an important first step in managing weed resistance. Possibilities include:
- Using individual herbicides allows multiple modes of action with premix or tank-mixed herbicides.
- Rotating from one crop to another will also allow for rotation to herbicides with different modes of action for a particular field.
- Ensuring that we plant into weed free fields is essential; be that done with herbicide or cultivation.
- Although cultivation is often important to successful crop establishment, this mechanical form of weed control can also aid in efforts to prevent resistance.
- Scouting fields and surrounding boarders/turn-rows for weeds can help in weed management efforts.
- Catching young, actively growing weeds will improve whatever control option is selected and prevent a new distribution of weed seed.
As always, the herbicide label is the law. Not only should we always follow the label for application rates, but we should also follow suggested amounts of carrier to apply herbicides and include any recommended adjuvants.
Relying on a single mode of action herbicide may very well be a low-cost, flexible weed management option, but continued reliance on this type of system will guarantee that a neighbor’s problem or the problem of a grower a few counties away will soon be your problem.
Palmer amaranth produces up to one million seeds per plant, so it doesn’t take long for a single resistant weed to cover a field with other resistant weeds. As planting time approaches, growers should begin making crop selection decisions and applying appropriate pre-emergent herbicides.