Herbicide resistance seems to be all that I talk and write about lately, and there is a new resistant weed to discuss.
The University of Arkansas has a testing program that helps growers and consultants determine why weed control efforts have failed. Plants can be harvested and sent in or seed can be harvested in the fall for testing.
Most testing is done by Jason Norsworthy or Nilda Burgos in Fayetteville. This year, Jason tested around 30 barnyardgrass samples sent in as a result of control failures.
One tested positive for resistance to clomazone (Command) herbicide. The population was two to three times as tolerant to clomazone as the susceptible population that was tested. The biotype is also resistant to propanil.
Since its introduction, Command has rapidly became the foundation of many weed control programs for Arkansas rice. For the money, it is hard to beat the level of grass control that we get from a single application of Command. There are concerns over crop injury, but once a rate is established for a given soil type, the injury is usually manageable.
Thus far, no real resistance problems have been encountered, making it an excellent resistance management tool.
There are two major challenges in a Command-based program. One, it needs rainfall or a flush for activation. Without that you might as well save your money for postemergence grass control products. Two, Command is weak at the rates used in rice on most broadleaf weeds and is especially weak on sedges.
This second weakness on the sedges and broadleaves is the focus of many of our protocols in the rice plots this year. We are evaluating two new numbered compounds, as well as some established products that can be used as tank-mix partners or possibly in a pre-mix with Command.
Some of them look quite promising; however, loading up the weed control budget on a pre-emergence treatment also means that you have to flush, or you will not get the benefit and will end up with a more expensive program by the end of the year.
Since the introduction of the new 3ME formulation of Command herbicide, drift has not been a problem. In fact, some of the tank-mix data that I have seen indicates that Command actually reduces the drift potential of its tank-mix partners.
In addition to the Command 24C label for aerial application in Arkansas, we now have multiple other products with their own 24C labels for tank mixtures with Command by air. I believe we should do away with these labels and that Command should be granted a full Section 3 label for aerial applications with any tank mix that is labeled for rice. We currently have other products causing more drift concerns than Command and no Section 24c is required for them.
The ability to put Command out by air has increased its popularity as a rice herbicide in Arkansas and other states. It is no surprise that a population of clomazone-resistant barnyardgrass has been found. Anytime you use the same herbicide on a large acreage, you are applying selection pressure on your weed populations.
I expect this will be a manageable problem. One reason is that we typically use at least two modes of action on most rice fields in Arkansas during the course of the year. In addition, we now have several options for herbicides with differing modes of action for barnyardgrass in rice.
If you are currently relying solely on one herbicide for weed control in any production system, it may be time to consider herbicide rotation along with all your other management decisions.
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