Resistant Weed Species: An Overview and Strategies for Immediate Action (Regarding Palmer Amaranth)
In recent years, the invasive weed species Palmer amaranth has become a significant concern for farmers in Ohio. This pigweed variant, originally native to the southwestern United States and northern Mexico, has caused substantial losses in crop yield and farm income, as well as a permanent increase in the cost of herbicide programs.
The weed is more widespread in several areas, making it crucial to prevent additional infestations. The Ohio State University (OSU) weed science program is leading the charge, with a primary goal of stopping the spread of Palmer amaranth in the state.
To combat this resilient weed, it's essential to apply postemergence herbicides when Palmer plants are less than 3 inches tall. However, Palmer amaranth has a broad period of emergence, ranging from April to August, making timely intervention a constant challenge.
The small, well-adapted seeds of Palmer amaranth are particularly problematic, as they thrive in minimum and no-tillage conditions. The presence of Palmer seed in cotton-derived feed products transported from the South into Ohio can lead to the introduction of Palmer amaranth. To mitigate this risk, it's advisable to avoid the use of cotton feed products or hay that might contain Palmer amaranth seed.
Female Palmer plants are prolific seed producers, producing 100,000 to upwards of 500,000 seeds. This rapid reproduction, coupled with a dioecious reproductive system that encourages obligate outcrossing, results in rapid spread of herbicide resistance.
Most populations of Palmer amaranth in Ohio are resistant to glyphosate (group 9) and ALS inhibitors (group 2), making diversification of herbicide programs and preventing escapees from going to seed essential to prevent the development of resistance to additional sites of action.
In addition, populations in the South have developed resistance to site 14 herbicides (fomesafen, Cobra, etc.), and are appearing to develop resistance to glufosinate (Liberty, Cheetah, Interline).
To combat the spread of Palmer amaranth, it's important to scout recently seeded CREP, wildlife, and similar areas for the presence of the weed. Scouting should also begin in mid-July for the presence of Palmer that may have escaped herbicide programs.
When Palmer amaranth is detected, it's crucial to remove the plants promptly. Plants without mature seed should be pulled out or cut off just below soil and removed from the field, and then burned or buried at least a foot deep or composted. Plants with mature seed should be bagged and removed from the field.
The Ohio Department of Agriculture (ODA) will test seed lots for the presence of Palmer seed, and they must pick it up from your operation. Movement of equipment from Palmer-infested areas in Ohio can contribute to the spread of Palmer amaranth, so it's important to be aware of this risk when purchasing used equipment, especially if it comes from Palmer-infested areas.
Most counties shown as "infested" on the map have only a few populations of Palmer amaranth, with some cases of complete remediation. This highlights the importance of vigilance and proactive measures in the fight against this invasive species.
Including residual herbicides in corn and soybean programs can help control the early emerging Palmer plants. By taking these steps, farmers can help protect their crops, their income, and the overall health of Ohio's agricultural industry.
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