Update from the Field: Path of Least Resistance
Posted by David Dyson, Agronomist on March 19, 2018
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The last couple of years we have been inundated with horror stories of herbicide-resistant weeds taking over the countryside. Some of us have had the pleasure of fighting nasty, resistant weeds from burndown to harvest. Some of the weeds that have been documented to have some level of herbicide resistance include Palmer amaranth (Figure 1), waterhemp (Figure 2), lambsquarter (Figure 3), marestail (Figure 4), and giant ragweed (Figure 5). First, we have to make a correct identification in order to determine what weed we are dealing with.
Palmer amaranth and waterhemp are very similar in appearance. Both are in the pigweed family of weeds, so their structure will be the same. Palmer amaranth is a new weed to the Midwest. Most of the areas surrounding The Andersons’ Farm Centers are not affected by this invasive weed, but we need to be on the lookout for it. Palmer amaranth and waterhemp will be completely hairless, which will set it apart from redroot pigweed. The difference between Palmer amaranth and waterhemp will be the petiole length. The petiole is the slender stalk by which a leaf is attached to the main stem. Palmer amaranth’s petiole will be longer than its leaf. Waterhemp’s petiole, on the other hand, will be shorter than its leaf. The difference is minor and means little. Both weeds can be Roundup resistant, and will appear later in the growing season.
Lambsquarter is a weed with leaves that are slightly toothed and coated with a white substance, especially on new leaves. The white substance will give this plant a silvery shimmer around the growing point where the new leaves are emerging. Lambsquarter has been documented to be resistant to the triazine chemical family, which includes sencor, atrazine, and simazine. Post-emergent applications can be a little tricky, as lambsquarter can produce a protective waxy layer over the leaf surface during hot or dry conditions. This will prevent most foliar herbicides from entering into the leaf. The use of OverPass® SF can help penetrate the waxy cuticle and allow the herbicide into the plant.
Marestail, or some call it horsetail, can be easily burned down. Beware though: if you don’t kill it off during your burndown application, you will fight the same plant all season. Marestail’s roots have an amazing ability to regenerate. If any part of the root system survives an herbicide application, the roots will grow multiple main stems. A bush is a lot harder to kill than a single stem plant. Hitting this plant early and hard is the only way to prevent a big problem later in the growing season.
Giant ragweed has been a problem in the Midwest for as long as I can remember. Stalk-boring insects are not helping the situation. Once a European corn borer or a stalk borer enters inside this plant and hollows out the main stem, any herbicide application will be useless. Ensuring the diameter of a giant ragweed never gets bigger than a school pencil will help prevent the insect from entering this weed.
In conclusion, alternating seed genetics will help with weed resistance due to the changing herbicides being applied to fields. If we can use multiple modes of action whenever we apply herbicides, resistance will take longer to get a foothold. Starting out with a clean field prior to planting is key to reducing problems with resistant weeds during season.
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Figure 1: Palmer amaranth
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Figure 2: Waterhemp
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Figure 3: Lambsquarter
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Figure 4: Marestail
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Figure 5: Giant ragweed |
