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Dawn Nordby Dawn Nordby
Extension Specialist - Weed Science
Department of Crop Sciences
(217) 333-4424
Aaron Hager Aaron Hager
Associate Professor of Weed Science
Department of Crop Sciences
(217) 333-4424
Patrick Tranel Patrick Tranel
Associate Professor of Molecular Weed Science
Department of Crop Sciences
(217) 333-1531

As If You Didn't Have Enough Weed Problems Already

Following the commercialization of glyphosate-resistant crops, the choice of which herbicide to use for controlling weeds in corn and soybean has been relatively "simple" for many farmers. However, a few weed species have become more prevalent with the adoption of these technologies, including waterhemp, lambsquarters, giant ragweed, and horseweed (marestail). Although only one of these weed species (horseweed) has been confirmed to be resistant to glyphosate, the consistency of control of the others (waterhemp, lambsquarters, and giant ragweed) has not been as great in recent years as during the "early years" of glyphosate-resistant crops.

Waterhemp populations in Illinois already have been identified with resistance to three types of herbicide chemistry (ALS, PPO, and triazine). Indeed, at least one Illinois waterhemp population demonstrates resistance to all three of these herbicide families. In this particular situation, only one option (glyphosate) remains for postemergence control in soybean. The use of a soil-applied herbicide (flumioxazin, pendimethalin, trifluralin, etc.) can be very beneficial in decreasing waterhemp populations, thus giving the crop an early competitive advantage and help toward shading out late-emerging waterhemp.

Lambsquarters is becoming increasingly prevalent throughout the state, and (perhaps) in more than one "form": there are numerous species of Chenopodium (the genus comprising the lambsquarters species) in Illinois. There have been many reports of inconsistent lambsquarters control with postemergence applications of glyphosate, with many of these being attributed to inclement weather (cold or hot spell) prior to herbicide application. However, given the diversity of the Chenopodium complex, we should not be surprised if certain species, subspecies, biotypes, etc. show decreased glyphosate sensitivity.

Giant ragweed has been plaguing many Illinois farmers for years. More recently, it was discovered that an extended emergence pattern has contributed to this weed escaping preemergence applications and subsequently being too tall when postemergence herbicide applications are made to control it (as well as other weeds that may exist in the field). Cloransulam applied preemergence has excellent control of giant ragweed, although growers should be aware that ALS resistant giant ragweed populations will not be controlled. A planned preemergence herbicide in combination with a postemergence herbicide application when giant ragweed are less than 6 inches tall has proven most effective at controlling this species. Because of giant ragweed's biology, it may not be prudent to wait until "all the weeds are up" to spray your post application if this species is present in your field.

Horseweed, otherwise know as marestail, has previously not been a threat to notill acres in Illinois. However, if we look to the east and south, we can see the impact that glyphosate-resistant horseweed has had on growers in these areas. Horseweed is most easily controlled prior to planting. The inclusion of 2,4-D into the burndown program can control horseweed, and also will reduce the likelihood of glyphosate-resistant populations.

Waterhemp Lambsquarters Giant Ragweed Horseweed
Giant Ragweed
Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension
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