No. 15 Article 4/July 7, 2006

Instances of Inconsistent Weed Control with Glyphosate

During the past week, there has been a noticeable increase in callers reporting "missing" weeds with an initial postemergence application of glyphosate in soybean. The species most commonly mentioned include waterhemp, horseweed (marestail), giant ragweed, common ragweed, and common lambsquarters. We have observed a similar "decreased performance" from glyphosate-containing products for each of these species during past seasons (lambsquarters in 2001 and 2005, horseweed in 2003, giant ragweed in 2004, etc.), but it seems that inconsistent weed control with glyphosate may be as widespread in 2006 as in any previous season.

What factors might be contributing to these weed escapes? If the reason were clear-cut in each situation, finding a solution would be (relatively) easier. Unfortunately, it seems that myriad factors might be contributing incremental portions to this problem. For example, weed control practitioners have known for years that common lambsquarters can be more difficult to control with glyphosate than other broadleaf weed species, particularly under dry/drought conditions. Waterhemp is a species with multiple emergence events, so there typically exists a wide range of plant sizes in fields when the postemergence application is made. Obviously, larger weeds can be expected to be more difficult to control because they are older plants and also because spray coverage can be limited. Application rate, volume, and spray additives are important factors to keep in mind, especially if you are attempting to achieve good spray coverage on larger weeds.

Similar to previous seasons, insect feeding within the stem tissue of various weed species has been very noticeable in some areas of Illinois and may have contributed to poor control following the application of a postemergence herbicide. Weed species that harbor these insects included waterhemp, giant ragweed, horseweed/marestail, annual smartweed species, common ragweed, and common lambsquarters. Researchers have identified insects in the Lepidoptera (Papaipema nebris, Ostrinia nubilalis, Epiblema spp.) and Coleoptera (Rhodibaenus tredecimpunctatus, Lixus spp., Dectus spp., Hippopsis lemniscata) orders present in these weed species as either larva or adults. Insect tunneling is most frequently observed in stems of large weeds (plants 6 or more inches high) compared with stems of smaller weeds.

Past, present, and near-future weather conditions can influence herbicide performance by affecting how much herbicide enters the plant and, to some extent, how extensively the herbicide translocates within the plant following absorption. Dry soils coupled with hot, low-humidity days tend to reduce the amount of herbicide absorbed by plants. In contrast, weeds growing with adequate soil moisture typically absorb applied herbicides faster and often more thoroughly.

What about the possibility that some of these weed escapes may suggest the occurrence of a weed population with resistance to glyphosate? Since the commercialization of glyphosate-resistant crops, the question of whether glyphosate-resistant weeds will or will not be selected has been extensively bantered around by individuals involved in virtually all phases of production agriculture. Those in academia have generally agreed on at least two points: (1) the potential for selecting weed biotypes resistant to glyphosate is less than that associated with selecting biotypes resistant to other herbicide families, and (2) never say it will never happen. Whichever position you might have taken on this question during the early years of glyphosate use in-crop, some facts to consider include these: glyphosate-resistant weed populations have been selected in the United States and in several other countries of the world; these resistant populations represent more than one weed species; several states, from the Midwest to the East Coast, have reported instances of glyphosate-resistant weeds; and we have no evidence to suggest that Illinois will be immune to this phenomenon.

Researchers at the University of Missouri have recently reported two populations of waterhemp that have consistently survived after glyphosate applications under field and greenhouse conditions. News releases from Missouri reported some of these waterhemp plants survived up to 6 lb acid equivalent glyphosate (a rate approximately equivalent to 170 fluid ounces of Roundup Original Max). If glyphosate-resistant weeds such as waterhemp can occur in other states, it seems likely that resistant weeds can be selected in Illinois.--Aaron Hager and Dawn Nordby

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