Weeds

The volume of inquiries about how to control large (taller than 12 inches) horseweed (a.k.a. marestail) and waterhemp in soybean has remained consistent over the past 10 days.  The answer can be summarized as follows: there are NO postemergence herbicides that will consistently control these very large weeds in soybean, especially if these weeds are resistant to glyphosate.
Glyphosate-resistant waterhemp can be controlled by foliar-applied PPO inhibitors (such as lactofen (Cobra), fomesafen (Flexstar) or acifluorfen (Ultra Blazer)) in conventional or glyphosate-resistant soybean varieties,…

We would like to take this opportunity to once again extend the invitation to attend the 2013 University of Illinois Weed Science Field Day, to be held Wednesday, June 26th at the University of Illinois Crop Sciences Research and Education Center, located immediately south of the main campus.  Coffee and refreshments will be available under the shade trees near the Seed House beginning at 8:00 a.m.  The tour will provide ample opportunity to look at research plots and interact with weed science faculty,…

We are pleased to announce that, with continued financial support from the Illinois Soybean Association, we are offering free screening for herbicide resistance in waterhemp during the 2013 growing season.  To date, over 1000 plant samples have been screened through this collaborative effort between the weed science program at the University of Illinois and the Illinois Soybean Association.  This valuable service helps farmers and researchers better track and understand the evolution of herbicide resistance in waterhemp across the state.…

Palmer amaranth (Amaranthus palmeri) is a summer annual broadleaf weed species taxonomically related to other pigweed species (waterhemp, smooth, redroot) common in Illinois agronomic cropping systems.  Palmer amaranth is not indigenous to Illinois, but rather evolved as a desert-dwelling species in the southwestern United States including areas of the Sonoran Desert.  Genotypic and phenotypic adaptability have allowed Palmer amaranth to expand its distribution and colonize the vastly different agricultural landscapes across much of the eastern half of the United States,…

The past several days have experienced a noticeable increase in questions about options to control marestail after crop emergence.  Many have reported poor marestail control from herbicides applied prior to planting (primarily no-till soybean), especially when these burndown applications contained only glyphosate or glyphosate and 2,4-D.  The increasing frequency of glyphosate-resistant marestail populations, the rush to plant whenever field conditions were conducive, and the less-than-ideal environmental conditions when many burndown applications were made, have contributed to a challenging situation for which a good solution might not be available. …

Earlier this month, we discussed advantages of and techniques for controlling emerged weed vegetation prior to planting.  Several winter annual weed species have reached maturity while others, such as cressleaf groundsel (Figure 1),
began to flower within the past 10 days across areas of central Illinois.  Control of these mature plants with herbicides or single passes of tillage equipment will continue to be challenging.
Preplant control is also needed for emerged summer annual grass and broadleaf weed species. …

A potential scenario of planting delays will be corn planted in fields where no soil-residual herbicide was applied.  If the corn has not yet emerged, the soil-residual herbicide can be applied as originally planned.  But, what if the corn has emerged and the soil-residual herbicide has not been applied?  Can the application proceed as planned, or will a different product need to be selected?  The answer depends on the respective herbicide.
Most, but not all, soil residual herbicides can be applied after corn has emerged. …