ISSUE 9   July 10, 2008


Evaluating weed control studies makes the benefits of preemergence herbicides in managing early-season weed competition and herbicide application timing in soybean clearly visible. In several field trials, we applied an array of preemergence broadleaf herbicides, which were followed by postemergence applications of glyphosate. The intent of these preemergence herbicides was to provide initial suppression of broadleaf weeds so that glyphosate applications could be delayed. The delayed glyphosate applications would then be closer to soybean canopy closure and we hope this will avoid the need for a second glyphosate application. Several of the preemergence herbicides were at reduced rates to keep the treatment costs affordable. The degree of weed control or suppression varied among the herbicides in this trial as expected because some herbicides were inherently more effective on different weeds. Still, the tested herbicides such as acetochlor, Permit, Authority First or Sonic, Authority MTZ, Authority Assist, Gangster, Spartan, Valor, etc. all provided significant weed suppression and allowed glyphosate applications to be delayed. The overall level of weed management will be greater than a single application of glyphosate. In your fields, you should be able fine tune management systems by selecting herbicides to match the weed species in the field. The benefits of this approach may be greater for crops planted in rows as compare to drilled, which canopy sooner.

Beyond the benefit of these two pass programs for improved weed management for this year, this approach should also hold longer-term benefits. Consider your current status of glyphosate-resistant weed development. Expression of weed resistance is a "numbers game", which means that the greater number of weeds sprayed with glyphosate increases the probability that a resistant weed might be found. In terms of our RR corn, soybean, canola, and sugarbeet fields and the high levels of common lambsquarters, common ragweed, and kochia, it is clear that a greater number of these weeds will be sprayed with glyphosate. Therefore, the risk of selecting a glyphosate-resistant biotype (examples of glyphosate-resistant weeds exist in other states) is greatly reduced where the effective preemergence herbicides were used.

Although the option to use preemergence herbicides in soybeans is long gone for this season, it may be worth considering if or where it may have a fit in your management for next season as glyphosate applications are soon to be completed.



While herbicide advertising often talks about full-season weed control, for most weeds we only need herbicides to control weeds until the canopy has developed sufficiently to suppress any late-emerging flushes. Weed survival rates can be greater at earlier emergence dates but biomass and seed production of weeds emerging at mid-season can be suppressed more than 90% compared to weeds emerging with the crop.

Many fields are likely to have greater problems with late-emerging weeds than normal. There are two primary causes for these infestations: 1) dry and cold conditions in May will suppress activation of soil-applied and excess rain in June will reduce the length of control provided by residual herbicides, and 2) poor canopy development due to cool temperatures, saturated soils and reduced stands will provide a favorable environment for weed growth.

Fields should be carefully evaluated prior to spraying weeds in large crops. Weeds that emerge significantly later than the crop are at a competitive disadvantage with the crop due to the cropís head start. While these weeds may survive and produce seed, their impact on the yield should be minimal unless they are thick enough to create a sod. For example, if you have 3 to 4 inch weeds in 30 inch corn it probably doesnít warrant an additional weed control trip. Spraying late season crop may cause more damage to the crop than would be gained by eliminating late-emerging weeds. Most importantly, the majority of herbicide labels prohibit application to larger crop due to problems with crop tolerance or efficacy. Glyphosate has restrictions in RR crops also.

Everyone expects weed-free field at harvest, sometimes it is best simply to live with the hand thatís been dealt. While late-emerging weeds in most fields are unlikely to impact yield, the seed produced by these plants will increase weed densities the following year. This increase in weed populations should be taken into account when developing weed management plans for 2009.

Rich Zollinger
Extension Weed Specialist

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