Rain Causing Herbicide Programs to Change
There are planted corn fields that were not treated with preemergence herbicides before the rain started. The good news here is that most preemergence corn herbicides can be applied to emerged corn, and some of them have enough foliar activity to control small, emerged weeds without the need to include postemergence herbicides. In addition, if corn planted is resistant to glyphosate and/or glufosinate (Ignite), these can be combined with preemergence herbicides to control weeds emerged at the time of application.
An early postemergence application of foliar plus residual herbicides can be just as effective at preventing yield loss due to weed interference, compared with a program of consisting sequential applications of preemergence and postemergence herbicides. However, early postemergence treatments may not provide adequate “season-long” control of weeds that tend to emerge late, such as grasses and the tough-to-control giant ragweed. Fields treated early postemergence should be scouted later in the season to determine if an additional postemergence herbicide is needed. Some considerations for an early postemergence approach:
1) Many preemergence corn herbicides are also labeled for application to to emerged corn — ALWAYS check the herbicide label prior to making an application to emerged corn. Corn herbicide descriptions in the 2010 “Weed Control Guide for Ohio and Indiana” contain information on maximum size of corn for postemergence application of preemergence herbicides, available at http://www.btny.purdue.edu/Pubs/ws/ws-16/
2) Be sure to check labels or consult manufacturer representatives, local agronomists, etc for information on the use of adjuvants in postemergence applications. The addition of surfactant or crop oil concentrate will often be needed to ensure control of emerged weeds, but use of inappropriate adjuvants can increase the risk of crop injury. Control of emerged grasses with atrazine will require the addition of crop oil concentrate.
3) Most corn herbicides cannot be applied using 28% as the spray carrier after the corn has emerged.
4) Fields should be treated after the first flush of weeds has emerged, but before most annual weeds exceed 2 to 3 inches in height, to avoid yield loss due to early-season weed interference. When applying within two to three weeks after corn planting, OSU weed scientists suggest using full rates of preemergence corn herbicides. It is possible to reduce rates somewhat when the early postemergence application stretches out to 3 weeks or more after planting, but avoid reducing preemergence rates by no more than 30% even then. Where the plan is to definitely make another application of postemergence herbicides, lower rates can be used.
5) Treatments that contain atrazine will control many small, emerged broadleaf weeds. Emerged grass weeds tend to be more of an issue. Atrazine is a good option for a preemergence herbicide that also has activity on emerged grasses, and it is most effective when applied at high rates to very small (less than one inch) grasses. Larger grasses will require the addition of postemergence herbicides. Gyphosate and glufosinate are not the only choices. Impact, Laudis, etc are effective and can control emerged grasses as well as many broadleaf weeds; consider tankmixing with atrazine.
The preceding article was adapted from a 2009 article written by OSU Extension Weed Specialist, Mark Loux. The full article is available at http://corn.osu.edu/newsletters/2009/article?issueid=290&articleid=1732
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