Source: Ohio State University
There are undoubtedly some fields of corn planted within the last week or so that were not treated with preemergence herbicides before the rain or wind started. Fieldwork may be difficult to accomplish this week, due to wet soils and a forecast for more rain, so it's a good bet that in some of these fields, the time between planting and herbicide application could stretch to 10 days or more. It's possible to forge ahead and just apply the intended preemergence herbicides regardless of how long it's been since tillage and planting, or without consideration for what the weeds are doing. However, a little thought and possible adaptation of the herbicide program can result in more effective control and a lower ceiling on herbicide costs.
We specifically want to caution about applying preemergence herbicides a week or more after tillage and planting, when there is no rain in the immediate forecast. Tillage stimulates a new flush of weed germination and emergence. It's essential that within a week or so after tillage, preemergence herbicides have been moved an inch of two down into the soil where they can be taken up (absorbed) by the shoots and roots sprouting from weed seeds. The problem scenario occurs when it's been a week or more after tillage, and the weeds are close to emerging from the soil. Preemergence herbicides applied at this time will reach maximum effectiveness only if rain moves them down into spoil before weed shoots emerge. Once the shoots have emerged, herbicides that act strictly through residual activity have lost most of their effectiveness, unless you are a big believer in "reachback activity." So, our suggestion in this situation would be that you reconsider applying preemergence herbicides where it's unlikely to rain before the weeds emerge (you can check by digging down to see what the weed are doing), and consider switching to an early postemergence approach. 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, the majority of the corn planted in 2008 is resistant to glyphosate and/or glufosinate (Ignite), and 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, giant ragweed, and waterhemp. They also will not provide adequate control of weeds that are not well controlled by preemergence herbicides, such as shattercane, johnsongrass, and burcucumber. 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) Most preemergence corn herbicides are also labeled for application to to emerged corn. Notable exceptions are products containing isoxaflutole without a safener (Balance, Radius, Epic - Balance Flex contains the safener and can be applied early POST) or simazine. Corn herbicide descriptions in the current "Weed Control Guide for Ohio and Indiana" contain information on maximum size of corn for postemergence application of preemergence herbicides.
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. Degree and Degree Xtra are the exceptions to this rule. These products can be applied in 28% to corn up to 6 inches tall as long as air temperatures do not exceed 85 degrees.
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, we 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 we suggest 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. However, keep in mind that the difference between full and half rates of atrazine premix products can be as little as $8 per acre.
5) Treatments that contain atrazine will control many small, emerged broadleaf weeds. Among preemergence herbicides, Lexar/Lumax and mixtures of SureStart plus an atrazine-containing product provide the broadest spectrum of broadleaf weed control, especially as weeds get larger. Emerged grass weeds tend to be more of an issue. Corvus will control small emerged grasses and has a good fit in this situation, but will require the addition of atrazine or another herbicide with foliar activity for most emerged broadleaf weeds. Atrazine is the only other preemergence herbicide that 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 here. Impact and Laudis can control emerged grasses at a cost similar to glyphosate and glufosinate, and they also control many broadleaf weeds. Impact and Laudis should be mixed with an atrazine-containing product for most effective control, especially of grasses.
Source: Ohio State University