Source: Ohio State University Extension
By Mark Loux
While a variety of rainfall and soil moisture conditions can be found around Ohio, a shortage of rain following application of residual herbicides seems to be common. We are hearing about weeds emerging early in the season even where residual herbicides were applied, which is an indicator of inadequate herbicide “activation,” or lack of downward movement into the upper inch or two of soil where weed seeds germinate.
Herbicides vary in the amount of rain required for activity, due to differences in water solubility and adsorption to soil, and whether absorption into the plant occurs via roots or shoots. Ignoring all of this though, the general rule is that 1/2-1 inch of rain is needed within about a week after application to ensure activity. Some considerations for this situation:
• Later rain will cause residual herbicides to eventually become active, but by then weeds may have already emerged through the thin layer of herbicide on the soil surface. There is the possibility of “reachback activity” from some herbicides, where emerged weeds are controlled, due to uptake of herbicide by roots. In our experience, this is erratic and not that reliable unless weeds are very small.
• Escaped weeds will usually require treatment with post-emergent (post) herbicides. Applying post when weeds are small ensures more effective control and prevents interference with the crop. Keep in mind that where residual herbicides have largely failed, we defer to the basic principles of weed removal timing for total post systems. Weeds in corn should be removed before they exceed 2 inches in height, and in soybeans before they exceed 4-6 inches in height (giant ragweed will typically be larger than the rest of the weeds). There is probably some flexibility in these sizes where the residual herbicides had enough activity to substantially reduce weed populations, so this can be somewhat of a judgement call.
• Be sure the post-emergence herbicides fit the weed population. Residual herbicides typically remove many weed species, leaving one or two that are then targeted with post herbicides (e.g. giant ragweed, marestail, morningglory). Where residual herbicide activity is reduced, the full complement of annual weeds can be present, requiring a more comprehensive post herbicide treatment.
Applying Residual Herbicides to Emerged Corn
There are also fields with emerged corn where no herbicide has been applied yet.
Most residual 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 add post herbicides. In addition, the majority of the corn hybrids are resistant to glyphosate and/or glufosinate (Liberty), which can be combined with residual herbicides to control weeds emerged at the time of application.
It’s also possible to mix in some other post herbicides such as Impact, dicamba, 2,4-D, Capreno and others to control emerged weeds, instead of glyphosate or Liberty.
Some issues to be aware of with regard to post-emergence application of residual corn herbicides follow.
• Only Degree and Degree Xtra can be applied using 28% as the spray carrier once corn has emerged. All other herbicides should be applied in water. Degree Xtra and Degree can be applied in 28% on corn up to 6 inches tall, when air temperatures are less than 85 F. Expect some leaf burn from these mixtures.
• There is usually a maximum corn size specified, which can be based on growth stage or corn height. This can be as small as the V2 stage for some herbicides, such as Corvus and Balance Flex. This information can be found in the herbicide description section of the "Weed Control Guide for Ohio, Indiana, and Illinois.”
• Most premixes or tank mixtures that contain atrazine can adequately control small broadleaf weeds, and especially those that contain another broadleaf herbicide (e.g. Lexar, Lumax, Instigate, SureStart). Grasses more than about an inch tall will require the addition of glyphosate, Liberty or other herbicide with effective grass activity.
• Follow adjuvant recommendations closely to minimize the risk of injury and do not assume that it is always possible to use an adjuvant once corn has emerged.
• Based on our research with this type of approach to herbicide management, herbicides should be applied when weeds are less than about 2 inches tall to ensure they’ve been prevented from causing yield loss.
• We find that herbicide labels can lack enough information on adjuvants and tank-mix partners for this situation. Be sure to check with dealers and manufacturer/distributor agronomists to get specific information when necessary.
There is a brief in-field video covering post application of residual corn herbicides on Youtube. You can subscribe to the Youtube page and be automatically notified of future videos. Another recently added video takes a first look at studies on various strategies for management of marestail in Enlist and Xtend systems.
Residuals on Soybeans
The situation in soybeans is vastly different than in corn, since most residual soybean herbicides cannot be applied once the soybeans have emerged. Failure to apply residual herbicides in soybeans typically results in the need to apply post earlier in the season, which can result in the need for a second post application to control late-emerging weeds. This can sometimes be avoided by applying an early post treatment that has some residual.
Several residual soybean herbicides can be applied early post, and several post herbicides do have some residual activity. These include the following: Pursuit (also in premixes with glyphosate such as Extreme and Thundermaster); FirstRate; Flexstar and the generic equivalents (also in premixes with glyphosate and metolachlor and Pursuit); metolachlor (Dual, Parallel, etc.); and acetochlor (Warrant). None of these provide residual control of most marestail populations, and it’s about impossible to obtain enough residual control of all the giant ragweed that can emerge after an early post application. Making a second post application is a more effective strategy for giant ragweed usually.