The weather is beautiful, and planting is in full throttle. However, due to the lack of rainfall, soil-applied herbicides have not been or will not be properly activated for adequate weed control. Dry weather can affect both soil-applied and postemergence herbicide performance. All soil-applied herbicides require rainfall to mobilize them for effective weed control.
In general, rainfall should occur within 7 to 10 days after application or before weed emergence. As a general rule of thumb, a ½ inch of rain is considered the minimum depending on current soil moisture levels and the herbicide used; ¾ to 1 inch is ideal.
More water soluble products such as Callisto, Outlook, Spartan/Authority, and metribuzin require less rainfall (1/4 to 1/3 inch) to activate them while the less mobile materials (Balance Flexx, Prowl, atrazine, simazine, Valor, Zidua) and deeper germinating weeds (e.g. yellow nutsedge, cocklebur, velvetleaf, ragweed, etc.) will require more rainfall for effective mobilization and activation into the seed germination zone. Keep in mind that many small-seeded annual weeds can germinate with minimal moisture. Thus, sometimes you may observe weed emergence before enough rainfall has activated the herbicide for effective kill.
If 10-14 days have passed without rainfall following a pre-treatment and weeds are starting to break, start planning for a post herbicide rescue application (or where possible, consider using a rotary hoe or that old cultivator that you have been thinking about trying again). Some “reach back” or “recharge” can be expected on small annual weeds (esp. broadleaves) with some herbicides when rainfall occurs, although depending on this may be a little like gambling.
In particular, the HPPD (Group 27) herbicides (Acuron, Balance, Corvus, Lexar, Lumax, etc.) tend to have better “reach back” potential than some other herbicides, and escaped grass control is probably of greater concern. The Group 5 herbicides (Photosystem II inhibitors) like atrazine, simazine, and metribuzin will also control small emerged susceptible broadleaves via root uptake. As for post herbicide applications, remember that small annual weeds are easier to kill than large ones and examine adjuvant options to maximize activity under dry weather conditions.
However, there are some situations in which the corn is already coming up and no herbicides including a burndown have been applied. (Some of these post herbicide options can be used to control escaped weeds from a pre-application, but just make sure to adherer to maximum herbicide load per season for some product active ingredients, namely atrazine (Group 5) and the HPPD (Group 27) herbicides.) Unless it’s Roundup Ready or Liberty Link corn, the options for broadspectrum burndown are very limited. (We do not recommend application of Gramoxone even if the corn is in spike stage but would suggest other herbicide tank mixes or using 2-pass herbicide programs.)
There are several herbicides, including residual products that can be applied after planting up until corn and weeds reach a certain size or growth stage. The greatest risk of failure comes with trying to control annual grasses such as foxtail and panicum as they are emerging without including a foliar-applied herbicide. Products like Prowl, Dual, Harness, Outlook, Zidua, etc. do not control emerged weeds, so additional herbicides will need to be included in the mixture that control existing weeds.
With many acres of Roundup Ready (glyphosate) and Liberty Link (glufosinate) corn, we have more flexibility in how we manage weeds after emergence. In addition, several “conventional corn” products are available to control emerged grasses (e.g., Accent Q, Basis Blend, Capreno, Impact/Armezon, Laudis, Resolve Q, Shieldex, Steadfast Q, and a few others) and even more are available for broadleaf weed control.
In most cases, these post or foliar-applied herbicides can be tank-mixed with residual products to provide several weeks of control. For most products, do not apply in a liquid fertilizer carrier if corn has emerged or injury may occur. Maximum corn and weed sizes vary for early post herbicide applications in corn depending on the product. Herbicides such as Princep and Verdict must be applied before corn emergence.
Balance Flexx and Corvus contain a safener and can be applied up to early POST (V2 growth stage) to corn while Anthem Maxx and Zidua SC can be applied up to the V4 growth stage. Acetochlor-containing products such as Degree (Xtra), Harness (Xtra and Max), FulTime NXT, Keystone NXT, Resicore, and SureStart II can be applied to corn up to 11 inches tall. Those herbicides that can be applied to corn up to 12 inches tall include atrazine, Acuron, Bicep II Magnum, Cinch ATZ, Lumax EZ, Lexar EZ, and Outlook. Resolve Q and Python can be sprayed on 20 inches tall corn.
And finally, Acuron Flexi, Halex GT, Prowl H2O, and Zemax can be applied up to 30-inch-tall corn while Dual II Magnum can be applied to corn that is 40 inches tall. Keep in mind, when tank-mixing with other pesticides follow the most restrictive product label. For a listing of additional herbicides and maximum corn heights and information on maximum weed sizes for these products please refer to Table 2.2-10 in the Penn State Agronomy Guide and check the most recent herbicide label for specific use guidelines. (Or refer to the herbicide label for additional use information from these sites— CDMS or Greenbook or Agrian).