The wet weather has made it difficult for timely herbicide applications, to say the least. In some cases, the corn is already coming up and no herbicides including a burndown have been applied. 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 a number of 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. Product like Prowl, Dual, Harness, Outlook, etc. do not control emerged weeds, so additional herbicides will need to be included in the mixture that control existing weeds.
With the increasing acres of Roundup Ready (glyphosate) and Liberty Link (Liberty 280) corn, we have more flexibility in how we manage weeds after emergence. In addition, a number of "conventional corn" products are available to control emerged grasses (e.g., Accent Q, Armezon/Impact, Basis Blend, Capreno, Laudis, Resolve Q, and Steadfast Q) 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 Balance Pro, Prequel, Radius, and Princep 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. Other herbicides such as Bicep II Magnum, Bullet, Dual II Magnum, etc. can be applied to corn up to 5 inches tall. Acetochlor-containing products such as Degree (Xtra), Harness (Xtra), FulTime, Keystone (LA), Resicore and SureStart II/TripleFlex II can be applied to corn up to 11 inches tall. Herbicides including Atrazine, Lumax, Lexar, Guardsman Max, and Resolve can be applied to corn up to 12 inches tall (20 inches tall for Resolve Q).
And finally, Prowl H2O and Halex GT can be applied to 30 inch tall corn or less. 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 weeds sizes for these products please refer to Table 2.2-10 in the Penn State Agronomy Guide or Table 2-15 in the Mid-Atlantic Field Crop Weed Management Guide and check the most recent herbicide label for specific use guidelines. Furthermore, you can find additional use information on the herbicide label on either the Crop Data Management System or the Greenbook Data Solutions websites.
Stressed crops and herbicide injury:
Due to the rainy and cloudy weather, the crops have been struggling to grow and likely will be more sensitive to postemergence applications. Certain formulations such as EC (emulsifiable concentrate) formulations as compared to encapsulated formulations (CS and ME) can cause more injury. For example, Degree Xtra/Warrant and Prowl H2O are generally safer on emerged corn as compared to their counterparts Harness Xtra and Prowl 3.3EC, respectively. Also there is an increased crop injury potential when combining residual herbicides with loaded glyphosate products or adding additional adjuvants to the spray mix. Regarding application timing after environmental stress, the general rule of thumb is to allow a few sunny days to pass after coming out of a rainy, cool period before applying herbicides (but we may not have that luxury this season). Since the plants are stressed, this allows them time to build up a thicker leaf surface and to get their metabolic processes functioning at a faster pace to detoxify the herbicide. Also, with all the moisture, warmer temperatures, and eventual sunlight the plants will be growing very quickly and are succulent, so consider using nonionic surfactant (NIS) instead of crop oil concentrate (COC) or methylated seed oil (MSO) as the spray additive.
Heavy rainfall and herbicides:
In those areas with regionalized heavy rainfall and flooded fields, if residual herbicides were already applied and water (and soil) is moving off of the field, some of the herbicide will likely be taken with it or it will simply degrade and lose activity. Over the next few weeks monitor those fields for lack of weed control or weed escapes. Post herbicide applications may be necessary for appropriate weed control. If a field needs to be replanted to corn or another crop, remember to adhere to the crop rotation guidelines of the most restrictive herbicide that was applied. Refer to Table 2.1-11 in the Penn State Agronomy Guide or Table 1.5 in the Mid-Atlantic Field Crop Weed Management Guide or the product label.