Applying Preemergence Herbicides to Emerged Corn By Kevin Bradley

I’ve received several calls recently from growers and retailers indicating that in the rush to get done with corn planting, many fields never received a preemergence herbicide application. Now, with the recent rainfall received in many areas of the state, it seems likely that some growers will not be able to get back on these fields until well after the corn has emerged. Luckily, many of our preemergence corn herbicides have labels that allow them to be applied as a postemergence treatment. Exceptions to this are Axiom, Balance Pro, Epic, Princep, and Stalwart C. These herbicides cannot be applied to emerged corn or crop injury can occur. Table 1 provides a list of the preemergence herbicides that can be applied as a postemergence treatment to emerged corn, as well as information about the maximum corn height and/or stage at which these applications can be made.

When preemergence herbicides are applied to emerged corn, it is important for growers to recognize that weed control will often be less consistent with these programs when compared to traditional preemergence herbicide applications. This is because some weeds are likely to have germinated along with the emerging corn crop, and many of these products provide little to no control of emerged weeds. One herbicide that does have some postemergence activity on weeds is atrazine, which is a component of most preemergence corn premixes. Generally, atrazine will provide control of labeled weeds less than 1.5 inches in height, and the addition of crop oil concentrate will greatly enhance the foliar activity of this herbicide.

Another consideration for growers who have missed the preemergence herbicide "window" is to change their plans to a total postemergence herbicide strategy. This may be the best choice in those fields where the weeds have gotten too large for a preemergence herbicide application. When choosing a total postemergence program, best results will usually be achieved by combining herbicides that have both foliar and residual weed activity. This will provide control of the weeds that are present at the time of the application, and will provide some degree of residual weed control for weeds that may emerge later in the season. Information on these and other kinds of corn herbicide programs is also available in the 2005 Missouri Pest Management Guide for Corn, Soybean, and Winter Wheat (Extension Publication M171)

Kevin Bradley, Extension Weed Scientist 573-882-2801

Table 1. Maximum Corn Height/Stage for Preemergence Herbicide Applications to Emerged Corn
Active Ingredient(s)	Herbicide Trade Name(s)	Maximum Corn Height/Stage for Broadcast Applications	Maximum Weed Height/Stage for Broadcast Applications
acetochlor	Degree, Harness, Surpass, TopNotch	11 inches	Apply before weed emergence. Will not provide postemergence weed control and requires a tankmix partner.
alachlor	MicroTech	5 inches	Apply before weed emergence. Will not provide postemergence weed control and requires a tankmix partner.
atrazine	Aatrex, Various	12 inches	Apply before weeds exceed 1.5 inches in height. When applied with crop oil, apply before weeds reach 4 inches in height.
atrazine + acetochlor	Degree Xtra, FulTime, Keystone	11 inches	Apply prior to weed seedling emergence or in a tank mixture that controls emerged weeds.
	Harness Xtra	11 inches	Apply before grass weeds reach the 2-leaf stage or in a tank mixture with a herbicide that controls emerged weeds.
atrazine + alachlor	Bullet	5 inch	Apply until weeds reach the 2-leaf stage. Applications made beyond the 2-leaf stage will not provide satisfactory weed control.
atrazine + dimethenamid	Guardsman Max	12 inches	Apply before weeds are greater than 1.5 inches tall or in a tank mix with products that control emerged weeds.
	G-Max Lite	12 inches	Apply before broadleaf weeds are greater than 1.5 inches tall and before grass weeds emerge.
atrazine + metolachlor	Stalwart Xtra	5 inch	Apply until weeds reach the 2-leaf stage. Applications made beyond the 2-leaf stage will not provide satisfactory weed control.
atrazine + Smetolachlor	Bicep II Magnum, Cinch ATZ	5 inch	Apply until weeds reach the 2-leaf stage. Applications made beyond the 2-leaf stage will not provide satisfactory weed control.
atrazine + metolachlor + mesotrione	Lexar	12 inches	Apply to broadleaf weeds less than 5 inches in height. Will not provide adequate control of most grasses that are emerged at the time of application.
	Lumax	12 inches	Apply to broadleaf weeds less than 5 inches in height. Will not provide adequate control of most grasses that are emerged at the time of application.
dimethenamid	Frontier, Outlook	12 inches	Apply before weed emergence. Will not provide postemergence weed control and requires a tankmix partner.
	Guardsman	12 inches	Apply before weeds are greater than 1.5 inches tall or in a tank mix with products that control emerged weeds.
flufenacet	Define	5-leaf stage	Prior to emergence. Will not provide postemergence weed control and requires a tankmix partner.
flumetsulam	Python	2 inches or before first leaf is unfurled (spike)	Apply before weed emergence. Will not provide postemergence weed control and requires a tankmix partner.
flumetsulam + clopyralid	Hornet	20 inches or V6 stage	Apply to specified weeds up to 8 inches in height.
mesotrione	Callisto	30 inches or 8-leaf stage	Apply to weeds that are less than 5 inches in height.
metolachlor	Dual II Magnum, Cinch	40 inches	Prior to emergence. Will not provide postemergence weed control and requires a tankmix partner.
metolachlor + mesotrione	Camix	5 inches	Apply before grass emergence but will control some broadleaf weeds less than 3 inches in height.
pendimethalin	Prowl H20	30 inches or 8-leaf stage	Apply before weed emergence. Will not provide postemergence weed control and requires a tankmix partner.