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Maximum Corn Sizes for Postemergence Herbicides

May 18, 2001
With more than 90% of the Illinois corn crop in the ground and more than 80% of it emerged and rapidly growing, we need to start considering what types of options we have for weed control on those acres where the soil-applied herbicides didn't get applied or are not working to their full potential due to lack of rainfall. Two issues ago we discussed options for using some of the soil-applied herbicides postemergence to control some of the early-emerging weeds (issue no. 6, "Early-Season Weed Control in Corn"). With this window rapidly closing in most areas in the state, we need to start thinking about how we are going to control those later-emerging grass and broadleaf weed species. One thing we need to keep in mind when planning these postemergence herbicide programs and rescue applications is the maximum corn size to which these herbicides can be applied. Many times if these restrictions are not followed there can be substantial injury to the crop that can sometimes lead to yield reductions. So it is always important to keep these maximum sizes in mind when planning postemergence herbicide applications. We will discuss some of these maximum sizes in the following text, and Table 3 contains maximum sizes and comments on all postemergence corn herbicides.

2,4-D­Containing Products. Herbicides that contain 2,4-D require the use of drop nozzles when corn is over 8 inches tall. 2,4-D esters will volatilize. Use amine formulations when temperatures approach 80 to 85°F. Also do not spray corn between tassel and dough stage with 2,4-D. However, this herbicide can be used as a harvest aid after corn is in the dent stage. Shotgun also contains 2,4-D and requires drop nozzles when corn is greater than 8 inches. However, the maximum height for applications with drops with Shotgun is 12 inches due to the atrazine component. Remember there are many susceptible plants to 2,4-D and certain precautions need to be made to avoid drift to these species.

Atrazine-Containing Products. Herbicides that contain atrazine must be applied before corn is 12 inches tall. These products include Basis Gold, Buctril + atrazine, Liberty ATZ (Liberty-Link corn), ReadyMaster ATZ (Roundup-Ready corn), and Shotgun. Marksman also contains atrazine, but the size limit is 8 inches or 5-leaf corn, whichever is more restrictive due to the rate of dicamba.

Dicamba-Containing Products. Herbicides that contain dicamba have rate cutoff restrictions. The application rates for Banvel and Clarity need to be lowered if applications are made to corn over 8 inches or 5-leaf. Maximum corn size at lower rates depends on the proximity and stage of soybeans and other susceptible plants. The maximum size is 24 inches if nearby soybeans are over 10 inches or are blooming, and if soybeans are smaller or are not in proximity, the maximum size is 36 inches. Distinct and Celebrity Plus have a minimum corn size of 4 inches, and Distinct rates change based on corn size (4- to 10-inch corn, 6 oz/A and 10- to 24-inch corn, 4 oz/A). NorthStar also has a minimum corn size of 4 inches and a maximum of 20 inches or V6 corn, whichever is more restrictive, unless drops are used; then it is 36 inches. Remember there are many susceptible plants to dicamba and certain precautions need to be made to avoid drift to these species.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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