Rates for Residual Herbicides in Corn

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

What rate of residual herbicide to use? In soybeans, use the full labeled rate because you need 4 to 6 weeks of residual control if spraying 3 to 4 weeks prior to planting, or you need the burndown activity at these rates because the weeds have gotten large. In my opinion, there is no clear cut answer for corn. A number of issues need to be considered:

Is this a no-till or conventionally tilled field?

· If it is no till you may need the higher rates because the amount of plant residue in the field;

· How far ahead of no-till planting are you spraying (so how long do you need to have residual control); and

· If spraying ahead of no-till planting, will you be coming back with additional preemergence herbicide at planting

Are you planning on a POST application? In our work, a preemergence followed by postemergence is the most effective and most consistent approach. A total preemergence approach can work if you have light weed pressure, have a dry year that limits a second flush of weeds, do not have large seeded species (morningglory, Texas panicum, cocklebur), do not have perennial weeds (horsenettle, yellow nutsedge, bermudagrass), are able to get your preemergence herbicides incorporated with timely rainfall or irrigation, or you are just plain lucky. Otherwise, most fields will need a postemergence spray.

My point of reference for a solid, one pass herbicide program is either Lexar or Lumax with additional atrazine; or a full-rate of a premix with atrazine and grass herbicide (Bicep, Harness Xtra, etc) PLUS pendimethalin (Prowl) or rimsulfuron (i.e. Basis). Any of these programs can include simazine. And I have seen all of these programs requiring postemergence treatments for acceptable weed control. None are consistently effective for full-season control.

If you need to spray postemergence then it makes sense to use a less intensive program at planting. But keep in mind, the less intensive soil-applied herbicide approach you use, the more likely you will need to spray earlier, may need to use more than one herbicide postemergence (particularly to provide residual control), and will need to be more timely with your postemergence treatment. If you can spray your postemergence herbicide before the weeds are 3 to 4 inches tall, and you are willing (able) to spray herbicide combinations (i.e. not rely solely on glyphosate), then reducing soil-applied herbicides by up to 20 to 30%, or not including all of the tankmixes, is a sound decision.


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