University of Illinois

No. 19/August 1, 1997

Blow the Whistle? Time to Quit!

Before you apply any herbicides in cropping situations this late in the season, you must answer several questions: Why? When? and How?

Why apply? Will late weeds create greater yield losses than a herbicide-stressed crop? Will these weeds create harvest problems, market discounts, or future weed problems (resistant weeds, noxious weeds!)? Should I use herbicides during crop reproductive stages, that is, soybean flowering to pod fill or corn tassel to dough? Contact herbicides may stress the crop, but large weeds may only be suppressed and re-leaf, creating "stick weeds." Should you apply translocated herbicides during reproductive stages? Translocated herbicides move with the "food supply" to "high-demand areas." Will this affect reproductive processes? Will it affect seed germination or residue tolerance?

When? How late can I apply? Check the herbicide label for preharvest interval (PHI) and the recropping interval (RCI). All soybean "post" herbicides, except Basagran (re-registration coming), have a PHI established; and several have restrictive RCI.

How to apply? Spot treat or broadcast? Directed or over-the-top? Ground or aerial application? To summarize, in most cases, it is too late to apply herbicides in soybeans as well as corn.

Table 4. Soybean postemergence herbicides and their restrictions.

Broadleaf herbicidePHI, daysRCI, months*Important premixes //Copacks
BasagranNone?NoneNoneNoneSee below.
Blazer or Status50 NoneNone NoneGalaxy, Storm// Conclude, Manifest
Flexstar or ReflexPrebloom41018Tornado
Classic or Skirmish603 9 9Concert, Synchrony STS, Reliance
Pursuit8548.518None "post" in soybeans!
Roundup on RR14NoneNoneNone
Roundup on non-RRPre-podNoneNoneNone

*Check label closely, as recropping intervals may change with circumstance; that is, soil pH, rate-applied, time-applied, drought, or corn type.

Marshal McGlamery and Aaron Hager, Department of Crop Sciences, (217)333-4424