University of Illinois

No. 9/May 23, 1997

Weather and Postemergence Herbicides

Rain-free intervals: Most postemergence herbicides have label statements indicating how much time after application is needed prior to precipitation to maintain effectiveness. Touchdown requires 4 hours, whereas Roundup Ultra requires 1 to 2 hours.

Corn herbicides: A rain-free interval of 1 to 2 hours after application is adequate for Buctril, 2,4-D ester, Lightning, Resolve, Resource, and Tough. Most other postemergence corn herbicides require a 4- to 8-hour rain-free period. See Table 3 in the May 16 issue of this Bulletin for a complete listing of rain-free intervals for postemergence corn herbicides.

Soybean herbicides: One hour rain-free is adequate for the soybean "post grass" herbicides, as well as Classic, Cobra, Pinnacle, Pursuit, Resource, Stellar, and Synchrony STS. Soybean herbicides requiring between 4 and 6 hours without precipitation after application are Basagran, Blazer, Flexstar, Galaxy, Reflex, and Storm.

Low temperatures: Last week, several counties in central and northern Illinois recorded temperatures that were low enough for frost. Temperatures below 40 degrees F slow weed and crop growth. Many postemergence herbicide labels recommend that applications be made to "actively growing weeds" and warn of the possibility of crop injury under widely fluctuating temperature conditions. At the other end of the spectrum, air temperatures above 80 degrees F increase the activity of postemergence herbicides on both weeds and crops, especially if accompanied by high humidity.

Relative humidity and moisture: Arid conditions increase the probability of poor weed control, while humid conditions increase the probability of better weed control, but also crop injury. High humidity increases the hydration of plant tissues, allowing better herbicide penetration. Contact herbicides are much more active under high temperatures and high relative humidity, increasing the possibility of crop injury.

Wind and "sandblasting": With the recent high winds, "sandblasting" of small corn plants occurred in many areas. The abrasive action of soil particles can cause injury to the plants, as well as increase the chances for crop injury from postemergence herbicides. Recent windy weather has increased the concerns about herbicide drift. All postemergence herbicides have statements concerning drift on their labels.

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