The later an effective application is made, the less likely weeds will become established following treatment. Because glyphosate will control larger weeds than most other herbicides, some growers delay application in the hope of avoiding a second glyphosate treatment. This simple approach has an “invisible” cost. While weeds may be controlled, delayed glyphosate applications allow weeds to compete with the crop, extracting a yield penalty. This “tax” due to early competition can be estimated with WeedSOFT™, a weed management computer program. WeedSOFT will calculate the yield loss due to early competition and highlight the penalty for late glyphosate applications. This yield penalty is approximately 2% for each crop growth stage delay in weed removal. The early competition penalty will vary with weed species, density, size, crop stage, row spacing and other factors.

What approaches can be used in RoundupReady crops to prevent early season weed competition?

The best approach is to apply a preemergence treatment for early season weed control and follow with a glyphosate treatment. Match the preemergence treatment with the weeds in the field. This practice has an additional important benefit -- herbicide resistance management. Using herbicides with different modes of action helps prevent the development of herbicide resistant weeds. A second approach to prevent early season competition is to apply an early glyphosate treatment and follow with a second treatment later. This approach will prevent yield loss from early competition and control later developing weeds. The disadvantage is that relying entirely on glyphosate for weed control increases the risk of developing glyphosate resistant weeds.

The important point to remember is that to harvest a full yield, a crop must be protected from early season weed competition as well as later weed growth.

Alex Martin
Extension Weeds Specialist, Lincoln

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.

Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.

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