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Residual Control in Roundup Ready Crops

Bill Simmons Bill Simmons
Associate Professor
Department of Natural Resources and Environmental Sciences
(217) 333-9649
fsimmons@illinois.edu
  Loyd Wax Loyd Wax
Professor
Department of Crop Sciences
USDA/ARS
(217) 333-4424
l-wax@illinois.edu

Soil-applied herbicides and postemergence herbicides with soil-activity are important parts of many weed control programs. Glyphosate-tolerant soybean culti-vars have allowed for successful weed control with use of the various glyphosate products, often with no help from other herbicides. Soil-active herbicides may be a valuable tool for delaying the application of glyphosate until closer to canopy closure or for reducing weed emergence following a glyphosate application.

Two current topics of study are:

  1. the effect of application timing of glyphosate alone or mixed with imazethapyr or imazaquin on weed control and soybean yield, and
  2. the use of reduced rate soil-applied herbicides in Roundup Ready Corn systems.

Consider weed spectrum to ascertain the need for commercial mixes of glyphosate and a residual herbicide. Is the glyphosate rate high enough in the mix to adequately control difficult species? What late-season emerging weeds are to be addressed with the residual herbicide? Are herbicide-resistant weeds present in the field, and will the mix control them? Glyphosate-based weed control systems offer soybean producers several options for excellent weed control. Application timing and the need for residual control are the two management questions that need to be addressed.

Controlling 4- to 6-inch weeds in Roundup Ready soybeans

Figure 1. Controlling 4- to 6-inch weeds in Roundup Ready soybeans
may provide a mulch to conserve moisture and suppress further weed emergence.

Soybean and corn vary in their tolerance of early season weed competition and, to some extent, their ability to recovery from early season injury. Our studies show that weed control in corn must begin early if yield potential is to be preserved. However, in soybean, weeds do not become important economically until they exceed a height of 6 inches.

We are also studying the influence of late-germinating weeds that might emerge after a glyphosate application and measuring the effect of those weeds on final grain yield. Limited weed re-infestations have not measurably reduced grain yields in our studies on dark silt loam soils in central Illinois. Delayed planting, low-water-holding soils, and the presence of highly competitive weeds such as giant ragweed and cocklebur all may increase the need for earlier weed control.

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