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Postemergence (POST) herbicide in-crop applications should be made early when weeds are small. In a glyphosate-tolerant corn system, a Roundup® brand glyphosate-only agricultural herbicide should be applied before weeds exceed 2 to 4 inches tall to avoid excessive crop competition.
Research conducted in glyphosate-tolerant corn over 2 years across multiple sites and states suggested that the optimum timing for initial glyphosate application to avoid yield loss was when weeds were less than 4 inches tall, no more than 23 days after planting, and before the V4 growth stage of corn growth (Figure 1).1 These are only guidelines, and it is important to understand that timely early-season weed control is critical to protect corn yield potential.
Figure 1. The left plot received two applications of Roundup PowerMAX®, one early-POST (V3) and the other late-POST (V7) when weeds were 2 to 4 inches tall. The right plot received only one application of Roundup PowerMAX late-POST (V7) when weeds were 6 to 12 inches tall (trial conducted at Michigan State University in 2010). 13;10;
Research has shown that weeds can compete for nitrogen (N) resulting in corn yield losses.2 Grass weeds can take up N at about the same rate as corn early in the year. However, once grass weeds exceed 4 inches in height, they accumulate N more rapidly than corn. This is a reason why weeds can become very competitive with corn if not controlled early in the growing season. Note that the corn looks much more N deficient in the later treated plot on the right in Figure 1.
There can also be greater risk of yield loss when using total POST herbicide application programs. Weeds grow rapidly, and weather can delay POST applications. Surveys have shown that when weeds are managed with only POST herbicides, application timings often occur too late to protect the full yield potential of corn.3 The use of preemergence (PRE) and POST herbicides in sequential applications or in tank mixtures can be the best approach to weed control in corn. A two-pass PRE followed by POST program can provide better consistency and a wider window for the POST application. Roundup® brand glyphosate-only agricultural herbicides can be used in glyphosate-tolerant corn from early- through late-POST applications. The products can be applied at recommended rates over-the-top through the V8 growth stage or 30-inch corn, whatever comes first. Drop nozzles are recommended to be used in 30- to 48-inch corn to avoid spraying into the whorls of the plant.
When making early-POST herbicide applications, you should target small weeds (less than 4 inches tall), which usually coincides when corn is in the V2 to V3 growth stage. Residual herbicides should also be used in the weed management program, either sequentially or in tank mixture with POST herbicides. Programs that provide multiple sites of action also help in the management of weed resistance.
Visit www.RoundupReadyPLUS.com for more information on weed management solutions.
1 Gower, S. et al. 2003. Effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn: results of a 2-yr multi-state study. Weed Technology 17:821-828.
2 Johnson, B. 2006. Nitrogen accumulation by annual grass weeds in Roundup Ready corn production. Purdue University Extension Publication. www.btny.purdue.edu.
3 Fickett, N., Boerboom, C., and Stoltenberg, D. 2013. Predicted corn yield loss due to weed competition prior to postemergence herbicide application on Wisconsin farms. Weed Technology 27:54-62.
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