Early-Season Weed Management

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Crops are most vulnerable to weed competition at planting and as new plants emerge. Significant yield is at risk if weeds are allowed to compete with crops during the first several weeks after planting. If not controlled, they can also decrease harvest efficiency and produce seed, which can impact future crops.

Ideally, weeds should be controlled several weeks prior to planting in order to allow for decomposition of plant material. Planting into existing weeds, or heavy weed residue that has not had time to decay, can interfere with seed placement and reduce emergence due to poor seed-to-soil contact. If a burndown application is delayed, planters should be adjusted to compensate for the increased plant residue.

In addition to starting with a clean field, removing weeds after planting when weeds are less than 4 inches tall is necessary to preserve yield potential. University research shows an average yield potential of 3 bushel per day is at risk for every day 3- to 4-inch weeds are left uncontrolled after V3 (3 leaf collars) to V4 growth stage corn.1

Controlling glyphosate-resistant kochia in western Kansas after emergence has been difficult. Weed management should begin prior to kochia emergence.2 Large flushes of kochia may emerge in late February to early March and into April. If allowed to emerge, postemergence herbicide applications often will not provide adequate control. According to Kansas State University, to effectively manage kochia at germination, a PRE herbicide application may need to occur in January through the first week of March, but prior to kochia emergence, which can vary season to season. If farmers wait until later to apply a burndown and preemergence herbicide in the same application, glyphosate-resistant kochia may be larger and may not be controlled. The choice of residual herbicides for effective preemergence control of kochia in February and early March will vary depending on subsequent cropping intentions.

To address glyphosate-resistant kochia in fields to be planted to corn or grain sorghum, consider an early PRE combination of a Roundup® brand agricultural-only herbicide (minimum of 0.75 lb ae per acre) (if weeds have emerged) with herbicides that have PRE and POST activity on kochia. Tank mixing 8 to 16 oz/acre of dicamba with 1 to 2 pints/acre of atrazine will control existing broadleaf and grass weeds, and will provide extended PRE control of kochia often into May.2

Key components of a successful weed management program:

  • Start with a clean field and control weeds early by using a burndown herbicide treatment or tillage in combination with a PRE residual herbicide.

  • Environmental conditions affect the rate of weed growth, crop development, crop tolerance to herbicides, and herbicide performance. In the spring, day and nighttime temperatures can fluctuate. The efficacy of a burndown herbicide application can be reduced by cold temperatures. It is recommended to wait on applying herbicides until nighttime temperatures are above 40 °F and daytime temperatures are in the high 50s to low 60s. Weed control may be even more effective if there are several days of warmer weather prior to herbicide application rather than applying on the first warm day of the season.
  • Sufficient moisture is required to activate residual herbicides; however, too much rain can hasten herbicide breakdown and may cause leaching below the weed seed germination zone. Typically 0.5 inch of rain received within 7 to 10 days after a PRE herbicide treatment is sufficient for good activation.4 However, the amount can vary depending on the soil type and moisture content prior to the rainfall event. Dry soil can require more rain because moisture is needed to wet the soil before significant movement of the herbicide can occur.
  • Multiple herbicide sites-of-action with overlapping weed spectrums in rotation, sequences, or tank-mixtures should be used.
  • The full labeled herbicide rate and proper application timing (weed size) for the hardest to control weed species present should be used.
  • Scout fields after application to observe if acceptable control has been achieved. Position an overlapping residual herbicide in your POST application for tough to manage weeds.

Sources: 1 Pocock, J. 2011. 5 Tips for corn weed management | Start with a clean field—then control weeds early as they reach 4 inches. Corn + Soybean Digest. http://www.cornandsoybeandigest.com/issues/5-tips-corn-weed-management-start-clean-field-then-control-weeds-early-they-reach-4-inches; 2 Thompson, C. and Peterson, W. 2018. Late-winter preplant applications for kochia control. eUpdate. Issue 669. Kansas State University Department of Agronomy. https://webapp.agron.ksu.edu/; 3 Hartzler, B. 2003. Is your weed management program reducing your economic return? Iowa State University Weed Science. http://www.weeds.iastate.edu/mgmt/2003/economics.shtml; 4 Hartzler, B. 2002. Absorption of soil-applied herbicides. Iowa State University Weed Science. http://www.weeds.iastate.edu/mgmt/2002/soilabsorption.htm. Web sources verified 01/20/18. 180122185804