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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:
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