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Control marestail in soybeansBy David G. Hallauer Meadowlark District Extension Agent It's one of the more challenging soybean weeds we face. Historically considered a winter annual, marestail can germinate, and has done so increasingly, in spring or summer, acting more like a summer annual. Tiny seed is easily dispersed by wind and can germinate quickly--or lay viable in the soil for many years, making control via crop rotation difficult. We're also seeing populations with glyphosate resistance, as well as some resistance to ALS inhibiting herbicides. According to Kansas State University Weed Scientist Dallas Peterson and Southeast Area Extension Agronomist Doug Shoup, the herbicide program effectiveness depends on growth stage and plant size. Catch it in the rosette stage since once it bolts and exceeds four to six inches in height, control becomes difficult. Combined with the fact that its long germination window means multiple applications may be needed (particularly in no-till), and the reason for its success is obvious. Peterson and Shoup suggest fall treatment programs--a must if you have a history of marestail. They can be effective even in to December as long as applications are made to actively growing weeds during a stretch of mild temperatures. Consider a November treatment to allow most of the fall germinating winter annuals to emerge. Don't expect residual herbicides to last until next spring or summer. Plan instead for early spring treatments (March/April) to help further. Marestail control options after bolting get pretty thin, particularly in situations where herbicide resistance is found in the population, making a fall application all the more important. Next week, I'll note some of the herbicide programs recommended by Peterson and Shoup to combat this troublesome weed. Date: 11/5/2012
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