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Importance of early season control of herbicide-resistant kochia


Researchers writing in the latest edition of the journalWeed Science are providing new insights into the control of herbicide-resistantkochia, a weed that competes with both dryland and irrigated crops across theGreat Plains states.

Kochia typically begins to emerge in late February or earlyMarch before other summer annual weed species. It then disperses its seedsbroadly by tumbling across the landscape.

Since the timing of weed emergence and seed persistence inthe soil can influence the selection of effective weed control practices,researchers conducted a two-year study to explore kochia emergence patterns andseed persistence. They harvested kochia seed from sites in five Midwest states,buried packets of seed and exhumed them at six-month intervals to evaluateviability.

Emergence densities varied widely across the plots and studyyears -- from as few as four to almost 380,000 seedlings per square meter.Cumulative growing degree days needed for 10% emergence also varied widely. InKansas, 168 days were needed, but only 90 in Wyoming and Nebraska. Researchersfound that more than 95 percent of kochia seed failed to persist for more thantwo years.

The findings have important implications for weed control.The authors say preemergence weed control should be initiated in the fall or byearly February to address the first flush of seedling emergence and seedproduction. Fall-established cover crops can create a canopy that suppressesthe density of kochia emerging in the spring. Early season tillage can also beeffective. Researchers found that seeds unearthed during tillage had a very lowpercentage of viability.

Growers may even want to adopt stale seedbed techniques andallow weed seeds to germinate so they can be treated prior to planting crops.Researchers caution, though, that kochia continues to emerge into mid-summer,which means an extended management program may be needed.

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