Herbicide-resistant kochia is a big problem in the U.S. Great Plains states, and has appeared in limited numbers in Manitoba over the past few years.
Now researchers, writing in the latest edition of the journal Weed Science, are beginning to reveal more about how the weed works.
Kochia typically begins to emerge in the U.S. in late February or early March, before other summer annual weed species. It then disperses its seeds broadly by tumbling across the landscape.
Since the timing of weed emergence and seed persistence in the soil can influence the selection of effective weed control practices, researchers conducted a two-year study to explore kochia emergence patterns and seed persistence. They harvested kochia seed from sites in five Midwest states, buried packets of seed and exhumed them at six-month intervals to evaluate viability.
Emergence densities varied widely across the plots and study years — from as few as four to almost 380,000 seedlings per square metre. Cumulative growing degree days needed for 10 per cent emergence also varied widely. In Kansas, 168 days were needed, but only 90 in Wyoming and Nebraska. Researchers found that more than 95 per cent of kochia seed failed to persist for more than two years.
The findings have important implications for weed control. The authors say pre-emergence weed control should be initiated in the fall or very early in the growing season, to address the first flush of seedling emergence and seed production.
Other management techniques may include cover crops and even waiting for kochia to emerge before seeding.