Tumbleweed illustrates tillage conundrum

A photo of tumbleweed

Glyphosate-resistant kochia, commonly known as tumbleweed, has become a case study in the US for the devastating effects of herbicide resistance on cropping.

Dr Phillip Stahlman of Kansas State University in the US said kochia (also known as fireweed, burning bush or tumbleweed) was first reported to be glyphosate resistant in 2007 in western Kansas. In crops, kochia grows up to 50 centimetres high, while the roots extend to a depth of two metres. Kochia is a prolific cross-pollinator, which contributes to the wide genetic diversity of the species.

The base of the plant breaks off at maturity and tumbles and bounces across the landscape. Seed is dispersed every time the dried plant touches the ground.

In 2010, there was an exponential increase in glyphosate-resistant kochia, particularly among growers using no-till. Dr Stahlman says several crops he visited were almost not worth harvesting because of the level of kochia infestation.

By the end of 2011, he and his team had confirmed glyphosate resistance in more than 50 populations of kochia from Oklahoma to Kansas, Nebraska, South Dakota, North Dakota and north into southern Alberta in Canada. A visual survey of 1500 winter wheat stubbles in western Kansas in late 2011 revealed that 64 per cent had been sprayed and 31 per cent had been tilled.

Photo of a man

Dr Phillip Stahlman

Nicole Baxter

In late 2012, an online survey of 52 crop consultants (representing about 500,000 hectares) showed 70 per cent had no problem controlling kochia before 2007. But by 2012, 70 per cent of paddocks had kochia growing and a third were found to be glyphosate resistant.

“We have developed a serious problem very quickly that now threatens many of our production systems throughout the central and northern US,” Dr Stahlman said.

He said tillage had become the most cost-effective, but hopefully short-term solution, for many growers. His worry is that if tillage remains the only option, it threatens the gains made through conservation agriculture and this, in turn, questions long-term farm sustainability.

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Dr Phillip Stahlman,

Region Overseas, North, South, West