Soil bacteria eat into herbicide efficacy

A photo of a man at a microphone

Dr Dale Shaner from the US Department of Agriculture explored the implications of the accelerated breakdown of triazine herbicides for Australian growers at the GRDC Grains Research Update for Advisers in Goondiwindi, Queensland.

 PHOTO: Clarisa Collis

Faster breakdown of triazine or Group C herbicides, and its implications for growers around the world, was a central subject at the recent GRDC Grains Research Update in Goondiwindi, Queensland.

Keynote speaker Dr Dale Shaner, from the US Department of Agriculture, said it was a global phenomenon and Australian grain growers may also need to rethink their conventional use of triazine herbicides for long-lasting weed control.

Dr Shaner said the decline in herbicide efficacy had been seen in simazine, cyanazine and ametryn, but the faster rate of degradation was particularly notable in atrazine.

He said the rapid degradation of triazine herbicides had resulted from an adaptation in soil bacteria, which was first detected in atrazine-contaminated soil collected from the US and Canada in the mid-1990s.

Since then, researchers have identified a diversity of soil microbes across six continents that can quickly degrade atrazine, and other closely related triazine herbicides, into carbon dioxide and nitrogen.

A small piece of DNA contains the genetic material that enables the soil microbes to consume triazines.

For growers, this development means the residual effects of triazines in soil have been cut short by at least 30 per cent, leading to ineffective weed control.

Dr Shaner has developed a laboratory assay that shows the adaptation has reduced the half-life (time it takes for half of the herbicide to degrade) of atrazine from more than 10 days to between one and three days.

In other words, atrazine applications at recommended label rates provide effective weed control for about a week, he said.

On-farm influences

The rate of herbicide breakdown is also influenced by on-farm factors, such as soil pH, frequency of atrazine use and environmental conditions.

In the past, triazines had a tendency to degrade more rapidly in acidic soils, but now the reverse is occurring. Research shows they are breaking down faster in alkaline soils, with a pH greater than 6.2.

In 2002, Dr Shaner found atrazine was being degraded more rapidly in paddocks where there had been a history of its use.

This finding was supported by a survey that revealed atrazine or simazine degradation was accelerated in about 80 per cent of paddocks in the US where it had been used in the past.

In situations where triazines had not been applied to paddocks for about five years, their breakdown was delayed for about three to four weeks, he said.

The half-life of triazine herbicides is also said to become shorter with each subsequent application during a season.

On a more optimistic note, Dr Shaner said faster triazine breakdown also had potential benefits, such as reduced risk of carry-over to sensitive follow-on crops, which could allow better crop sequencing options.

Decreased risk of triazines leaching into groundwater was another benefit resulting from the actions of the soil bacteria.

More information:

Dr Dale Shaner,
dale.shaner@ars.usda.gov

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