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Thirty years ago, herbicide resistant weeds were a mere academic curiosity, herbicides dominated weed control (and always worked well) and GM stood for General Motors.

The present reality is that weeds have evolved resistance to 21 of the 25 known herbicide modes of action that underpin 148 different herbicides. Herbicide resistant weeds have now been reported in 63 crops in 61 countries.

In another significant shift, GM is now intimately associated with ‘glyphosate resistant’ soy, cotton, maize and canola, which in the US and Canada has become the most rapidly adopted technology in the history of agriculture.

Glyphosate resistant weeds now dominate herbicide resistance research, with 24 species across 18 countries reported as resistant to glyphosate.

No longer an academic curiosity, herbicide resistant weeds represent a major annual threat to crop production, profitability and sustainability across the globe.

Glyphosate lost to the US

A recent US and Canadian survey revealed that almost half of all growers in these countries suspect they have glyphosate resistant weeds in their GM cropping systems. In the US, 25 million hectares – about the total area of cropping in Australia – is infested with glyphosate resistant weeds.

The biggest weed problem in the US is Palmer amaranth, a succulent that emerges over seven months, grows at a rate of three to 10 centimetres per day and generates one million seeds per plant. Glyphosate resistant Palmer amaranth is now in 26 US states and is spreading at a fast rate.

In Argentina, glyphosate resistant Johnson grass is also a rapidly growing issue.

Many now believe glyphosate is lost to the US, with Argentina and Brazil likely to suffer the same fate given their high adoption of glyphosate resistant crops.

In an ironic twist, some US growers have had to resort to the age-old practice of hand-weeding and deep tillage to manage herbicide resistant crop weeds.

In Tennessee, where nearly all growers practised no-till five years ago, more than half have now had to return to deep tillage to bury herbicide resistant weeds.