Weeds pose the greatest risk of crop loss for wheat and barley farmers, but the increased use of herbicides to kill these weeds has meant that certain species are now resistant to herbicide chemicals, recent research has found.

Researchers in New Zealand have assessed the risk of herbicide resistance in 101 weed species known to affect wheat and barley crops on Kiwi farms.

While most species (55) were considered to be low-risk of developing resistance, 30 were medium-risk, and 16 were considered high-risk.

The top ten scored weeds were:
Echinochloa crus-galli
Poa annua
Lolium multiflorum
Erigeron sumatrensis
Raphanus raphanistrum
Lolium perenne
Erigeron bonariensis
Avena fatua
Avena sterilis
Digitaria sanguinalis

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Seven out of ten high-risk weeds were grasses.

The most used herbicides were synthetic auxins, an enolpyruvylshikimate-phosphate synthase inhibitor, acetolactate synthase (ALS) inhibitors, carotenoid biosynthesis inhibitors, and long-chain fatty acid inhibitors.

Herbicides containing acetolactate synthase inhibitors were found to pose the greatest risk of causing resistance for more species than other kinds of weed killers.

Despite pre-emergence herbicides being known to delay resistance, New Zealand farmers only applied flufenacet and terbuthlazine with high frequency, researchers said.

The researchers concluded that New Zealand farmers should be informed about the high-risk weed and herbicide combinations to avoid future crop loss.

"Based on our analysis, surveys for herbicide-resistant species should focus on the high-risk species we identified".

Read the full report on Plos One here.