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Australian Herbicide Resistance Initiative (AHRI)

Think outside the drum

Nature landscape of grass trees and sky

If herbicides alone were the answer to all of our weed problems we would have eradicated crop weeds years ago. Most of us now realise that to achieve true weed control success we need to add non-herbicide tools into the mix. It doesn’t matter which weed, which crop, or which country we are talking about, the benefits of good cultural practices apply everywhere.

Some excellent long term Canadian research has confirmed how important it is to combine good cultural practices with herbicide use. The Agri-Food Canada trials show that after nine years of full herbicide rate, high barley seeding rate, and a diverse crop rotation, there were zero wild oats in trial plots. Any reduction in seeding rate, diversity of rotation, or herbicide rate always resulted in more wild oats in crop and a higher wild oat seed bank.

Increasing crop competition with weeds results in more crop and less weeds. Why not try increasing the cereal seeding rate on a weedy paddock this year?

Wild Oat is Western Canada’s worst crop weed, and now over 20% of cropland has herbicide resistant wild oats. Canadian researchers John O’Donovan and Neil Harker from Agriculture and Agri-food Canada established four long term trials in Western Canada in 2001. Two of these trials continued until 2009.

Treatments investigated:

  1. Semi-dwarf v tall barley cultivars
  2. Two barley seeding rates – normal (200 seeds /m2) and twice normal
  3. Two rotations – continuous barley or a diverse rotation of barley-canola-barley-field pea.
  4. Herbicides applied at 25%, 50% or 100% of recommended rates.

Table 1. Summary of key research findings from the Beaverlodge site. Wild oat seed bank was measured by taking soil core samples and sieving in 2009. Barley yield was measured in 2009 at the completion of the trial. All plots were sown to barley in 2001 and 2009.

Table for yield based on herbicide rate

It is a pretty simple message. Full herbicide rate combined with crop competition and sound crop rotation results in more crop and fewer weeds.

Anecdotal evidence by the author suggests that herbicides tend to be more effective when applied to small plots in ideal conditions compared to those applied on a vast scale on large farms. The reduced herbicide rates in this trial are an attempt to simulate resistance and/or the variability of the efficacy that can occur at the farm level due to factors such as weather, stages in weed development and general application errors. Other research has confirmed that reduced herbicide rate often results in faster resistance evolution. These trials were conducted on weeds with very low resistance levels.

Cultivar – there was no difference in wild oat biomass or seed bank between semi-dwarf or tall barley varieties. The semi-dwarf variety used was a better performing variety than the tall variety. The semi-dwarf, well adapted variety competed equally with weeds as the taller variety.

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