EDMONTON – Recent surveys suggest 40 percent of farmers in Western Canada have applied herbicide at lower-than-recommended rates at one time or another.
Eric Johnson, weed biologist with Agriculture Canada at Scott, Sask., said the practice can pay off, but farmers must be aware of how to minimize the risks.
Johnson said recent research with Ken Kirkland at Agriculture Canada’s research centre in Scott and Rick Holm at the University of Saskatchewan in Saskatoon on reduced rate weed control looked at the effect of Horizon application rates and timing for control of wild oats. Wild oats were sprayed at the two-, four- and six-leaf stages using the full rate, two-thirds the recommended rate and one-third the recommended rate.
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Johnson said the plots at Scott had relatively low wild oat populations, while plots in Saskatoon had high populations.
“There was a reduction in yield as they delayed spraying,” he said.
“It’s been well established, the importance of early weed removal, but at these low (weed) populations, we were getting pretty good control even at a one-third rate.”
Researchers did not tank mix Horizon with broadleaf herbicides.
“Broadleaf herbicides will antagonize this grassy herbicide, so one-third rate would be too low,” Johnson said.
The economic benefit from cutting application rates on low weed populations was about $4 per acre at the two-thirds rate and $12 per acre at the one-third rate.
“In Saskatoon, with a higher wild oat population, the timing was much more critical. Going past the four-leaf stage, the yields declined dramatically. Cutting to a one-third rate saw a definite reduction in yield. We only had a $2 per acre economic advantage applying Horizon at the two-thirds rate when wild oat populations were really high.”
Johnson said Agriculture Canada researcher Tom Wolf of Saskatoon has been looking at different water volumes and droplet sizes and the effect they have on Horizon, a Group 1 product.
He uses a special type of sprayer that maintains the same pressure while adjusting the water volume.
“Water volume didn’t have a huge effect on the activity of Horizon at full rate, but once you go to a coarse droplet size, you start seeing some reduction in control, particularly when we go below 7.5 gallons per acre,” Johnson said.
“When we cut the rate in half, it’s much more variable, but you can see low-rate, low-water volume, very coarse spray provides some of the lowest levels of weed control.”
In experiments with Achieve at the Scott plots, Johnson saw results similar to what Wolf did with Horizon, though he said rates cannot be reduced to one-third and still achieve adequate weed control. In Saskatoon, with high weed densities, yield declined in step with application rates and delayed
application.
“With water volumes, on Achieve we didn’t see a problem going from 10 gallons to five gallons of water per acre, but at all rates, when we started going below five gallons of water, we started to see higher levels of wild oat biomass and lower levels of weed control.”
Johnson said Achieve is sensitive to ultraviolet light and researchers saw benefits in spraying in the evening when they started cutting rates.
However, this does not apply to all herbicides.
Researchers have also looked at the economic advantages and disadvantages of cutting the Achieve rate in half in different parts of Alberta. Results ranged from an advantage of less than $10 per acre to a disadvantage of $50 per acre.
“Their average economic disadvantage for cutting the rate was a loss of $20 per acre,” Johnson said.
“Cutting these rates is not without risk. The other thing they did find was that there was a three- to five-fold increase in the amount of wild oat seed produced per sq. metre.”
