EDMONTON Ñ Recent surveys suggest as many as 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 know how to minimize the risks.
“In the more humid areas, we find a higher percentage of growers that will reduce their herbicide rates than in the drier areas. That makes sense,” said Johnson.
“Producers cut rates more in wheat and barley. In canola, they’re more likely to use the recommended rate.”
Read Also
VIDEO: Green Lightning and Nytro Ag win sustainability innovation award
Nytro Ag Corp and Green Lightning recieved an innovation award at Ag in Motion 2025 for the Green Lightning Nitrogen Machine, which converts atmospheric nitrogen into a plant-usable form.
He thinks that’s because half of all the planted canola is Roundup Ready, and the recommended application rate for Roundup is low. As well, he said the price for Roundup isn’t that high compared to other herbicides.
“In the Liberty Link varieties, there’s not much room to be cutting the Liberty rate. So where you see growers cutting rates are typically in cereal crops.”
Johnson said crop health and competitiveness have a big impact on how well herbicides work. A crop gets the best chance to emerge and compete when there are high plant populations, fertilizer is put where crops can access it and weeds can’t, and seedlings emerge quickly.
Herbicide rates can be altered by spray application timing, susceptibility of weeds, droplet size, application volume and the use of certain chemistries.
Johnson said a study on canola vigour index, which is percent germination x 1,000 kernel weight, linked ground cover and crop yield.
“Moving from two grams per 1,000 seeds to 3.5 grams per 1,000 seeds provides a considerable difference in ground cover. The higher 1,000 seed weight provides more cover and is more competitive with weeds, so it’s important to use high quality, high vigour seed.”
Research in Alberta from Lethbridge and Lacombe by Neil Harker and Bob Blackshaw shows the importance of good growing conditions for weed control. Johnson said Harker and Blackshaw measured the rate of wild oats growth 24 hours before a herbicide application, under a range of conditions.
“As the growth rate increased, up to an inch of growth over 24 hours, weed control increased dramatically. They only saw differences when they cut to a quarter rate. At full or even half rate, they didn’t see much difference. That rate has been set to work under a wide number of environments.”
With good growing conditions, they were able to get about 65 percent weed control at one-quarter of the recommended herbicide rate. Johnson said while the industry standard for commercially satisfactory weed control is greater than 80 percent, there was still weed control at a greatly reduced rate.
Johnson said new spray nozzles on the market can have an influence on reduced herbicide rate applications.
“We’ve gone from a flat-fan nozzle that produces a fine droplet size and very good coverage, to a wide array of nozzles designed to reduce spray drift. Throw in a variable where we’re looking at coarser droplets and you can run into situations where coverage is lower, particularly when cutting water volumes.”
Johnson said farmers will see problems with grasses more than with broadleaf weeds because of leaf orientation and leaf size. Grassy weeds have a small leaf area so they are less susceptible to spray than broadleaf weeds, which tend to have more horizontal leaves.
“The herbicide rate affected weed control more frequently than spray quality (fine versus coarse). Grassy weeds are more sensitive to application methods than broadleaf weeds. Groups 1, 9 and 10 were most sensitive to application methods with grassy weeds. Group 6 and 9 for broadleaf weeds,” he said.
“The Group 2 products were the least sensitive for both grassy and broadleaf weed control and appear to be well-suited for low drift sprays. A lot have some soil activity and seem less affected. More frequently we saw reduction in control when we went to coarser sprays in Group 1 compared to Group 2 sprays.”
The grass versus broadleaf issue is especially obvious with Liberty on the InVigor hybrid
canolas.
“If you’re going to have failures with Liberty, you’re going to have failures on grasses more so than the broad leaves, because coverage is very important. Going to a very coarse droplet size or lowering your water volume, you start seeing a reduction in control. Liberty is one product where I would not go below eight gallons of water per acre.”
Johnson said a study at the Scott research station with the herbicide Everest looked at the full rate and the two-thirds rate, with 10, five and three gallons of water per acre.
“There was some slight reduction in control, but not significant when going as low as three gallons of water per acre. And there was no difference in control when we go from full rate down to two-thirds rate,” he said.
“The reason, we think, is Everest is pretty hard on the crop at that full rate. We feel at the two-thirds rate, we were getting less crop injury. The company agrees with us and they are looking at registering that rate for Everest for wild oat control.”
Johnson said by lowering the water volumes and herbicide rates and going to a coarse droplet size, there was little effect on weed control.
Everest has some broadleaf weed activity. He said it’s good on wild mustard and redroot pigweed and has some activity on wild buckwheat.
In 2003 and 2004, Johnson compared crop competitiveness, early versus late herbicide applications and droplet size.
He wanted to see if a more competitive crop would compensate for reduced coverage with large droplet sizes.
“If you have a coarse spray, you have much more flexibility in timing. You can spray earlier or in windier conditions. The hypothesis was, if we go out early in competitive crops, maybe we lose a little bit of wild oat control, but maybe we can keep the yields up.”
In 2003, Johnson used Polish canola and barley as the competitive crops, with flax as the not so competitive choice.
“We did get an interaction of crop type and spray quality. In the barley, it didn’t matter what sort of droplet size we had. This was with Group 1 herbicides. Even on the canola it didn’t make a difference. But in the less competitive flax crop, we started getting higher levels of wild oat biomass when we went to a very coarse spray. We didn’t see an interaction of crop type and yield, provided we sprayed early,” he said.
“We had an interaction of spray quality and timing Ñ spraying seven days or 17 days after emergence. The very coarse spray yielded as well as the coarse or fine at seven days. It even yielded better than the fine spray at 17 days after emergence.”
In 2004, Johnson looked at four crops, barley, semi-dwarf wheat, canaryseed and flax, but this time used the same herbicide combination in all of them.
“Puma Super and Buctril M are not registered in canaryseed, but we are looking at getting them registered. And Puma Super is not registered in flax, but Fusion is and it contains fenoxaprop, which is the active ingredient in Puma Super,” he said.
“We got similar results as we did in 2003. In the competitive crops, the very coarse nozzles did not have any impact as far as weed control was concerned. But in canaryseed and flax, we started seeing a reduction in weed control when we went to very coarse droplets.
“And we did find an interaction in timing. Canaryseed and flax are very slow to establish and we based our spraying time on the one- to two-leaf stage on barley. We did see in the slower establishing crops that we could spray a little too early. There was no interaction between nozzle and timing, but there was a reduction in yield if we sprayed later. The bottom line is, we can maintain yields with these coarse sprays, even though we don’t get quite as good wild oat control, provided we apply early.”
More information on herbicide application rates will be provided in coming weeks.
