Producers are told to spray when weeds are growing, but with both weeds and crop both reeling from a string of stresses and Mother Nature not co-operating, that spray window has been hard to pin down.
Why it matters: Hardened weeds and poor spray conditions may leave producers with less effective weed control than they expect this season.
Temperatures plunged in late May, leading to widespread complaints of frost damage and crop stress. From May 26-28, agro-Manitoba posted overnight lows down to -8.9 C in parts of the Interlake, with frosts lasting up to 11 hours. Across southern Manitoba, temperatures May 26-27 widely hit near or below -2 C with many frosts stretching well over six hours.
Producers were still gauging frost damage when temperatures swung to the opposite extreme days later. By June 2, all of agro-Manitoba was under a heat advisory. By late morning June 4, Environment and Climate Change Canada was forecasting 30 C in parts of central Manitoba, and temperatures were expected to hit the mid-30s on both Friday and Saturday, well above the safe window for spraying. Temperatures were expected to come down into the second week of June, but still hover in the high 20s.
Kim Brown-Livingston, Manitoba Agriculture's weed specialist, urged producers to check their product labels for temperature limits. Most labels set out a temperature window between 21-28 C, producers heard during a Crop Talk webinar put on by the province June 2. Producers ignoring those limits risk both greater crop injury and less control over weeds, Brown-Livingston warned.
Between the two extremes, came the wind. Agro-Manitoba commonly reported wind gusts over 40 kilometres an hour between May 28 and June 1.
Sprayers 101, an online spraying resource for farmers, puts maximum wind speed for spraying around 20-25 kilometres an hour.
On top of that, Brown-Livingston noted, crops were starting the year stressed due to extremely dry conditions and, with the exception of one rain event in recent weeks, have remained dry.
“To me, that’s the big one that we’re dealing with right now,” she said. “We’ve had frost, we’ve got wind, we’ve got extreme heat, but we’ve been dry the whole time through all those three things.
“This is an ugly period for spraying,” she said. “It’s very ugly.”
Producers are now fighting weeds that have been hardened off by those weather events.
Those spraying at the top end of their product label’s safe temperature limit will face weeds that are more tolerant to the product, Brown-Livingston warned. Slower plant metabolism both limits how much product weeds take in, while crops are also metabolizing the product more slowly, increasing the chance of injury. At the same time, weeds will produce more wax layer on leaves during heat, limiting the effectiveness of the herbicide.
Tom Wolf of Sprayers 101 called early weed removal “absolutely essential,” leaving the producer dealing with a smaller, easier to kill and less hardened adversary.
“We’re producing weeds that are a little bit tougher, they’re going to be a little bit harder to control and that probably means that the basics matter,” he said. “I don’t think that there’s a single silver bullet that will help the situation, but probably if we pay close attention to a few smaller things, it does add up and put the odds back in favour of the weed control.”
There is nothing that can be done about the heat — over a certain temperature, spraying should no longer be on the table, Brown-Livingston said.
“We can mitigate wind to some extent,” she said. “We can use low-drift nozzles. We can use coarser sprays, we can drop the booms low, you just have to make sure you’re getting the proper overlap... we still have to be prudent and good stewards and do all of that properly, but the heat, there’s not a lot we can do about.”
Once temperatures drop back into the high end of that safe range, however, the provincial weed specialist did have some advice on best practices.
Producers spraying during the morning will take advantage of lusher plants not yet stressed by the day’s heat, she noted.
Tweaking water and application rates may also give a better chance for success. She urged producers to use the maximum possible water rate for both good coverage and crop health. Systemic herbicides should veer to the high end of the labelled rate, she said, while contact herbicides should lean the other way.
“If we’re under stress, we want to make sure that herbicide works as good as it can on the weed, but we also want to make sure it’s as easy as it can be on the crop, and the big thing you can probably do with that is water volume,” she said.
The producer may also want to consider an adjuvant or surfactant to help bolster uptake on the leaf.
Hard water and water high in bicarbonates also impact some herbicide performance, while things like iron and manganese can lead to buildup that plugs nozzles and screens, the province has said.
“We ought to be aware of what’s in our water and take the best corrective action,” Wolf said. “In most cases, the water that you have, you’re going to be able to use it, but you might have to add a conditioner.”
Less popular for many producers, he noted, is paying attention to uniformity in the field. Driving faster with a higher boom creates patches of over- and under-dosing, he said, while experts have long noted that a lower boom helps limit risk of drift.
Droplet size can also determine if the product even makes it to the plant, since smaller droplets will dry faster. The provincial weed specialist urged producers to keep spray as coarse as possible, according to the label.
Sprayers 101 has been pointing producers to Delta T, rather than just relative humidity, for gauging evaporation risk, Wolf said.
Delta T is measured as the difference between the dry bulb temperature and the wet bulb temperature, “which basically gives you a direct index of how quickly water evaporates,” Wolf said. “Relative humidity doesn’t do that because the risk of water evaporation at any one relative humidity depends on the air temperature.”
Sprayers 101 recommends an upper Delta T limit of eight.
Wolf also urged producers to pay attention to both the quality of their water, and the quality of their spray.
“Oftentimes, a sprayer will be equipped with one set of nozzles for their post-emergent work, and we might need to fine-tune that a little bit,” he said.
A finer droplet is better for smaller and grassy weeds, although Wolf acknowledged Brown-Livingston’s point on evaporation of fine droplets during heat.
“For the most part, let’s be realistic, we’re doing a one-pass herbicide situation with a tank mix or a product that has a wide spectrum, and we are targeting both grasses and broadleaves at the same time,” he said. “The grasses and the small weeds are always going to be the limiting factor, with relatively few exceptions, so we are in a tough spot.”
Spraying in the cooler part of the day, while dodging many of those issues, often pushes into periods of the day that experts typically tell producers to avoid, due to the risk of inversion or drift.
If the window won’t open, however, the biggest answer might be productivity, Wolf said.
Much of his research involves getting as many acres covered in the small periods that a producer might have suitable to spray, while still keeping driving speed slow.
“The answer’s always been to look at how you spend your time and to get rid of the time-wasters,” he said, citing things like filling, waiting on trucks or cleaning.
“A lot of small things add up and before you know it, a half-hour’s gone,” he said. “How much can you do in half an hour? You could do 50 acres in half an hour, easily.”
Making the best of it
Even with the best practices, however, Brown-Livingston warned, this year might be about what’s possible.
The right staging window is tight, she acknowledged, and the risk of losing that window due to weather is very real. Some producers, she said, will not be able to wait for better conditions.
“We’re doing the best we can, and I think we just have to be realistic that there might be some escapes, because some of these weeds are going to be bigger. They might be hardened off. We might not get great control,” she said.
Brown-Livingston turned producers to resources such as the SPRAYcast forecasting system, through the Manitoba Potato Network, or Manitoba Agriculture and Resource Development’s network of weather stations, to hone in on a spray window ahead of time and be prepared to go when conditions allow.