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Controlling weeds in dry conditions

Most of the Prairies saw little rain last fall and had very little snow cover. 
Get ready for the challenge of managing weeds in dry weather

There are very different considerations for managing weeds under dryer conditions. Some herbicides may be less effective and crops are generally less competitive. With winter precipitation levels already well below normal, if the dry conditions of last summer and fall persist into this spring, farmers may have some decisions to make when it comes to their weed management programs this year.

“Under dry conditions your agronomics under planting become even more important because you’re not going to get the crop competition under drier conditions that you would under good growing conditions. That’s going to put more of a premium on your weed management program,” says Len Juras, crop protection research and development scientist with Dow AgroSciences Canada Inc.

Weeds in dry weather

Weeds are more challenging to control in dry conditions than in wet. Just as crops need moisture to grow, so do weeds. But when moisture is limited, weeds use internal survival mechanisms to help them retain as much moisture as possible.

“If we get dry conditions, what happens in a general sense is weeds tend to harden off,” says Juras. “So they grow more slowly. Their cuticles (the waxy protective covering of the leaves) and their tissues get thicker and less responsive. It’s tougher for a herbicide to get into the plant. And once it gets into the plant, its metabolism changes so it’s harder for the herbicide to get to the active sites it needs to target.”

Which weeds?

During dry conditions, how likely is it that farmers will see different weed species emerging than they had under wetter conditions?

“With the extremely wet years that we’ve had, we’re going to need more than one dry year before we see a major change in weed species,” says Eric Johnson, weed biologist with Agriculture and Agri-Food Canada (AAFC) at the Scott Research Facility in Saskatchewan. “In fact, if you look at data from our longer-term studies, we tend to have high weed populations in the spring when we have above average rainfall in August. That wasn’t the case last year, so I don’t expect that the weeds will be as prevalent during burn-down unless we get a lot of snow and rain in the spring. So perhaps not as many winter annual-type weeds are a possibility, and we may see a delay in their emergence.”

Perennial weeds on the other hand, unless they’ve been managed in the fall, might have an advantage if there is little spring moisture.

“There’s likely still going to be lots of subsoil reserve moisture even if there are surface shortages, so you are may see perennials doing quite well because they are already rooted and tapped into that soil moisture that is down below,” says Clark Brenzil, Saskatchewan Ministry of Agriculture weed wpecialist. “They’re going to have the jump right away, if we happen to have a dryer than normal spring.”

Perennial weeds need to be controlled in the fall when they’re laying down their growth for the following year, adds Brenzil, although most farmers do tend to manage them on an ongoing basis as part of their overall weed management strategy. “If you don’t wait until they’re out of hand, and try to manage them as a part of an ongoing program, they won’t tend to be as big an issue as they could escape to be,” he says.

Zero- or minimum-till systems may also help with drier conditions by retaining moisture in the soil. “Under conservation tillage we tend to have a pretty good start to the season,” says Juras. “So I expect that the usual customers will come up, but it will be what happens after that which will determine weed management strategies.”

It’s likely that annual weeds in the C4 class, which originate from arid areas and are better adapted to drought, will be more prevalent during dry weather conditions. This includes plants such as kochia, Russian thistle, lambs quarters, redroot pigweed, green foxtail and barnyard grass.

“C4 plants have an adaptive system that allows them to photosynthesize down to very low carbon dioxide levels,” says Brenzil. “The way that a plant gets carbon dioxide into its leaves in order to photosynthesize is it allows it in through the stomata (pores) in its leaves, which also allow moisture to escape. A C4 plant can close its stomata to reduce moisture loss while still continuing to photosynthesize. This allows it to draw down the carbon dioxide inside the sealed leaf to much lower levels even while the stomata is closed, giving it an advantage over a plant without the same C4 system.”

In dry conditions, weed germination can be hampered by lack of moisture, just as with crops, which also means that crops will be less competitive than usual against weeds. Exceptions are Russian thistle and kochia, which, because of their different physiology, can geminate at lower moisture levels than crop plants. Kochia has evolved to be able to draw water out of saline and dry conditions and Russian thistle, when it germinates, has a miniature plant already well developed inside the seed, which means it can emerge within just a few days even when conditions are dry.

Green foxtail is another weed that may become more prevalent with reduced moisture. Because it doesn’t compete well with cereals under wet, cool conditions, many farmers have opted to skip spraying their green foxtail in recent years and allowed the crop to suppress the weed. During drought conditions, thinner, stressed crops may not be able to out-compete green foxtail.

Control Options

Under dry conditions weeds can be harder to control for a number of reasons.

Almost all herbicides carry a warning stating that they are more effective when weeds are actively growing. Under drought conditions weeds are not actively growing, so sometimes herbicides simply won’t work as well. “If you’ve got drier conditions, then the likely case is that both crops and weeds are under stress. As a result the weeds are going to be harder to kill,” says Brenzil.

Trying to decide which herbicide product to use will start with careful and early scouting to determine which weeds are emerging and likely to become problematic. “You really have to know your weeds during drier conditions because mistakes can cost you a little bit more,” says Juras.

Once weeds are identified, it’s down to choosing a product that offers good control.

“On the product label I look at weed control versus weed suppressed, height and leaf stage,” says Juras. “If a product can control wild buckwheat at the one- to two-leaf stage then I maybe tend to stay away from it, but if I have something that says it can control up to five- or six-leaf stage then I have a little more comfort that that product is going to perform under more challenging conditions, because it has an innate ability to control that weed more easily.”

Systemic herbicides will generally be less effective under hot, dry conditions, which reduce a plant’s metabolic and transport processes, making it difficult for the herbicide to penetrate and move through the plant.

Contact herbicides (such as those in Groups 5, 6, 7, 10 and 14) which are applied to foliage, only kill plant tissue in the immediate vicinity of where the spray droplet lands. They can be more effective than systemic herbicides in dry conditions, but this can also lead to crop injury, especially during the first few hours after spraying.

Timing herbicide applications can be trickier under dry conditions. Farmers must find the balance between applying early enough to prevent weeds from out-competing the crop, but not too early, when weeds could be too small for the herbicide to be effective. As a general rule of thumb spraying at the earlier end of the recommended leaf stage range for a particular product is recommended wherever possible.

In most cases a spring burn-off will get producers of to a good start, especially if winter annuals or early annual weeds are starting to emerge. “Typically what happens under drought conditions is that your early germinators tend to get established and as it gets progressively dryer they have the advantage because they have germinated and established, so you typically don’t get a second flush,” says Juras. “Farmers will probably help themselves quite a bit by making sure they have a good burn off under drought conditions because they want to save any available soil nutrients and moisture for the crop.” If winter annual or early-germinating weeds are allowed to grow they will use up these resources and they will not be there for the crops. University of Saskatchewan research has shown that an early spring burn-off yielded higher than later burn-off, even if the crop was not seeded immediately after.

In areas that have little residual or surface moisture early on, it may be a case of wait and see. “If it’s dry, farmers will have to evaluate if they need that early application,” says Neil Harker, research scientist, weed ecology and crop management at AAFC’s Lacombe Research Centre. “They might want to delay their herbicide application if it’s very dry and the crop is up but there are no weeds yet and wait for some timely rains to get those weeds going. Most herbicides have very little residual activity so if you spray and there’s not much weed material there it’s of little value.”

Application considerations

1. Application rates: The number one thing to remember is to not cut application rates. “When the crop’s not growing as well and the weeds are under stress, you had better keep your application rates high and follow the application parameters in terms of water volumes,” says Johnson.

2. Water volumes: Spray coverage becomes much more of an issue in dry conditions when using contact herbicides. Using the highest recommended water volumes and maintaining larger droplets will help ensure better coverage and efficacy. Even systemic herbicides under drier conditions will perform better at higher water volumes.

3. Water quality: Water quality deteriorates in drought conditions, so using sloughs as a water source may affect some herbicides. Saskatchewan Agriculture’s website gives information about how herbicide effectiveness can be reduced by some water sources. Water quality factors of main concern are cleanliness and mineral ion content. “Water that is not clean and contains suspended silt and organic matter can reduce the activity of the following herbicides: diquat (Reglone, Reward), paraquat (Gramoxone) and glyphosate (Roundup, Rustler, Credit, Factor, Glyfos, Maverick, Renegade, Touchdown, Vantage). It is important to use only clear, clean water for mixing these products. It should be noted that the same kind of inactivation can occur when these products are applied to plant surfaces that are covered with a layer of dust. Dust kicked up during the spray operation may also result in reduced control, especially directly behind the sprayer,” says the website.

4. Adjuvant application: The plants’ defences to moisture loss, like a thicker wax layer or more hairs on leaves, also act as barriers for the herbicide to reach and penetrate the leaf surface, so it’s equally important to apply adjuvants at the maximum recommended rate to help the spray stick to the leaf and not run off.

5. Temperature: In-crop applications with contact herbicides should be avoided during the hottest part of the day to prevent crop injury, which can occur at temperatures above 27 C. Spraying in the evening will lessen the risk of crop injury and allow more time for uptake and diffusion of the herbicide in the target weeds before they are activated by sunlight the following morning.

Herbicide residue

Herbicides break down in warm, moist soils through microbial and chemical processes. When the soil is dry, these processes slow down or sometimes stop. Herbicides that have residual characteristics in the soil can have higher-than-normal carryover following application in dry years. Even if fall moisture is received, after residual herbicide has been applied during dry summer conditions, there is a risk that herbicide carryover could affect the following crop. So it may be advisable to choose herbicides without residual characteristics if soils are dry and a dry weather pattern is already well established at the time of spraying.

Other considerations

“If there is a risk that we’re looking at reduced soil moisture, farmers need to make sure they build a healthy root system on the crop plant,” says Brenzil. “That means don’t skimp on the starter fertilizer. And phosphorus in particular will get it off to a quick start.”

In a dry year some crops may be so stressed that they will end up as livestock feed. Herbicide applications during the growing season, however, may restrict options for grazing or forage uses. Feeding restrictions on many herbicide labels have not been developed. If there is a chance that the crop will end up as forage, farmers should check the product label or check with the manufacturer or a local agronomist. †

About the author


Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at or follow her on Twitter @angelalovell10.



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