Drought can complicate weed control efforts

Slow-emerging weeds make for tough spraying decisions, while crops that are slow to develop also present challenges

Slow crop development, low weed pressure and dry soil conditions are adding a stressful wrinkle this year to in-crop weed control efforts.

Clark Brenzil, provincial weed specialist with Saskatchewan Agriculture, said some farmers are unsure about when to spray annual weeds due to poor soil moisture.

Others are delaying in-crop herbicide applications or considering skipping them altogether, at least until they see what the short-term weather forecast holds.

Brenzil said June 12 that one of the biggest weed-control challenges growers are facing this year is rooted in the fact that annual weed growth has been slow and future weed pressure is hard to predict.

Populations are low and, in many cases, the summer annuals that have emerged have not advanced quickly.

This has growers weighing the economic benefits of applying more chemical to fields that may have limited yield potential.

“There’s some uncertainty with producers about whether they need to spray, or if they should spray,” Brenzil said.

At the same time, growers are concerned that weed pressure could ramp up quickly, especially in areas that receive rainfall.

Growers who are uncertain about whether to apply in-crop herbicides under drought conditions should keep a few things in mind.

For starters, assess the staging of the crop relative to weed development.

At a certain point in a crop’s development, the crop will out-compete weeds that are small or recently emerged.

“If your crop is at the six-leaf stage or later … the odds of any weeds that come after that … actually having an impact on yields … are relatively low,” Brenzil said.

“It’s largely just a factor of the crop being so far ahead that any weeds that come after that point are non-competitive to the crop.”

In instances where the crop has emerged but is slow to develop, decisions about whether to apply in-crop herbicides become more difficult.

Tammy Jones, provincial weed specialist with Manitoba Agriculture, said it’s generally recommended that growers control annual weeds as early as possible, even in thin crop stands.

Thin stands are less competitive and are therefore more prone to yield loss, especially when competing against weed species that are more drought tolerant than the crops being protected.

Different crops have different critical weed-free periods, added Jones, who prepared an online information sheet that can be found at bit.ly/2wQwJyz.

During critical weed free periods, it’s important to minimize weed competition in order to maximize yield potential and reduce absorption of nutrients and soil moisture reserves by unwanted competitors.

Weeds are generally easier to kill when they are smaller and less advanced.

Drought conditions so far this year warrant a close examination of the weed species being targeted.

Some annuals that are short in stature and look to be slow developing might actually be more physiologically advanced than they appear.

In those cases, weed populations may have already advanced beyond the point where they can be effectively controlled by a specific herbicide product.

Applying chemicals under extremely dry conditions is never an ideal practice, Jones said.

Applications to moisture- or heat-stressed crops can injure the cultivated plants and will often produce less-than-satisfactory results in terms of weed control.

If weeds are dormant or are not growing aggressively, herbicide movement in the plant is usually reduced because of slower rates of translocation and metabolism, Jones said.

Some weed species, such as lamb’s quarters, may develop a thick wax layer on the leaf surface, which acts as a barrier to herbicide absorption, she added.

When selecting a herbicide, growers should also be cognizant of the fact that contact herbicides and systemic herbicides —depending on environmental conditions — may provide different weed control results and create different production risks in terms of crop injury.

Contact herbicides, for example, may cause more crop injury when applied at temperatures higher than 25 C.

Jones said systemic herbicides are best applied early in the morning, after plants have recovered from the heat of the previous day.

Contact herbicides are best applied in the evening after daytime temperatures have dropped and more moderate temperatures will result in increased chemical efficacy.

Across much of the West, the most prevalent weed species affecting this year’s crops are warm-season annuals that thrive under hot conditions and are typically more drought tolerant than the crops with which they’re competing.

In drought-affected areas of Manitoba, weed species such as kochia, redroot pigweed and foxtails are prevalent.

High populations of Russian thistle and flixweed have also been reported.

Weed species that are more drought-tolerant than the crop in which they’re found can cause significant yield losses.

In addition, failure to control those species will add new seeds to the weed seed bank, creating additional challenges in subsequent crop years.

Brenzil said the transition from a string of relatively wet years to a series of relatively dry years has prompted many growers to reclaim low spots and slough bottoms in their fields.

Reclaimed areas may produce heavy populations of saline tolerant weeds such as green foxtail, kochia and Russian thistle.

“It follows that a lot of the weeds that we will see this year are warm-season weeds that generally tolerate dry conditions as well,” said Brenzil.

“Kochia and Russian thistle are two weeds that are quite well-adapted to dry conditions and will actually germinate under conditions where a crop won’t.”

He said growers in drought-stricken areas may be inclined to take a wait-and-see approach before determining whether a herbicide application is warranted.

This can be a good strategy, but results are not always guaranteed.

If growers are fairly confident in the weather forecast, a decision to postpone herbicide applications for five to seven days — until after a rainfall — will allow plants to begin actively growing and should result in better herbicide activity, Jones said.

Herbicide labels should provide information on the appropriate crop stage for application, as well as the target weed size for optimal efficacy.

Always read the label carefully and keep in mind that weeds that are not controlled this year will set seed and become a greater problem in future crops.

Growers should also refer to provincial crop protection guides for information about economic thresholds for control of different weed species.

In areas where extremely dry conditions persist, growers should also consider production risks associated with chemical residues and carry-over injury.

“Because we haven’t had a lot of precipitation in some areas, a lot of those herbicides will still be bound to the soils, so if we do get a decent rainfall … a lot of that stuff will get liberalized from the soil and it will become available for the plants to take up,” Brenzil said.

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