Volume 38, Number 9 | April 2, 2012
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PRACTICAL PRODUCTION TIPS FOR THE PRAIRIE FARMER
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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 BY ANGELA LOVELL
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here 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
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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
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.
Systemic herbicides will generally be less effective under hot, dry conditions, which reduce a plant’s metabolic and transport processes 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
“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
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PHOTO: KEITH TOPINKA, U. OF ALTA.
Typical drought symptoms in canola, with the bluish wax building on the newer canola leaves as the plant tries to retain more moisture under drought. In volunteer canola, herbicide droplets may roll off the waxy leaves, or not penetrate the leaves as well. Some weeds also develop waxy leaves under drought stress.
PHOTO: KEITH TOPINKA, U. OF ALTA.
Water-deprived plants are shown on the left, versus properly watered plants on the right. Growth is slowed when canola plants are deprived of water, and the crop offers less competition to weeds.
In This Issue
Wheat & Chaff ..................
2
Features ............................
5
Crop Advisor’s Casebook
8
Columns ........................... 22 Machinery & Shop ............ 36 Cattleman’s Corner .......... 40
Mix it up to avoid herbicide resistance LEE HART PAGE 6
Weed control in flooded acres
DANELL VAN STAVEREN PAGE 10
FarmLife ............................ 49