Your fall weed control checklist

Attention to fall weed control is needed as harvest nears its end.

Speaking during a Manitoba Agriculture webinar Sept. 25, provincial weed specialist Kim Brown singled out weeds that thrive in salinity, which concerned many producers this spring.

Kochia is perhaps the most infamous weed threat on that list. A prodigious spreader cursed for its tendency to bypass herbicides, kochia is a persistent late-season menace.

“We’re always talking about kochia in the fall,” Brown said. “It stays a really long time. It interferes with harvest.”

She has heard of large areas left unharvested due to proliferation of the weed.

“We don’t want to put that green material through a combine. We also don’t want to get those seeds back out onto the land to grow again for the next year or the next couple of years.”

Fortunately, kochia seeds have a short lifespan, usually only two years. If salinity is helping a kochia population thrive, producers should think carefully about their field work strategy, she warned, because tillage is likely to make the problem worse.

Manitoba Agriculture soil management specialist Marla Riekman has repeatedly noted that salinity is a water problem, and management should involve practices that will draw down a high water table. Growing weeds will do that, particularly in areas so saline that nothing but weeds will grow.

In a Co-operator article last year, she suggested mowing could keep kochia from going to seed while also growing in a saline patch.

Foxtail barley is another saline-tolerant enemy, and pigweed species are another potential threat, the big ones being Palmer amaranth and waterhemp. They can regrow after harvest, Brown noted.

“They’ll be very, very small plants, but they can still set some seed.”

Waterhemp drew Brown’s particular attention. Reports of the noxious weed have grown this year, helped by the warm fall.

Round-leaved mallow, a low-growing weed, is also an issue, especially in wide-row corn crops.

“These are big weeds. They’re really more of a harvest nuisance right now for the crops that we are still harvesting,” Brown said.

Plan of attack

Producers are advised to wait four to six weeks post harvest before spraying. Temperatures should be above 8 C for a few hours after application, and the best times are mid-morning or early afternoon.

After a frost, spray if 60 per cent of weed leaf tissue is green and pliable. Low rates of glyphosate won’t be effective if there is less, so it’s best to wait two to three days after a frost to assess leaf survival, Brown said.

It’s also best to wait at least three days or longer before tillage, to allow herbicide to reach the roots of weeds.

She offered the following product-specific advice.

Blackhawk: pyraflufen-ethyl (Group 14) and 2, 4-D (Group 2): Apply on many broadleaf weeds that are less than two inches in height or width.

Distinct: dicamba (Group 4) and diflufenzopyr (Group 19): Apply on biennial wormwood, cocklebur, common and giant ragweed, kochia, Canada thistle, round leaf mallow, lady’s thumb, dandelion, lamb’s quarters, perennial sow thistle, pigweed, volunteer canola, wild buckwheat and waterhemp.

Express SG: tribenuron-methyl (Group 2): Use on narrow-leaved hawksbeard, flixweed, dandelion, cow cockle, lamb’s quarters, wild buckwheat and volunteer canola.

Express FX: tribenuron-methyl (Group 2) and dicamba (Group 4): Use on Canada fleabane, volunteer flax, dandelion, volunteer canola, cleavers, wild buckwheat, chickweed, Russian thistle, common ragweed, white cockle, flixweed, cow cockle, kochia, pigweed, lamb’s quarters and narrow-leaved hawksbeard.

Express Pro: tribenuron-methyl and metsulfuron-methyl (Group 2): Use on Canada thistle, white cockle, dandelion, cow cockle, night-flowering catchfly, wild buckwheat, narrow-leaved hawksbeard, volunteer canola, and cleavers. Apply late in the season to gain control in spring.

Heat: saflufenacil (Group 14): Can be mixed with glyphosate (use Merge) to deal with cleavers, wild buckwheat, common ragweed, volunteer canola, dandelion, round-leaved mallow, kochia, narrow-leaved hawksbeard, lamb’s quarters and flixweed.

Korrex II: dicamba (Group 4) and florasulam (Group 2): is used at 56 acres/case for dandelion, perennial sow thistle, kochia, cleavers, chickweed, cow cockle, narrow-leaved hawksbeard, shepherd’s purse, wild buckwheat, smartweed, stinkweed and volunteer canola.

Paradigm PRE: halauxifen (Group 4) and florasulam (Group 2): Use at 60 acres per jug on dandelions, shepherd’s purse, chickweed, cleavers, cow cockle, night-flowering catchfly, Canada thistle, pigweed, sow thistle, perennial sow thistle, smartweed, light infestations of kochia, volunteer canola, white cockle, barnyard grass and wild buckwheat.

PrePass XC and Flex: florasulam (Group 2) and glyphosate (Group 9): Use at 80 acres a jug for PrePass Flex 80 and at 40 acres a case for PrePass XC. They can be used on Canada fleabane, chickweed, cleavers, common ragweed, cow cockle, dandelion, flixweed, kochia, lamb’s quarters, narrow-leaved hawksbeard, pigweed, shepherd’s purse, smartweed, wild buckwheat and volunteer canola.

Smoulder: saflufenacil (Group 14) and metsulfuron (Group 2): Use for dandelion, Canada thistle, kochia, cleavers, Canada fleabane, lamb’s quarters, stinkweed, redroot pigweed, round-leaved mallow, narrow-leaved hawksbeard, volunteer canola, wild mustard and wild buckwheat.

Authority 480: sulfentrazone (Group 14): This requires a minimum of half an inch of moisture to activate and spring snowmelt can activate it early. It’s used on kochia, Russian thistle, cleavers, eastern black nightshade, lamb’s quarters, purslane, pigweeds, wild buckwheat, and waterhemp. It should be used at the last pass across the field and at an 88-millilitre rate for mustard and wheat, and a 118 ml rate for fava beans, peas, flax, sunflower and soy. It should not be applied to coarse-textured soils (sand, loamy sand, sandy loam) or fine textured soils or fields treated with sulfentrazone products within the last 24 months, including applications in fall.

Authority Supreme: sulfentrazone (Group 14) and pyroxasulfone (Group 15): This also requires a minimum of half an inch of moisture to activate and is used on kochia, cleavers, eastern black nightshade, common ragweed, lamb’s quarters, purslane, pigweeds, waterhemp, wild buckwheat, barnyard grass, downy and Japanese brome, green and yellow foxtail and wild oats. It should be applied in the last pass of the field, and under the same rules for soil and sulfentrazone products as described for Authority 480.

Fierce: flumioxazin (Group 14) and pyroxasulfone (Group 15): This requires the same moisture to activate as Authority and works on dandelion, Canada fleabane, cleavers, common ragweed, kochia, lamb’s quarters, pigweeds, smartweed, volunteer canola, waterhemp, wild buckwheat, barnyard grass, foxtail barley, green foxtail, wild oats and downy and Japanese brome.

Focus: pyroxasulfone (Group 15) and carfentrazone (Group 14): Has the same moisture requirements as products noted above and is used on kochia, cleavers, lamb’s quarters, pigweeds, stinkweed, waterhemp, wild buckwheat, wild mustard, barnyard grass, foxtail barley, downy and Japanese brome, green and yellow foxtail, wild oats, Italian ryegrass and annual bluegrass. It should be used in the last pass across the field at 33 acres per jug if soil has more than three per cent organic matter, and 40 acres per jug if organic matter is less than three per cent. It should not be applied if the pH is under 7.8 or on sandy soils with higher organic matter.

Valtera: flumioxazin (Group 14): Requires a quarter-inch of moisture to activate and is used to treat Canada fleabane, cleavers, chickweed, common ragweed, dandelion, kochia, lamb’s quarters, nightshades, pigweeds, waterhemp, wild buckwheat, green foxtail and volunteer canola.