Deal with weeds in the fall for a fresher spring

Source: Saskatchewan Agriculture and Food

If Mother Nature has been good to you and your crops are largely in the bin, October is a good month to think about fall weed control.

Clark Brenzil, provincial specialist in weed control with Saskatchewan Agriculture and Food (SAF), says there should still be some time for control measures before the snow flies.

"The challenge that arises with producers harvesting larger acreages in fall is that, unless the weather co-operates, they may not get done until very close to freeze-up," he stated. "At that point, it may be too late for some perennial weeds. But this year we saw many crops come off in mid-summer, which may present some opportunities for fall weed control."

Brenzil says that, for some weeds like Canada thistle, which are fairly hearty in the face of cold weather, there may still be good opportunities if the plants are in good condition and there haven't been many hard frosts yet. "We should still have some reasonably warm temperatures, and there's still a chance for herbicide to be absorbed by the plant and moved to the roots and developing buds underground. But for other perennial weeds, like dandelion, herbicide applications generally need to occur before October to be successful."

According to Brenzil, much of a producer's approach to fall weed control depends on the types of weeds being targeted. "If you're looking at a perennial that is more sensitive to frost, control needs to take place earlier in the fall, either with a pre-harvest herbicide application, or after harvest before there has been too much frost damage," he said.

"For winter annual weeds, later is better since they only begin to germinate in mid-September, and control needs to take place as late in the fall as possible to control them effectively."

The advantages of fall weed control are obvious when spring comes and your fields are already well prepared for the season. "Research is showing that the earlier perennial and winter annual weeds are controlled, the greater the yield benefit to the following crop," Brenzil said.

"If the weeds are left there until just before seeding, they use moisture and nutrient resources that could otherwise be used by the crop in that critical early development stage. If you can't get it done this fall, plan to control winter annuals and dandelion as soon as possible next spring."

Brenzil says that post-harvest perennial weed control may be an option for many producers this year on fields where harvest took place earlier in the summer. "Perennial weeds that were cut off during harvest will have had the four to six weeks they need for adequate top re-growth in order to provide a good target for the herbicide spray."

According to Brenzil, a common mistake producers should avoid is trying to use the same herbicide rate they would with a pre-harvest treatment.

"Because the mature standing weeds were cut off with the crop, the leaf surface area of the weed that is able to intercept herbicide droplets has been reduced significantly," Brenzil stated. "Since there are a lower number of droplets for each plant, the concentration of herbicide in the spray solution must be increased by increasing the application rate in order to get the same amount of active ingredient into each plant. The rate needs to be right the first time because the first effect of glyphosate is to stop nutrient (and herbicide) movement in the plant, making additional herbicide applications ineffective."

Brenzil estimates there are at least 12 glyphosate formulations available now from six different manufacturers, plus glyphosate mixes with other herbicides. No matter which brand you choose, though, there are important things to remember when spraying the chemical.

"Glyphosate-based herbicides can be negatively affected by cold conditions. The ideal time to spray is when several days are expected to be bright and sunny, with temperatures in the 15 to 20 degree (Celsius) range and overnight lows no less than five degrees (Celsius). If glyphosate is sprayed under cool, cloudy conditions, there is a high risk of it getting trapped in the leaves and being unable to translocate to the roots," he noted.

"Die-back of perennial plants treated with glyphosate in the fall is not necessarily a good predictor of control come next spring. If glyphosate is sprayed on a day when sugars are being rapidly moved to the roots, the plant may not show signs of death this fall, but will not emerge next spring either, and that is the goal of the exercise."

Because of the cooler temperatures in mid- to late October, glyphosate may not be the most appropriate herbicide for winter annual control in late fall, and should be saved for next spring. Winter annual weeds such as stinkweed, flixweed, whitlow-grass, pygmyweed and shepherd's purse can be effectively controlled just before freeze-up using a light rate of 2,4-D (0.2 to 0.28 millilitres per acre of 600 grams-per-litre formulation).

Other problem winter annual weeds, like narrow-leaved hawk's-beard, should be left alone until being treated with glyphosate early in the spring. Not only is 2,4-D ineffective on hawk's-beard, but it injures the plant enough that it makes the glyphosate applied the next spring ineffective, as well.

More information and advice on fall weed control can be found on the SAF website at http://www.agriculture.gov.sk.ca/, or by calling the Agriculture Knowledge Centre at 1-866-457-2377.

For more information, contact:
Clark Brenzil, Provincial Specialist, Weed Control
Saskatchewan Agriculture and Food
Phone: (306) 787-4673
E-mail: cbrenzil@agr.gov.sk.ca