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Timing is Critical for Fall Weed Control

By Clark Brenzil, PAg., Provincial Specialist, Weed Control, Regina

Harvest is roughly progressing at a normal schedule and this means that there may be options for fall weed control this year. Producers need to remember that the timing of fall weed control activities is critical to their success.

Winter Annual Control

Research done in the 1990s by Ken Kirkland, at the Agriculture and Agri-Food Canada Scott Research Farm, showed that the best control of winter annual weeds using the phenoxy herbicides – 2,4-D and MCPA – occurred when applications were made in mid to late October after most winter annual plants had germinated, or, first thing in the spring while plants were actively growing yet still in the rosette stage, in April or early May. Applications made in September and late May were less effective. 

The late October, applications of 2,4-D at rates of four to six oz. or, 113 to 170 g ae* (grams of acid equivalent) of active ingredient per acre, also provided excellent control of winter annual weeds while being safe to sensitive crops such as lentil, chickpea, canola, flax, sunflower, dry peas and field beans, where the same rates applied in the spring were not as safe.

Another interesting observation was a benefit to fall winter annual control. However, fall 2,4-D may not work on narrow-leaved hawk’s beard. It takes high rates (12 to 16 ounces per acre = 340 to 453 g ae per acre). A lower 2,4-D fall application may not provide control and could compromise efficacy of glyphosate applications on hawks beard the following spring.

In fields where hawk’s-beard is a problem, producers may be as well served to leave winter annual management until early the following spring. Small rosettes of hawk’s-beard are susceptible to glyphosate at 180 g ae per acre whereas, applications that are delayed until bolting, require 270 to 360 g ae per acre.

Glyphosate products provide effective control of winter annual plants in the spring, but since producers do not make two passes, one early and one pre-seed, most winter annuals will be at more advanced stages and require higher rates to control. This delay in treatment until just prior to seeding can also result in the actively growing winter annual plants depleting moisture and nutrients from the seedbed, resulting in lost crop yield, before it is even planted.

Perennial Control

Effect of photoperiod on Canada thistle
Adapted from Miller and Lym 1998

Fall is also an optimum time to apply herbicides for the control of biennial and perennial weeds. This is because these plants are storing energy resources to survive the winter and to provide energy for their reproductive drive the following year. For example, experiments done by Miller and Lym, in growth chambers at North Dakota State University, showed that 90 per cent of Canada thistle shoots remained vegetative when they emerge while average daily sunlight is less than 15 hours. They also found that there was a higher rate of movement of herbicides to the roots of Canada thistle in the vegetative rosette stage verses the bolting stage and as a result control was better.

Looking at sunrise/sunset tables issued by the National Research Council, day length dropped below 15 hours on the following dates in these Saskatchewan locations or those with similar latitudes:

Graph of sunrise sunset dates in Saskatchewan

This indicates that Canada thistle shoots emerging after the dates above will stay as rosettes and will be more susceptible to herbicide activity than prior to those dates. With six weeks of regrowth recommended after harvest this also indicates that post-harvest control options may not be practical in most years on Canada thistle much north of Regina.

Producers have long suspected that a light non-killing frost improved the level of control of their herbicide applications. Now there is research to prove this and explain what is happening in these situations. Researchers at the University of Nebraska measured the levels of various types of carbohydrates in both Canada thistle and dandelion before and after the first frost of the season. Researchers found that after the first frost the plants converted highly complex sugars and starches to less complex sugars and the simple sugar, fructose. Other earlier research had shown that these less complex fructans, 1-kestose (DP3), 1- nystose (DP4), & 1- fructofuranosyl-nystose (DP5), act as an anti-freeze, protecting plant cells against loss of membrane integrity from the formation of ice crystals at cold temperatures.

The University of Nebraska Research showed that applications of dicamba to Canada thistle and 2,4-D and dicamba to dandelion reduced the concentration of total sugars in the plant including the less complex fructans. The reduction in the concentration of sugars in the roots caused by the herbicides was substantially greater shortly after the first light frost, as was the control delivered by those herbicides, likely because they had stored less energy resulting in winterkill. Increasing rates of the herbicides also reduced the root sugars to a greater degree. Unfortunately, there is no reference to this concept applying to other herbicide classes other than synthetic plant hormones (Group 4).

The phenoxy herbicides and other plant hormone mimics, can be effective when applied right up until freeze-up, whereas the efficacy of amino acid synthesis inhibitor herbicides like glyphosate and Group 2s can be reduced by low temperatures.

Timing is everything when it comes to weed control and not necessarily calendar time. The plants and climatic conditions will dictate what times are appropriate for optimum control of weeds with herbicides and producers need to be weed watchers to get the most out of their herbicide applications.

*Herbicides formulated with several salt variants such as phenoxy herbicides (Ester, Amine, K and Na salts) or glyphosate (K, IPA, TMS, MA and DMA salts) use “acid equivalence” to compare rates between salt variants. The reason for this is the different cations (positively charged portion of the molecule) have widely varying weights, whereas the glyphosate acid (negatively charge portion of the molecule) is the effective part of the molecule and is the same weight regardless of which salt it is paired with.

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