By Matthew Struthers, AAg, Crops Extension Specialist, Moose Jaw
If you were unable to carry out fall weed control this year, don’t worry, there is always next fall. Make sure to plan early in 2023 to ensure you have time get those pesky weeds under control. Traditionally, these weeds would have been controlled by tillage late in the fall, but now that zero-till or conservation-tillage practices are more widely used, chemical control of these weeds is required. Thankfully, fall application of 2,4-D and other phenoxy herbicides have been proven to be an effective control method of weeds such as dandelion, stinkweed, flixweed and even narrow-leaved hawk’s beard.
Research done at the University of Saskatchewan found that post-harvest fall application of 2,4-D was a better option for dandelion control compared to an in-crop application of glyphosate during the summer. In the summer, it is possible for dandelion to be dormant, depending on the crop being grown, as the crop canopy blocks the sunlight, resulting in less effective control of the weed. Narrow-leaved hawk’s beard in the germination to rosette stage can be controlled by a fall application of 2,4-D if the weather is fair from mid-October to the beginning of freeze up. Flixweed is a strong competitor and rapidly grows in the spring. It often becomes too mature by the time in-crop herbicide applications are carried out, making them ineffective at controlling the weed. Far better control can be achieved by using fall 2,4-D herbicide applications when flixweed rosettes are small, actively growing and susceptible to chemical control.
The potential residual carryover effect of 2,4-D also allows for early control/suppression of weed seedlings which greatly reduces competition for nutrients and sunlight in the spring. A fall application of a phenoxy herbicide such as 2,4-D does not eliminate the need for a pre-seed or in-crop application of other products such as glyphosate in the spring, but it will reduce crop/weed competition. Fall applications of herbicides should be done as late as possible. Typically, from mid-October to freeze up is the ideal time, as this will allow for any late germinating weeds to emerge and greatly reduce the amount of weed seedlings that escape the herbicide.
Using low rates of 2,4-D or other phenoxy herbicides in the fall may still allow for the seeding of sensitive broadleaf crops such as oilseeds and pulses the following year, but caution must be used if applying during dry fall conditions on coarsely textured soils. In these conditions, there is a larger potential for injury and application is not recommended at any rate. Higher herbicide application rates in the fall or spring applications will not allow for these crops to be seeded; it is always recommended that re-cropping restrictions on product labels be followed. For tougher to control weeds such as narrow-leaved hawk’s beard it is recommended to use a higher rate of active ingredient to ensure adequate control. This will restrict re-cropping to mainly cereal crops which have good tolerance to 2,4-D and can be grown after a fall application.
Applying herbicides in the fall can save time in the spring and lower the risk of losing herbicide effectiveness to surprise spring rains. Plan ahead and ensure you are able to get into your fields and control winter annual weeds before they begin to spread across your fields. For more information regarding fall weed control please consult the Guide to Crop Protection or reach out to your nearest crops extension specialist.