Glyphosate will be more effective for different weeds at different times of the year.
Perennial weeds visible above and in the crop canopy are primary targets for a pre-harvest glyphosate application.
Prior to harvest, short days and cooling temperatures signal perennial weeds to start building root reserves. Glyphosate is a systemic herbicide — it translocates right to the roots where it is most effective. Don’t spray too early. Wait until the weeds are building winter energy stores.
Pre-harvest glyphosate application is appropriate in shorter growing season areas that generally have less time prior to a killing frost or even early snow cover for post-harvest applications. Pre-harvest glyphosate applications also make the most of the larger amount of foliage prior to harvest, able to capture the spray droplets and result in improved uptake by the plant. Canada thistle and quackgrass are ideal target weeds.
Keep in mind, pre-harvest glyphosate is primarily for perennial weed control. Don’t expect glyphosate to act like a desiccant to dry down green plant material. For pre-harvest glyphosate to work according to expectations, the recommendation is to apply to actively growing weeds and ensure that application is not significantly impaired by the crop canopy. Some weeds are better targeted post harvest, after they have been clipped and allowed to regrow.
Weeds growing closer to the ground under a dense crop canopy and now exposed after harvest are ideal targets for post-harvest glyphosate application.
These weeds need sufficient actively growing, residue-free leaf material to absorb the maximum amount of glyphosate. The key is to check for leaf condition and active growth, indicating prime environment for glyphosate translocation to the root material.
With cooler fall temperatures, weeds require time to generate enough new growth necessary for glyphosate uptake. Consider using a tank mix partner with glyphosate post-harvest for improved control, or to provide residual control, keeping cropping rotation restrictions in mind. Minor frost may actually improve glyphosate efficacy post-harvest, but always spray during the day when weeds are actively growing.
Controlling volunteer crops post-harvest may be effective to prevent insect and disease pests from surviving from one year to the next on living plant material. Foxtail barley, dandelions and winter annuals are target weeds post-harvest.
Winter annuals germinate in the fall, growing right up until the ground freezes. Winter annuals include stinkweed, shepherds purse, cleavers, narrow-leaved hawks beard, storks bill, chickweed and flixweed. A pre-seed burn off with glyphosate will also control winter annuals.
Overwintering annuals and spring germinating weeds that present prior to crop establishment are targets for a pre-seed application of glyphosate. The goal is to have no weeds in the field before crop emergence, as weeds will be very competitive and harder to kill than weeds that germinate with or later than the crop.
Perennial weeds that missed a fall glyphosate application will not be effectively controlled with spring glyphosate application. In the spring, perennial weeds are moving energy out of the roots. At best, glyphosate may only provide top growth control. On its own, glyphosate will not control volunteer glyphosate tolerant canola.
If this is the target weed, consider a tank mix partner that will act on the volunteer canola. Glyphosate tank mix partners can improve efficacy and are part of good stewardship that includes herbicide group rotation.
If few weeds have emerged when you’re ready to seed, consider a tank mix partner with a residual product to provide extended control. Watch cropping restrictions on residual products.
The clock is ticking if you’re waiting to spray a glyphosate burn off on a non-glyphosate tolerant crop in the pre-emerge stage. It can be done, but the pressure is on to ensure the sprayer beats the crop before it emerges. Fields seeded without a pre-seed burn off will still have overwintering annuals and spring germinating weeds. The recommendation is to control these weeds with the pre-emerge glyphosate as soon after seeding as possible or get caught trying to take down oversize weeds.
Plan to make in-crop applications on glyphosate tolerant crops early, when the crop is at the one- to two-leaf stage. Early emerging weeds have more impact on yield than weeds that emerge after the crop reaches the four- to six-leaf stage. Volunteer cereals and wild oats may have a second flush after the initial application.
To check if a second pass of glyphosate is required, scout the field 10 days after the first application, giving the glyphosate time to show signs of activity and for new weeds to emerge. If you are planning more than one in-crop glyphosate application, be cautious with crop staging and total rates of product applied. Spraying glyphosate on current varieties of glyphosate tolerant canola when the crop is at bud stage or later may impact yield and maturity.
Understand the most vulnerable growth stage of target weeds and your glyphosate applications will have a greater impact. †