Posted September 30, 2024 | By: Twyla Jones, Agronomic Innovation Lead, Canada
POST-HARVEST HERBICIDE APPLICATIONS HELP NEXT YEAR'S CROP GET OFF TO A BETTER START
Weeds have always been one of the biggest yield robbers for farmers. Any time growers can reduce weed pressure prior to putting a crop in the ground, they are ahead of the game — and it is why my husband, and I are planning to spray some of the fields on our farm with herbicides once the crops are harvested.
Fall 2024 has been open thus far with fall frosts and snow staying at bay, combined with moisture this September many growers are considering fall herbicide applications.
Spraying fields post-harvest can target weeds missed by in-season and pre-harvest applications such as perennial weeds and dandelion control which is most effective in late fall. It is also an opportunity to apply pre-emergent products which will be activated in the spring; an integral tool for herbicide laying which can have a significant impact while tacking weeds such as Kochia. Late fall applications with residual herbicides can help reduce spring workload and ensure early weed removal.
For those opting to a do a post-harvest herbicide application, there are a few things to keep in mind when choosing products. Determining which herbicides to use for each field will depend on the kind of weeds you want to target. Observations made during harvest are a terrific opportunity to assess weed pressure. It could be perennials, winter annuals, or resistance issues that you are seeing, and each situation should be approached differently. When planning which herbicides to use, often a multi-year integrated approach to reducing weed pressure is required, with post-harvest applications being part of the solution.
When it comes to managing weed resistance, layering of herbicides has been shown to be the most effective herbicide tool for resistance management for weeds such as Kochia. Late fall applications of these products can help eliminate the need for post-application rainfall in the spring for herbicide activation, as snowmelt can serve that purpose in addition to reducing spring workloads.
Post-harvest applications for perennial weed control need careful assessment to maximize results. A combination of regrowth and frost-free days is required. For Canada thistle, look for three to four new leaves. If possible, a couple of days without frost prior to application and daytime temperatures more than 10 C are best.
Once frosts start to dip below -5 C, the effectiveness for Canada thistle control diminishes — winter annuals and perennial grasses can withstand slightly harder frosts. Winter annuals are easily controlled with a tank mix of glyphosate and a phenoxy or Group 2 product. Applications of straight glyphosate should be avoided for resistance management.
Some herbicides applied post-harvest can have residual effects that can carry into spring, so growers need to be aware that the product selections they make in the fall could limit next year’s crop choices. Consulting their sales agronomist can help farmers ensure that their herbicide choices and crop rotation plans are always coordinated, maximizing the effectiveness of post-harvest weed control.
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