Q. What weeds should I target in post-harvest weed control applications?
A. Two general weed groupings can be effectively controlled with a post-harvest herbicide application.
Perennials: Controlling perennial weeds such as Canada thistle, dandelions, foxtail barley and quackgrass involves an integrated plan, and a post-harvest herbicide application that includes glyphosate is a pivotal part of the plan. Late-fall herbicide applications tend to be highly effective (compared to a spring application) because plants are building root reserves. Because of this reserve-building process, the glyphosate will actively move into the roots, enhancing chemical efficacy.
Post-harvest applications are especially effective for low-growing weeds such as toadflax and dandelion. These weeds can often be missed with a pre-harvest application because dense crop canopies can limit the herbicide from reaching the weeds.
Taller perennial weeds such as quackgrass and Canada thistle will be cut off with harvest operations. However, if there is adequate regrowth, a fall application is still a viable option. If the plant has generated three to four new leaves following harvest operations and weather conditions are favourable for growth, good efficacy can be expected from a glyphosate application.
Tip: Chemical control of perennials can be challenging. Be sure to follow the appropriate rate structure for the target weed and consider adding a surfactant to improve efficacy.
Winter annuals: This includes weeds such as stinkweed, shepherd’s purse, flixweed and narrow-leaved hawk’s beard. These weeds can germinate in the fall and begin re-growing early in the spring. By the time pre-seed herbicide application takes place, these weeds can be well advanced and difficult to control. Removal of these weeds is best achieved with a fall herbicide application.
Tip: Add a tank mix partner to enhance control, provide soil residual control and mitigate herbicide resistance. Ensure the tank mix partner is compatible with next year’s crop.
Norm Flore, P. Ag, CCA, is a manager of agronomic services for Nutrien Ag Solutions in southern Alberta.