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Pre- and post-harvest weed control

Q & A with CPS

Q: When should I start weed control for my crop?

A: A weed-free crop at harvest represents the cumulative effort of field management during the season. To manage many weed species, the season begins the previous fall.

Pre-harvest weed control tends to focus glyphosate application on perennial weeds such as quackgrass or Canada thistle. At this point in the season these species grow rapidly and will move glyphosate to the spreading root systems. There may also be some control of simple perennials such as dandelion and foxtail barley, and even late-emerging annual weeds. Be aware of proper crop staging to ensure the seed is fully mature so there is no impact on yield and no risk of herbicide residues above maximum residue limits (MRLs) in the harvested grain.

In pre-harvest herbicide application, there must be a distinction between weed control and crop desiccation. Glyphosate is intended for weed removal and is not a true desiccant. Be sure your goals align with the products you use.

Weed control after harvest depends on which weed species are targeted, weather conditions and the crop planned for next year. In fall, we target weeds that will survive winter. Perennials such as Canada thistle require time to regrow before control is attempted. Low-growing perennials such as dandelions are ideally sprayed soon after harvest when fully exposed. Winter annuals including stinkweed and narrow-leaf hawksbeard can be controlled later in fall to allow time to germinate and emerge. Weather will dictate timing as frost will begin to damage the leaves of weeds, depending on species. We often add another herbicide to glyphosate to improve weed control and introduce another mode of action. However, be sure that any added herbicide will not damage next year’s crop.

Fall weed control is a big step in reducing weed competition for soil, water and nutrients the following year.

Lyle Cowell, M. Agric., P. Ag, CCA, is a manager of agronomic services with Crop Production Services in northeast Saskatchewan.

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