Getting the most bang for your herbicide buck
/by Caitlin Congdon
Coming into the 2022 production season, we’re seeing the rise of many input prices, and herbicides aren’t an exception. Several factors have led to recent shortages in glyphosate, which is a key player in many herbicide programs. With the decrease in availability of such a popular product, more strain is put on alternatives, limiting supplies and driving up prices all around. So, what can you do to make sure you’re using your herbicides effectively and getting the most bang for your buck?
Identify the weed species present and hit them when they’re most vulnerable. Annual weed species such as ragweed, lambsquarters, and Redroot pigweed are most vulnerable in their seedling stage, when they’re actively growing but still small enough to control effectively without having to use high rates of product. However, they’re also more difficult to identify when they’re newly emerged, making it tough to choose the correct product and rate for the population present. Use a weed identification guide to help identify weeds or take pictures and get in touch with your agronomist or extension staff to make sure you know what you’re dealing with. Identification Guide to the Weeds of Quebec is an excellent resource with very clear photos of eastern Canadian weeds in both their seedling and mature forms.
Pick the right product for the weed population present. Is your weed population made up of grasses or broadleaves or a mixture of both? Make sure you check the product label and know what the target species are for that product. The Ontario agriculture department’s publication 75A, Guide to Weed Control in Field Crops has some useful tables that show the relative efficacy of herbicides on various weed species on a scale of 0-9. If the product you planned to use has a low rating for the weed that’s most predominant in your field, it’s worth looking at other options with higher efficacy ratings.
Pick the right rate for the weed population present. Herbicide labels often have different rates that can be used depending on the host crop, weed species, and weed stage. For example, one rate might be sufficient to control ragweed when it’s below a certain height but the weed requires a stronger dose when it surpasses that height. Using a lower rate than is listed on the label can result in poor control and the need for subsequent applications or different methods to minimize the impact on the crop yield. This may lead to increased cost of production, which may not be recovered through adequate weed control for increased yield. Read the product label closely to ensure you’re selecting the correct rate.
Pay attention to herbicide rotation. Herbicides are grouped based on the mode of action of their active ingredient or ingredients. Continuously using products with the same mode of action increases the risk of weed species developing a tolerance or resistance to the activity of those products. Groups 2 and 5 are the most common groups with associated resistant weed species in Canada. Products containing multiple modes of action may be members of multiple groups, increasing the spectrum of control and at the same time decreasing the likelihood of resistance development. However, once resistance has developed, continued use of a “multi-pack” product containing the mode of action with resistance may increase the risk of diminishing response to the other modes of action. If you see unexpectedly poor weed control in a field, make note of it so that you can review other factors that may have impacted the efficacy and track activity in the future.
We often talk about the “four Rs” of nutrient management, but the same principles can apply to herbicide use as well. Using the right product at the right rate during the right time in the right place can help to ensure efficient and effective herbicide use and reduce economic losses caused by ineffective applications.
(Caitlin Congdon is a field crops specialist with Nova Scotia’s Perennia Food and Agriculture Inc.)
