Wet field conditions across the province have many questioning the timeliness of their post-emergent herbicide application. Unfortunately, options are limited.  The stage of the crop and/or weeds dictate whether you  1) spray now and deal with the ruts later, or 2) hold off a few days in hopes of dryer weather.

Consider the following when prioritizing fields for herbicide application:

  • Crop and weed stage are critical. Applying herbicide(s) outside of the crop and/or weed stage indicated on the product label can result in crop injury and decreased herbicide efficacy. Be sure to check the condition of your crop before spraying, since stressed crops may be more susceptible to herbicide injury. The window for in-crop herbicide application varies by product and crop. Crops with relatively few herbicide options, like field peas, may have a small window of opportunity.
  • Crop competitiveness. The critical weed-free period indicates when and for how a crop needs to be kept weed-free to minimize yield loss. In general, the more competitive the crop, the shorter the critical weed free period. Therefore, your pre-seed burnoff, pre-plant or pre-emergent herbicide application may carry you further with competitive crops (e.g. cereals) compared to crops like soybean, corn or flax.
Picture3*Current University of Manitoba/Manitoba Pulse & Soybean Growers project to refine the critical weed-free period for soybean grown in MB.

  • Economic thresholds (ET). Thresholds can help determine if you need to apply an in-crop herbicide based on the density of a particular weed. For example, 6 to 16 plants/m2 of wild oat can result in less than 5% yield loss in spring wheat (actual ET depends on weed and crop staging; refer to page38 of the 2016 Guide to Field Crop Protection). Similar ETs exist for select grassy weeds in wheat, barley, canola, and flax, for kochia and biennial wormwood in sunflower and for volunteer canola in soybean (see below). The downside to weed ETs is that they are species specific and they don’t consider weed seed return to the weed seedbank.


A few other spray tips:

  • Don’t ‘save time’ by skimping on sprayer clean. Refer to the product label & page 15 of the Guide to Field Crop Protection for clean out instructions.
  • Check out SPRAYcast (www.weatherinnovations.com/spraycast.cfm) for a 3-day forecast of optimal spray times.

Happy herbiciding!

Submitted by: Jeanette Gaultier, Provincial Weed Specialist, Manitoba Agriculture

2016 Guide to Field Crop Protection: www.gov.mb.ca/agriculture/crops/guides-and-publications/#gfcp


Have a follow-up question?