Herbicide breakdown requires sufficient time under adequate moisture and soil temperature to support the growth of microbes that degrade herbicide molecules. Some herbicides are broken down quickly or are bound tightly to soil, preventing them from causing problems for crops that are planted the following season. Other herbicides take longer to decay, and as a result, persist into seasons following the year they were applied. These residues can injure sensitive crops that are planted in following seasons. Herbicides that have restricted recropping options are considered residual herbicides.
Rainfall and Herbicide Carryover Risk
In-season rainfall after herbicide application is the most important factor needed for the normal breakdown of herbicides in the soil. Less than normal rainfall can result in residual herbicides remaining in the soil at higher levels than expected, increasing the risk of herbicide injury to the more sensitive of the crops registered for planting in following year(s).
The map below is simply an alert to the possibility of greater carryover of herbicide than normally expected. However, rainfall events can be very localized. Due to potentially large distances between rainfall reporting sites used to create this map, and the smoothing process used, this map may either overestimate or underestimate the amount of rainfall at the individual field level. Producers should use their own rainfall records to estimate their specific risk of carryover injury from the herbicide they used the previous year to following crops, or if dry conditions have been experienced over multiple years in a row, potentially residues from previous few seasons as well.
Isolated areas of higher rainfall (green) within a larger area of lower rainfall (yellow to red) may also indicate a single heavy rain event that may not provide enough ongoing moisture for the breakdown of certain herbicides. Isolated areas of lower rainfall among areas of adequate rainfall are possible due to missing reports for rainfall events. Contact the herbicide manufacturer on whether rainfall in your area is sufficient to allow their product to break down. The map below indicates the amount of rain between early June and early September. Rainfall in this period is critical to the breakdown of herbicide residues in the soil. Precipitation outside of this window is not as effective because soil temperatures are cooler or frozen, minimizing the amount of breakdown that can occur.
The impact of the carryover of some herbicides on the following crop is affected by soil pH. The levels outside of the neutral soil pH range (6.5 to 7.5) may increase the risk of carryover of some herbicides. Other soil properties such as high clay or organic matter content may protect crops from the damage due to herbicide carryover. Clay and organic matter bind to some of the herbicide molecules making them less available to be absorbed by the crop. As a result, sandy soils with low organic matter are at greater risk of injury from herbicide residues.
The appearance of injury symptoms from herbicide residues in the soil are often preceded by a soaking rainfall, which releases the herbicide from soil particles and moves it into the rooting zone where it is taken up by the crop roots. Check the chart in the front of the Weed Control chapter of the Guide to Crop Protection for a list of residual herbicides.
Always follow label directions on what crops to plant following the application of any herbicide. When dry conditions exist, consult with the manufacturer for additional guidance.
How does 2024 shape up for potential carryover?
The following analysis relates to the Risk Maps for the 2024 growing season in the 'Related Items' below based on rainfall in 2023. Because soils remained warm until late September, end dates on all maps for 2024 risk are taken until September 25, 2023.
Rainfall needed for the breakdown of herbicides applied prior to seeding was adequate in several areas with deficient rainfall running in a swath from Weyburn to Coronach, Assiniboia west to the Alberta border, from the Maple Creek area to the Kindersley area, the Saskatoon area to the North Battleford area and the areas around Outlook and Moose Jaw. There is a ‘Very High’ risk of carryover in the RMs of Piapot, Happyland, Deer Forks, Clinworth, Chesterfield, Pinto Creek, Brock and Mariposa. (See the map below that shows May 2 to September 25, 2023, precipitation amounts).
Rainfall needed for breakdown of residual herbicides earlier in the in-crop spray season (May and early-June) was adequate north and east of Spiritwood and in a swath from Watrous to Yorkton and north, with the exception in the RMs of Cote, Livingston and Saltcoats where there is a moderate risk of carryover. There is a ‘Moderate to High Risk’ of carryover in the RMs of Star City and Flett’s Springs as well. Another area of adequate rainfall occurred between Lloydminster and Meadow Lake. Outside of these adequate areas the risk of extended carryover of herbicides was ‘Moderate to Very High’ with pockets of ‘Serious risk’ in the RMs of Happyland, Pinto Creek, White Valley, Bone Creek, Rudy and Rosthern. Two ‘Serious Risk’ areas with a zone of ‘Extreme Risk’ surrounding them can be found to the east of Maple Creek and in the RM of Tecumseh. (See the map below showing May 30 to September 25, 2023, rainfall).
For later applications (mid-June and onward) the area of the province short on rainfall for breakdown extended further north to envelope Spiritwood and Tisdale areas, but in areas near Humboldt and Wynyard, moisture remained adequate for breakdown. Other areas where moisture remained adequate for breakdown include the Hudson Bay, Prince Albert and Meadow Lake areas. Areas of ‘High’ to ‘Very High Risk’ run from Yorkton west to the Alberta border and south to the USA border, from the Alberta border to Saskatoon and north to North Battleford. New pockets of ‘Serious Risk’ emerge in the Coronach, Assiniboia, Weyburn, Moose Jaw areas. The area running from Leader to Val Marie is ‘Serious Risk’ for herbicide carryover, with ‘Extreme Risk’ pockets. (See the map below showing rainfall for June 13 to September 25, 2023.)
While the extended dry fall was good for harvest, the lack of any additional precipitation through harvest meant that any later season breakdown couldn't be counted on.
Not only do producers need to be aware of persistent herbicide they applied in 2023, but also those they may have applied in 2022 or earlier, particularly for those products that have multi-year plant-back restrictions. Much of the west side of the province was in very high to severe and even extreme deficiencies for herbicide breakdown going into 2023. (See maps for rainfall from June 1 to August 30, 2022, below.)
Saskatchewan still has large areas that are not well-covered for rainfall reporters and the ministry relies heavily on crop reporter observers to provide rainfall records for these maps. The Ministry of Agriculture is always looking for more volunteers to make data like this more complete and many rural municipalities currently do not have reporters. To inquire about participating, contact the Agriculture Knowledge Centre general inquiry line at 1-866-457-2377.