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 2022 shape up for potential carryover?
The following analysis relates to the Risk Map for the 2022 growing season in the 'Related Items' below.
Rainfall needed for breakdown of residual herbicides earlier in the spray season (May and early-June) was patchy in the northwest of Saskatchewan and short for a good portion of the rest of the province. The exception to this trend was the southeast of the province, confined by a line running from the U.S. border near Oungre to roughly Moose Jaw and then northeast from Moose Jaw to Kamsack. This area also received additional rainfall in the first two weeks of June that helped in the breakdown of products applied in May and June 9 to 12, 2021.
For later applications (mid-June and onward) the entire province was short on rainfall for breakdown to some degree, with the exception of small localized areas around Smeaton to Choiceland, south of the corridor between Moose Jaw and Regina, a small area between Vibank and Kendal north of Lipton, and a larger area east of Yorkton. A large area west of a line from Prince Albert to Moose Jaw and on to the border with the U.S., received less than four inches and in a large portion of that area, less than three inches was received. This places these areas at very high to serious risk of unexpected carryover that could impact the more sensitive crops listed as eligible for recropping of particular herbicides.
Even in the areas mentioned that received adequate rainfall for "normal" breakdown of most herbicides within the key months of June, July and August, much of this rainfall came in mid-August. Normally, rainfall received this late would contribute less to herbicide breakdown because of cooling soils, but a warmer, extended fall period will lessen that concern. However, soils were so dry by that date that moisture was moved from the surface to deeper layers very quickly. The layer near the surface where much of the breakdown occurs dried out relatively quickly after these rains.
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 2021, but also those they may have applied in 2020 or earlier. Those areas north of a line from Kindersley to Prince Albert and then from Prince Albert, southeast to Foam Lake and then east of a southward line, roughly following Highway 47 to Stoughton and north of Highway 13 east to the Manitoba border had adequate moisture for breakdown of residual herbicides in 2020 going into the 2021 growing season. Areas west of a line roughly transcribed by Highway 20 from Nokomis to Lumsden then Highway 6 from Regina to the U.S. border and south of a line from Kenaston to Rosetown and then to Leader, had high to very high risk of poor herbicide breakdown in 2020 going into the 2021 season. The remaining areas between these two were marginal for moisture needed for breakdown. This creates additional challenges for crop selection for the southwest of the province that will also have to consider herbicides applied in 2020 as well as 2021.
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.