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 2021 shape up for potential carryover?
The following analysis relates to the Risk Map for the 2021 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 and short for a good portion of the area south of Highways 7 and 16 east of Saskatoon and west of Highway 47. There were several areas in the southwest where the risk of poor breakdown of all residual herbicides was very high (<100 mm for June, July and August) as well as other smaller areas around Moose Jaw and Indian Head.
For later applications (mid-June and onward) the northern boundary of the inadequate area had moved north in the western side of the province to a boundary running north of Kindersley to Prince Albert and those areas in the southwest with very high consolidated into a larger regional trend with isolated areas of “Severe” risk beginning to emerge. On the eastern side of the province, the drier areas encompassed an area bound by Prince Albert, Melfort, Foam Lake and then southward along the original Highway 47 boundary until Stoughton where many areas to the south of Highway 13 east of Stoughton had inadequate moisture for breakdown of residual herbicides.
Areas to the east of Highway 47 and north of Highway 13 in the south had adequate moisture for breakdown for most of the season.
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 in those areas where inadequate breakdown would be a problem.
Saskatchewan still has large areas that are not well-covered for rainfall recorders 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 crop reporters 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.