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 2020 shape up for potential carryover?
The following analysis relates to the Risk Map for the 2020 growing season in the 'Related Items' below.
Rainfall needed for breakdown of residual herbicides was largely adequate across most of Saskatchewan, with the exception of an area around Kindersley and an area in the southwest (RM of Reno #51). However, because the rainfall in 2018 and 2017 was very deficient for herbicide breakdown in many areas, there may still be risks associated with herbicides applied in 2018 or 2017 that have multi-year restrictions on recropping.
Given that the soil temperatures dropped dramatically after an early-September snowfall and rain there would have been very little additional opportunity for herbicide breakdown beyond that date.
There are several gaps in rainfall reporting for many sites. Small, isolated locations of light green, such as small spots near the Richard/Speers area, southeast of Tisdale, near Allan and directly south of Shaunavon (RM 78), are likely a result of missed reporting. The depth of dryness reported in the Marengo area (RM 292) is also likely a result of missed rainfall data and is more likely at a high to moderate risk of herbicide carryover, similar to the surrounding area.
We are always looking for more crop reporters to make data like this more complete, and many RMs currently do not have reporters. To inquire about participating, contact the Agriculture Knowledge Centre general inquiry line at 1-866-457-2377.