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 seeded 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 maps below are simply an alert to the possibility of greater carryover of herbicide than would normally be 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 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. Contact the herbicide manufacturer on whether a single heavy rainfall is sufficient to allow their product to break down. Two maps are produced - early maps (early June to early September) and late maps (late June to late September). A shift from normal to elevated risk indicates a large rainfall event in early June. A shift from deficient to normal indicates a large rainfall late in the season. In either case these rains may not provide enough ongoing moisture for adequate breakdown.
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 risk of 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 2019 shape up for potential carryover?
The following analysis relates to the Risk Maps in the Related Links below.
Rainfall needed for breakdown of residual herbicides was inconsistent across Saskatchewan. It ranged from adequate in far northern portions of the grain-belt to inadequate for much of the areas south of a line between North Battleford and Melfort as well as a finger of higher risk pushing north into the Shellbrook/Canwood area.
A comparison between early rainfall (June 1 to September 1) and late rainfall (mid June to mid-September) shows a moist break in the middle of the later map. This was a rain that came during the 3rd week in August. Given that the soil temperatures dropped dramatically after mid-September there would have been very little additional opportunity for herbicide breakdown beyond that date. This would have allowed for about 3 to 4 weeks of breakdown before the system shut down entirely for the fall. Only short persistence herbicides would have adequate opportunity to break down in this short time and then only in very localized areas.
Therefore, the earlier map is likely a reasonable reflection of the relative risk of herbicide carryover in 2019.