42 Story Idea? Email Paul.Nolan@Farms.com Better Farming | March 2024 Ag Insights HERBICIDE CARRYOVER & ROTATION Some herbicides can stay active in the soil for months or years after application. By Erica Yaskowich Soil and weather conditions have been drier than average in the last few years, which has placed emphasis on herbicides that have residual activity. There can be consequences for previous herbicide application, making planning for the 2024 season vital. Some herbicides break down quicker or are bound tightly to the soil particles, while other herbicides take longer to break down and can persist in the soil into subsequent seasons. If the herbicides that take longer to break down come into contact with a sensitive crop, they can cause injury. Most soil residual herbicides, when applied at the recommended rates, will disappear in a few weeks and do not impose any restrictions on cropping options for the next year. This is different for each residual herbicide. Always follow the label. However, some herbicides can stay active in the soil for months or years after application. A delay in breakdown can be exacerbated by dry weather conditions. Herbicide residues are deactivated in a few different ways, including: Breakdown by chemical reaction, Breakdown by soil microbes, Volatilization or escaping into the atmosphere as a gas, Photodegradation or breakdown by light, Leaching, Binding to soil particles. Herbicides will often be deactivated by more than one of the methods above, so they won’t interfere with subsequent crops. Many herbicides are non-residual and are only temporarily bound to soil particles while they are broken down. This binding ensures that the herbicide is not available to cause crop damage. Generally, the breakdown process occurs more rapidly when the soil is moist and warm. During the winter, when the ground is frozen, and periods when the soil is dry, herbicide breakdown is greatly reduced. In-season rainfall after herbicide application is an important factor in the breakdown of soil-applied herbicides. If there is less than normal rainfall, it may result in the residual herbicides remaining active within the soil at higher-than-normal levels. From June to early September, rainfall is particularly important for breakdown of herbicide residues. If precipitation falls after September, then it is not as effective, as soil temperatures will be much cooler and/or frozen. Cold soil temperatures minimize the amount of breakdown. The pH of the soil also affects the carryover of soil herbicides. If the soil is out of the neutral pH range, which is 6.5 to 7.5, there is also an increased risk of herbicide carryover. Soil type also plays a part in herbicide carryover risk. If the soil has high clay content or organic matter, it could protect the crops from herbicide carryover damage. The clay and organic matter bind to some of the herbicide molecules which will make them less available to be taken up by the crop. Low-organic-matter sandy soils have a greater risk for herbicide injury. Rainfall is important for the breakdown of herbicide residues. Tracy Miller photo
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