It has been reported this season that some cotton fields have had issues with residual herbicide carryover with symptoms ranging from poor establishment, slow early growth or the need to re-plant. The function of residual herbicides is to provide a barrier to weed emergence by preventing weed seeds present in the soil from germinating. Some herbicides can remain active in the soil for weeks, months or years. This can be an advantage as it ensures good long term weed control. However, if the herbicide stays in the soil longer than intended it may damage sensitive crop or pasture species sown in subsequent years.

Residual herbicides are generally applied at rates well in excess of the rate required to provide weed control. This enables residual weed control to be maintained for several weeks or months after application, while the herbicide undergoes degradation. For many herbicides, small to modest changes in application rate may not lead to a proportional change in the level of risk posed to rotational crops.

Herbicide degradation will be strongly affected by temperature. Providing adequate soil water is present, degradation is far slower in the cool/cold months of winter and faster when temperatures are warmer. Without adequate soil moisture these processes are reduced, breakdown slows and herbicide persistence increases. When soil becomes very dry, herbicide degradation may stop completely until further rainfall (or irrigation) is received.

There are a number of factors which determine the life of residual herbicides in the soil which can complicate decisions around plant back periods and introducing susceptible crops into the rotation. Therefore, it is very difficult to provide consistent recommendations on the life of residual herbicides in the soil as it often varies from season to season. Factors that impact on herbicide longevity include, soil type, moisture, organic matter, soil pH and temperature. Two of the main degradation processes responsible for herbicide breakdown in the soil are microbial activity and hydrolysis (hydrolysis is only important for some herbicides). Both breakdown pathways rely on temperature and soil moisture. 

You also need to consider how rainfall has fallen over the warmer months (spring, summer and early autumn) since the herbicide was applied. If the soil, particularly the soil surface, has remained moist for many weeks during this period, then herbicide degradation is likely to be more rapid. If the soil surface has had extended dry periods, or the required rainfall has fallen as one or two large falls (with substantial dry periods in between), then herbicide breakdown is likely to have been significantly slower, and the risk posed to following crops higher. As microbial degradation is the primary breakdown mechanism for most herbicides, adequate soil moisture (and temperature) over time is required to build and support the microbial populations needed for this process. 

Additive effects can also increase the longevity of herbicide active ingredients; i.e. products with Imazapic (eg Flame) persist for long periods of time in the soil. If an “imi” tolerant crop is grown to avoid plant back issues and other Group 2 IMI herbicides are applied, the rate of breakdown will slow.  Be wary of compounding a residue problem by planting a herbicide resistant crop and spraying with more of the same Mode of Action. 

Dry conditions reduce herbicide degradation and is influenced by poor uptake of herbicides by crop plants and limited microbial or chemical degradation of herbicides in dry soil conditions. With so many factors involved, trying to provide a general ‘rule of thumb’ that would apply in most situations is impossible.

Written and developed by Eric Koetz, NSW DPI, CottonInfo Technical Lead (Weed Management)

Acknowledgement: Mark Congreve

Image by Graham Charles: label rate of Ally applied to left side of field in wheat, then long fallow back to cotton.

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