Residual herbicides - carryover and behaviour in dry conditions

Page last updated: Tuesday, 12 June 2018 - 2:14pm

The term 'residual' applies to a number of herbicides that have a long lasting activity in the soil. These herbicides are often applied directly to the soil prior to planting crops, pre-emergent. Residual herbicides rely on moisture and microbial activity to break down. So one of the consequences of a drier than average growing season is that the herbicides applied may still be active in the following season. In addition, residual herbicides can be applied in advance of dry sown crops and still be expected to have sufficient weed control activity when the season breaks.

Carryover of residual herbicides from season to season

One of the consequences of a growing season being drier than average particularly in the eastern and northern WA wheatbelt, is that herbicides applied in one season could still have significant residues in the soil, and affect crops planted the following year.

Sulfonylurea (SU), imidazolinone (IMI) or triazine herbicides are likely to cause the most concern. And residues, from the previous season may affect crop emergence or even kill sensitive crops or crop cultivars in the next season.

Diflufenican damage symptoms have also been reported following recent dry years on canola crops where products such as diflufenican (for example, Brodal® Options) were applied to lupin crops, or diflufenican/MCPA (for example, Tigrex®) or diflufenican/bromoxynil (for example, Jaguar®) were applied in wheat crops.

The soil pH will have an impact on which herbicides are more likely to persist. All other things being equal, imidazolinones will be more persistent on acid soils and sulphonyl ureas on alkaline soils. Triazines are very slightly more persistent on alkaline soils.

Dry conditions reduce herbicide degradation

The main factors contributing to residue carryover are poor uptake of herbicides by crop plants and limited microbial or chemical degradation of herbicides in dry soil conditions.

For simazine to be absorbed by plants, the required water content is 5% in sandy soils and 10-15% in heavy soils. Simazine has a half-life of 3-6 months during winter. (Half-life is the length of time it takes for the effectiveness of the herbicide to reduce by half).

In an average year, at least 95% of simazine applied in the crop will be degraded by the end of the season. Any remaining simazine will have no effect on the following crops.

The presence of root diseases in a cereal crop may exacerbate the effects of herbicide residues.

Summer rain can aid in the breakdown of residues but only if the soil surface remains wet long enough for the breakdown processes to occur.

In some instances, where there are large downpours of rain that infiltrate through the soil profile the water may move soluble residues (such as the SU’s) down the profile. This may be an issue of concern on duplex soils, where crop damage symptoms do not appear until the roots reach the zone where the residues have moved to.

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