December Agronomy Update

Warning for Farmers Regarding Re-Cropping and Plant Backs to Residual Herbicides Following a Dry Year.

As crop planning is well underway, farmers should consider developing a secondary plan in addition to their primary plan. This secondary plan should include drought-tolerant crops and flexible planting dates to mitigate risks from unpredictable weather, especially in cases where there is insufficient rainfall to plant the preferred or planned crop type.

The following information pertains to the re-cropping and plant back periods to residual herbicides, particularly following a dry year.

1. Herbicide Residuals: Residual herbicides may remain active in the soil longer than expected in dry conditions, potentially affecting subsequent crops. It is critical to assess the specific herbicide used and its documented residual effects.
2. Consultation Required: Farmers are advised to consult product labels, manufacturer guidelines, and your Agronomist before making decisions regarding re-cropping. Specific plant back periods must be followed to reduce the risk of crop injury and ensure compliance with legal requirements.
3. Risk of Crop Damage: Planting sensitive crops within the herbicide’s residual timeframe may result in crop damage or failure. This relates to rainfall and timeframe requirements. The risk is heightened following a dry year, as moisture conditions can affect herbicide degradation rates.
4. Rainfall Records: It is best to have accurate rainfall records for each block. Rainfall can vary greatly across a farm, and this will impact herbicide degradation and your decision processes. If you do not have accurate rainfall records, err on the side of caution and plan to install rain gauges or weather stations in more locations across the farm.
5. Microbial Degradation:  Microbial degradation of herbicides requires a moist and microbial active soil. Factors that can influence degradation – Soil type, moisture, temperature, pH, and the presence of organic matter can all affect microbial activity and herbicide degradation rates. The rate of degradation varies widely among different herbicides and soil environments, making it essential to consider these factors when planning herbicide application and crop rotations. Sandy soils with low organic carbon content can still be a risk with some herbicide even when the rainfall and time requirements are followed.

For further reading

Manning, B. (2019) Herbicide residues and drought. Local Land Services. NSW Government. Available at: Herbicide-Residue-Factsheet-2019.pdf

Agriculture Victoria. (2013) Avoiding crop damage from residual herbicides. Victoria State Government. Available at: Avoiding crop damage from residual herbicides | Chemical residue resources | Managing chemical residues | Chemicals | Farm management | Agriculture Victoria

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