In light of the dry summer, herbicide residues have had limited opportunity to break down in the soil, and if the dry spell continues, this will have a bearing on rotations and variety selection for this year.
Many commonly used herbicides including group B’s (imidazolinones), Lontrel® (group I) and Balance® (group H) can last in the soil for months. These herbicides can control future weed emergence, however, if conditions are not suitable they can negatively impact the subsequent crop.
Group B herbicides are broken down in the soil by microbes in wet, aerobic conditions. In soils with low organic matter, non-wetting sands, water-logged soils or after prolonged dry periods, soil residuals will persist and can affect susceptible crops.
In most instances a minimum of 150mm of rain (from the date of application) is needed to break down herbicide residues but the speed of breakdown can vary according to soil type, soil pH, soil temperature, microbial activity and the residual chemical used.
Growers should read chemical labels to accurately determine minimum re-cropping intervals, but generally Group B’s will persist for longer in high pH alkaline soils.
Growers should also note that one summer thunderstorm is not sufficient to breakdown some Clearfield herbicides, particularly if hot and dry conditions prevail. Five rainfall events of 20mm fortnightly is a much better scenario than a one off rainfall event of 100mm.
After a dry period, it is vital that growers take adequate precautions to prevent any potential effect in the following crop.
Last year some growers got caught, planning early and not implementing changes prior to sowing when still dry.
To work out what crop can be sown after using a residual herbicide:
- Determine the rate of herbicide application.
- Quantify the rainfall from the date of application up until sowing the plantback crop (excluding on off rainfall events during summer).
- Consult product label recommendations.
In paddocks where group B herbicides were used, wheat options include Kord CLF Plus (ASW), Elmore CLF Plus (AH) and Grenade CLF Plus (AH). Scope CL (Malt) is a group B tolerant barley.
Of the pulses field peas are the least sensitive, followed by lupins, vetch and chickpeas. Lentils are the most sensitive, the exception being lentil variety PBA Herald XT which has improved group B tolerance.
Damage to susceptible crops sown at the lower end of the plantback range can be minimised by taking the following steps:
- Root disease test. Crop effect will be magnified in the presence of root disease.
- Apply zinc to the seed to encourage early root development.
- Delay seeding to maximise the opportunity for chemical breakdown.
- Sow at the right depth to ensure adequate nutrition to promote rapid germination and emergence.
- Do not use another Group B herbicide in the plantback crop to avoid the potential of compounding herbicide effects.
- Avoid plant stress during the growing season. Any stresses encountered (such as poor growing conditions or insect damage) may exacerbate herbicide damage symptoms.
A comprehensive list of minimum re-cropping intervals and guidelines is available on the Pulse Australia website at: https://www.pulseaus.com.au
