It is important to keep an eye on residual herbicides when controlling weeds late in the season and over summer, as it could impact your choices for the next crop. This is even more important in dry seasons. The breakdown of herbicides is influenced by many different elements, the main ones being soil temperature, moisture, microbial activity, soil pH, soil organic matter, as well as the chemistry of the herbicide used and how it degrades.  

It is important to remember these few key points: 

• Soil microbial activity is critical for herbicide breakdown, and this can be considerably influenced by lack of moisture. A general rule to follow is if the top 10cm of soil isn’t moist, then minimal degradation would be occurring regardless of time.  
• Consider how the rain has fallen since the herbicide application, rather than simply calculating the total rainfall. A single small rainfall event into a dry soil doesn’t wet up the soil long enough to build up microbe levels, and a single large event can cause water to run. Therefore you need an event that will keep the soil moist for more than a few days. This will allow microbial populations to build up and begin to break down the herbicide.  
• Consider the mobility of the herbicide. Herbicides with a higher water solubility that don’t bind tightly to the soil can move down the soil profile after rainfall events and sit below the optimum microbial activity zone. This can increase persistence.  
• Always adhere to label recommendations on plant back periods to minimize the risk of crop damage.  

A couple of important ones to remember are:  

In-season:

• Paradigm – Sub Clover, Canola, Chickpeas, Faba Beans, Field Peas, Lentils, Lupins. Medic & Vetch – 8 months and 100mm  
• Ally – Chickpeas, Faba Beans, Lupins, Oats, Field Peas, Canola, Sub Clovers & Medic – 9 months. 
• Lontrol Advanced- Clover, Chickpeas, Faba Beans, Field Pea, Lentils, Lupins, Medic & Vetch – Rate up to 150ml/ha 9 months. 
• Intervix – Chickpeas, Faba Beans, Field Peas, Lupins, Non-Clearfield Wheat, Barley & Oats – 10 months 
• Sakura – Durum Wheat, Oats & Lucerne – 21 months and 550mm 
• Velocity – Canola, Clover, Chickpeas, Faba Beans, Field Peas, Lentils, Lupins, Medic & Vetch – 9 months and 250mm 
• Atlantis – Barley, Oats, Canola, Chickpeas, Field Peas & Vetch – 9 months 
• Faba Beans & Lentils – 11 months 

Summer Spraying:

• Ally – Wheat 10 days & Barley 6 weeks (as well as mentioned above)  
• Terrad’or – Canola 15-20g/ha 1 week – 21-40g/ha 2 weeks 
• Kamba 750 – Canola – 1 week at 135ml/ha & Lentils 14 days at 185ml/ha, Chickpeas at 125ml/ha 21 days.
• Amicide Advance – Canola – Rate up to 500ml/ha, 2 weeks 
• Clover, Chickpeas, Faba Beans, Field Peas, Lentils & Lupins – Rate up to 500ml/ha 1 week 
• Ester 680 – Canola – Rate up to 510ml/ha, 2 weeks, chickpeas, field peas, lupins – rate 510ml – 1000ml, 14 days, Lentils and Faba beans – rate 510-1000ml, 7 days
• Clover, Chickpeas, Faba Beans, Field Peas, Lentils & Lupins – Rate up to 510ml/ha 1 week 

The information above is just a snapshot. Remember to read the label for the full story on plant-backs. Please take into account other factors that will influence plant back, including minimal rainfall requirements.