Even if you think it shouldn’t be too much of a worry, it’s worth checking your plant back periods. As you’d know, keeping an eye on herbicides when controlling weeds late in the season and over summer is essential, as residues can cause crop damage and impact your cropping choices. Of course, different factors will influence how quickly herbicides get broken down. The main ones are; soil temperature, moisture, microbial activity, pH, soil organic matter, and the chemistry of the herbicide used and how it degrades. 

So, how do you ensure you don’t damage your crop or limit your cropping decisions? Firstly, it’s important to remember these four key points when it comes to residual herbicides:  

1. No moisture = no breakdown: Soil microbial activity is critical for herbicide breakdown, which can be considerably influenced by lack of moisture. A general rule is that if the top 10cm of soil isn’t moist, then minimal degradation would occur regardless of time.  
2. How the rain fell is more important than how much total rainfall: Consider how the rain has fallen since the herbicide application rather than simply calculating the total rainfall. A single small rainfall event in dry soil won’t wet the soil long enough to build up microbe levels, and even a single large event can cause water to run off rather than soak. Therefore, you need rain events that have kept the soil moist for more than a few days for proper breakdown. This will allow microbial populations to build up and begin to break down the herbicide.  
3. Chemical profile: It’s also important to 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 zone of optimum microbial activity, increasing persistence.  
4. Follow the label: Always adhere to label recommendations on plant back periods to minimize the risk of crop damage.  

It can be hard to remember some of the problems caused by chemicals you might have used, so we’ve listed a few of the common ones below: 

Summer Spraying:  

• Metsulfuron (Ally) – Wheat is 10 days and Barley is 6 weeks, plus all Legumes and Oats are 9 months i.e., Chickpeas, Faba Beans, Lupins, Oats, Field Peas, Canola, Sub Clovers & Medic. 
• Chlorsulfuron (Glean) – Barley is 9 months, and Canola, Beans, Peas & Non-Clearfield Lentils are 22 Months. 
• Terrad’or – Canola at 15-20g/ha rates are 1 week, and at 21-40g/ha rates are 2 weeks.  
• Dicamba (Kamba750) – Canola is 2 weeks at 135ml/ha, and Lentils are 1 month at 135ml/ha.  
• Amine 625 + Amicide Advance 700 – Canola with rates up to 500ml/ha are 2 weeks, and Clover, Chickpeas, Faba Beans, Field Peas, Lentils & Lupins with rates up to 500ml/ha are 1 week. 
• Ester 680 – Canola with rates up to 510ml/ha are 2 weeks, and Clover, Chickpeas, Faba Beans, Field Peas, Lentils & Lupins with rates up to 510ml/ha are 1 week.  

In-season: 

• Paradigm – Sub Clover, Canola, Chickpeas, Faba Beans, Field Peas, Lentils, Lupins, Medic and Vetch are 8 months and 100mm of rain.  
• Metsulfuron (Ally) – Chickpeas, Faba Beans, Lupins, Oats, Field Peas, Canola, Sub Clovers and Medic are 9 months.  
• Clopyralid (Lontrol Advanced) – Clover, Chickpeas, Faba Beans, Field Pea, Lentils, Lupins, Medic and Vetch with rates up to 150ml/ha are 9 months.  
• Intervix – Chickpeas, Faba Beans, Field Peas, Lupins, Non-Clearfield Wheat, Barley and Oats are 10 months, and non-Clearfield Canola is 34 months.  

Pre-Emergent 

• Sakura – Durum Wheat, Oats and Lucerne are 21 months and 550mm of rain. 
• Velocity – Canola, Clover, Chickpeas, Faba Beans, Field Peas, Lentils, Lupins, Medic and Vetch are 9 months and 250mm of rain.  
• Overwatch – Chickpeas, Oats, Sub-clover and Clover are 9 months and 250mm of rain, and Medic is 9 months and 350mm of rain. 
• Reflex **NEW** – Wheat, Oats, Durum, Chickpeas, Field Peas, Faba Beans, Lupins, Vetch, Lentils, Sub Clover, Medic, Canola and Barley are 9 months and 250mm of rain with rates of 1.5L/ha.
  *** NOTE: Take caution when sowing cereals into paddocks treated with Reflex the previous season. During the 2022 season, some paddocks treated with Reflex in 2021 displayed levels of crop damage. The majority were in WA, with a handful in SA and the Vic Mallee. Speak to one of our Agronomists if you plan to plant cereals into a paddock treated with Reflex in 2022 if:

• • It is a light-textured soil (sandy-sandy loam) 
  • The profile contains low organic matter 
  • The soil is non-wetting 
  • High use rates of Reflex were used in 2022 (>1L/ha, particularly if used PSPE) 
  • Minimal summer rain, where the topsoil failed to stay moist for numerous days at a time 
  • Soils have been ameliorated (ripping, spading, etc,) 

Do not plant canola the following season after Reflex, this is still a relatively touchy situation but, in most cases, we suggest being on the safe side and avoiding this if possible until we know more.  

Please remember, the information above is just a snapshot, so always read the label for the full story on plant-backs.  

This article was originally published on March 23rd 2022 but has been updated and revamped to ensure it is accurate and relevant for you.