Nov 2023
Chemical carry over – friend or foe?
Nick McKenna
Nov 2023
Chemical carry over – friend or foe?

Earlier this year I was at work and came across some DAP. It wouldn’t have been noteworthy, except that it was September and I found it ~5cm beneath the soil surface in the paddock. It’s pretty dry if 4 months after seeding the fertiliser granules haven’t dissolved! That most people have a crop to harvest is remarkable.

 

Figure 1. Rainfall records for Perenjori. Red line shows 2023 rainfall to date, dark blue line is median rainfall.

This begs the question- if fertiliser hasn’t had a chance to break down, what about the pre-emergent chemistry? Many chemicals we use in our broadacre cropping systems are broken down by microbes. Microbial activity is influenced by factors such as organic carbon content (low in many soils and falling), rainfall/time the soil is moist (low this year for many areas), and temperature. Given the dry winter there is a good chance that we will see increased herbicide carryover next year. Be aware of chemicals that are more likely to carryover and take steps to mitigate the damage now.

 

The label stated plant back periods are usually a good guide for crop safety but unfortunately, they are not always accurate. Herbicide carryover has translated into crop damage the following year despite meeting requirements listed on labels. Obviously, herbicides behave in a more nuanced way than the label would have us believe!

If you have an eye for detail the DT50 value for herbicides can shed further light on how likely a herbicide is to carryover. The DT50 value is essentially the half-life of the chemical, or the days of time that it takes for 50 per cent of the herbicide in the soil to breakdown. This is influenced by soil type (influencing how tightly the herbicide is bound to the soil), type of breakdown, and application rate. The rate of
breakdown varies between different soils and environmental conditions. For these reasons a range of values may be presented for this characteristic. If you are inquisitive, the DT50 value for different chemicals can be accessed from the third link provided at the bottom of the article.

These reported values should only be used as a guide; chemicals such as Mateno complete, Reflex, and Overwatch can all appear to carry over to the next season in some situations despite meeting label requirements and passing the DT50 value.

Reading only the reported DT50 you would be forgiven for thinking that there is no risk of carryover- after all, if 50% of a chemical is broken down in 85 days (Reflex), surely 12 months after application there should only be ~5% of the original dose left behind? This is making several assumptions, chief among them that there is ~4.3 half-lives between the application and the next crop planting. In our cropping systems with low microbial activity and prolonged dry spells it is no surprise that sometimes the DT50 value may not give a full understanding of the requisite plant back period. As such, they should be used as a guide only – it’s unlikely that this internationally accepted DT50 value has been ground truthed on Wodjil sandplain!

There is still considerable time before next seeding, and summer rain is a perfect chance for microbes to activate in the soil. Any rain you receive in the warmer months will be a great chance for microbes to fire up, degrade chemicals in the soil, and reduce the risk of carryover.

But this isn’t something you can plan for. Instead, it is better to take actions to limit the damage next year. These include.

  • Avoid applying again next year. Some chemicals are already a little bit damaging to the crop, so avoid applying again. This is especially true of herbicides with notable crop effect such as Overwatch and Mateno complete. Give the crop a fighting chance and go easy on it if it’s going to be contending with residues.
  • Sow on an angle and increase seeding rate. Increasing seeding rate to help compensate for some plant death will give you a fighting chance if you are afflicted with herbicide carryover. Youd rather have too many plants than too few! Sowing on an angle to 2023 will ensure you get at least some seeding rows in safe areas.
  • Ensure all other soil constraints are removed. If you have a herbicide that’s hanging around like a bad smell that’s going to hurt your crop, ensure that soil compaction, acidity, nutrient status aren’t going to compound the effect.

Finally, consider the reasons you used the herbicide in the first place. In some situations, such as where you have used Sakura/Mateno complete, the weed burden may have been almost unmanageable. After a bone-dry year it’s hard to tell how many weeds have been killed, so the problem may carry over into 2024.

2023 was a diabolical year in some parts, and there will be a legacy to contend with. Some herbicides are more than likely going to carryover (Overwatch into lupins, Reflex into canola and wheat.) Go through your paddock diary from 2023 and have a chat with your agronomist about any chemicals you used that may show up next year.

Resources and further reading.
10293-GRDC-Plant-back-guide-manual-LR-002.pdf

Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers – GRDC

https://sitem.herts.ac.uk/aeru/iupac/search.htm

 

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