Understanding herbicide behaviour vital for better management

Chemical residues from pre-emergent herbicides can be a major issue for subsequent crop growth. However, growers are now more aware of how to deal with the issue thanks to recent GRDC-funded herbicide workshops run in Narrogin and Perth.

Senior consultant with ICAN (Independent Consultants Australia Network) Mark Congreve said that microbial degradation is vital to the breakdown of most herbicides, particularly pre-emergents.

“Microbial populations are basically driven by temperature, moisture and a food source. In the absence of one of those they go into hibernation. They pretty much live on the organic matter in the soil, which is concentrated in the top five or ten centimetres. Moisture is the biggest limiting factor and, less so, temperature unless it’s winter when it can get pretty cold.

“Microbes use herbicide in the soil as another source of organic carbon and break it down. They might be toxic to some species, but conversely those that use herbicides as a food source actually proliferate. So you might change the balance, but almost invariably numbers go up if you’ve got moisture. So I spend a lot of time talking to agronomists about the importance of moisture to breakdown herbicides (and reduce chemical residues) so the soil is safe for crops.

“A lot of the plant back statements specify a certain number of months and amount of rainfall on the label that is required to break down herbicides with microbial activity. It’s probably more how the rain falls than the amount, or how many weeks the surface layer is moist for.

“The ability of microbes to break down herbicides can differ depending on the herbicide’s mobility. A relatively immobile herbicide is going to stay near the soil surface where there is lots of biological activity, so breakdown is effective. Our biggest challenge is mobile herbicides. With lots of rain they will stratify down the profile and out of the root zone, particularly if you’ve got subsoil constraints like acidity or plough pans. You can break down stuff near the soil surface relatively easily, but we might have some herbicides sitting at depth where there’s not much organic matter and therefore less microbial activity. They can be a problem if your crop’s roots get down to those layers.”

Similarly, residues can be an issue if you practice inversion tillage. This practice buries soil carbon and microbes at depth and brings soil with minimal microbes and organic matter to the surface. In this situation, breakdown will be much slower. Also, because many herbicides bind to organic matter and become unavailable to plants, the lack of this substance means herbicides will be much more available, while fewer microbes means it will be available for longer. An example is the commonly used Trifluralin which binds readily to stubble and rates have increased with stubble retention to get enough into the soil. If stubble and organic matter is suddenly removed, suddenly it’s much more available and growers get all sorts of crop damage.

Conversely, consistent use of the same herbicide can increase breakdown rates.

“If we use lots of the same herbicide relatively frequently, we’ll build up populations of microbes that break down that particular herbicide. The soil becomes conditioned to deal with that herbicide, so you end up with accelerated degradation and you don’t get the persistence with that herbicide.”

The key message from the workshops was to understand the chemistry of different herbicides to improve efficacy and reduce crop damage and risk of resistance.

“The idea of the workshop started with grower groups having 6 or 8 pre-emergent herbicide options and running trials to find out what worked best. They would get different results at different sites and thought they needed to do more and more trials to figure out what was going on. But they actually just needed to understand the chemistry and the environment or soil type and rainfall. Understand that and it becomes more predictable what will work and what won’t. It’s about getting a better understanding of what’s driving the chemistry.”

For more on herbicide behaviour, go to https://grdc.com.au/resources-and-publications/resources/herbicide-behaviour

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