Understanding herbicides for better management

Article by Sustainable Agriculture Manager Peter Clifton

Chemical residues from pre-emergent herbicides can be a major issue for future crop growth but the key to overcoming the problem, according to agronomist Mark Congreve, is understanding more about the chemistry of the various products available.

Mark – also a senior consultant with ICAN (Independent Consultants Australia Network)  -shared his insights at workshops in Narrogin and Bunbury a few years back and I’ve collected them up into an article for our reference.

His key message at the time was: “Understand the chemistry of different herbicides, soil type and rainfall to improve efficacy, reduce crop damage and risk of resistance.”

The importance of soil moisture

According to Mark, more soil moisture will ensure herbicides are broken down, reducing remaining residue and creating safe soil for new crops.

It’s not always as simple though, as reading instructions on the product label specifying how much rainfall or what period of time you should wait for herbicides to break down.

Here are some additional things to consider according to Mark.

Herbicides are broken down in the soil by microbes – microscopic living organisms, which like any form of life, are affected by temperature, moisture and the availability of a food source. In the absence of one of those, microbes go into hibernation.

The most important factor for ensuring lots of soil microbes, says Mark, is moisture.

Generally, microbes live on food or organic matter in the soil which is concentrated in the top 5 or 10 cm. Mostly herbicides provide another food source for them and while some forms of microbe might find a particular product toxic, there will be others which like it so thrive and proliferate.

So while rainfall amounts was important, Mark said sometimes it was how the rain fell and how many weeks the surface layer of soil remained moist for.

Herbicide Mobility

An immobile herbicide will stay near the soil surface where the microbes are busy feasting and creating lots of biological activity. A mobile one will move deeper down the soil profile with rain and right out of the root zone – especially if there are soil constraints like acidity or plough pans.

Therefore, how effectively the microbes can break down the herbicide and get rid of residue will be determined by the herbicide mobility. Mark said while immobile herbicides sitting near the surface could be broken down relatively easily, mobile herbicides that weren’t being broken down at deeper levels and leaving residue as a result, presented a real challenge, particularly if crop roots reached that depth.

The effects of tillage

Herbicide residues could also be a problem if farmers were practicing inversion tillage. This process buries soil carbon and microbes at depth and brings soil with minimal microbes and organic matter to the surface.

As a result, the process of microbes breaking down the herbicides as food will be much slower.

Rinse and repeat?

Mark says consistent use of the same herbicide can increase the rate of microbes breaking down herbicides and removing residue.

Using lots of the same herbicide relatively frequently will build up populations of those microbes which thrive on that particular product. The soil therefore becomes conditioned to dealing with that herbicide and the whole process of breakdown becomes a lot more efficient.

For more information on herbicide behaviour, go to this website

For more soil resources, visit the below Project Pages: