Plant Back Warning: Be Mindful Of Herbicide Residues

Unfortunately, this year many growers have been caught out with herbicide residuals which have affected their cropping flexibility. The dry conditions across the state, along with the increased reliance on residual herbicides, have created plant back issues in many areas. The issue is, for most herbicides to breakdown in the soil, they rely on two key mechanisms; microbial degradation or a chemical reaction known as hydrolysis. Both of these require soil moisture. Which is why insufficient rainfall or prolonged dry spells can significantly increase the duration of the required re-crop interval. So with these dry conditions we’ve experienced, herbicides are tending to remain in the soil and can cause issues for crop emergence.

Issues with plant backs:

Certain herbicides can persist in the soil for quite some time, whether that be weeks, months, or even years. Soil persistence can be a good thing, as it will provide weed control for an extended period. But on the other hand, this can be a problem if the herbicide stays in the soil longer than intended and creates the potential to damage sensitive crops or pastures in the following years.

For example, chlorsulfuron can be used in wheat and barley, but can damage legumes and oilseeds when it persists in the soil for several years.

How to identify if you have an issue.

The main difficulty is identifying potential herbicide residues before they cause a problem. Currently, the only methods for predicting carryover is the information given on product labels about climate and soil type. Herbicide residues are generally too small to be detected by chemical analysis. And even if it was possible, it would be too pricey to be a part of normal farming practices. When the crop has emerged, diagnosis is problematic because the symptoms shown can often be confused with nutrient deficiencies or disease.  Also, the effects of herbicide residues can make crops more susceptible to stresses such as those above, making it even harder to identify.

Which herbicides are residual?

The following herbicides all contain residual activity or certain planting restrictions. Growers need to be careful, as the labels do not have consistent terminology or layout warnings in the same place. It’s important to read every label in its entirety and check for these restrictions.

In general, Group B herbicides have the longest plant back issues.

The following herbicides contain residual activity and/or planting restrictions:

How do herbicides break down?

As mentioned above, herbicides are broken down by either chemical (hydrolosis) or microbial degradation.

Chemical degradation is a process that occurs naturally, but the speed is dependent on the soil type – whether it’s acidic or alkaline – and soil moisture and temperature, amongst other factors.

Microbial degradation is reliant on the population of suitable microbes, which live in the soil, to consume the herbicide as a food source, and the surrounding soil conditions.

Both processes are boosted by heat and moisture. But are hindered by herbicide binding to the soil. Binding depends on the soil structure, whether it’s pH, clay or sand, and other compounds such as organic matter or iron.

For example, triazines that are broken down by hydrolysis, break down much slower at high pH levels. Which is why we see these residuals persisting longer in alkaline soils. In this situation, microbial decomposition becomes the primary mode of breakdown.

It’s important that you are aware of your soil types and climate to understand the breakdown of different herbicides. It’s a very complicated topic, so speak with your agronomist if you’re unsure. Make sure to read labels and understand re-cropping periods for every paddock.

How can I prevent residual herbicide damage?

First, read every part of the label, even the fine print, as information on residues can be written anywhere throughout the label.

When choosing a herbicide, make sure it’s absolutely necessary for the weed population you have. Always keep in mind limitations for future cropping which the herbicide may have.

Keep detailed spray records. Make sure nothing gets sprayed then forgotten. Before seeding, go back and check records so you don’t sow a sensitive crop species into a paddock with potential residuals. Records should include weather conditions, spray dates, chemical rates, batch numbers, rainfall, soil type and pH. (Not forgetting about varying soil types within the paddock). This can give you something to work back on if unexpected crop damage occurs.

If you know you’ve used a herbicide with a plant back, or you know with certain conditions there is potential for residual to still be lingering in the soil, change plans to sow a crop type that is less susceptible to that chemical.

Try to ensure growing conditions that are free from stresses such as herbicide spray damage, disease, and nutrient deficiency. These will compound the problem. If the crop is stressed by any of these factors, it will then make the crop more susceptible to residue damage.

If you would like to know more, or are unsure if you may have plant back issues in your paddock, please call one of our agronomists.