Labels and plant-backs - what you need to know for 2018

Take home messages

• How residual herbicide products react with soil, water, temperature, light, organic matter and microbes determine their potential for plant-back risk.
• Drivers for pre-emergent residual herbicide breakdown are set by a product’s physical and chemical properties.
• Labels are legal documents that provide guidelines and key conditions that when met will prevent plant-back issues.

Discussion

Persistence of soil residual herbicides and their effect on plant-back crops are determined by a product’s physical and chemical properties and their interaction with the complex and highly variable environmental and biological systems in which they are used. This interaction can be simplified by looking at how these products interact on a basic level, then complexity can be added back into the system.

Where do we start? Herbicides are chemicals. When chemicals are applied to soil and plants in a particular environment, they interact. Just like a chemical reaction.

Product A + Product B + Conditions (Pressure/Heat) => Reaction => Product C

If we look at it through the eyes of an agronomist or grower:

Plant (G) x Environment (E) x Management (M) => Response => Yield

Agricultural chemicals and plants when used together produce a biological response leading to yield. Agronomists and growers need to make decisions based on their knowledge of how the desired crop will respond to the environment, the products and the management decisions implemented on the targeted paddock.

So what factors influence the persistence of residual chemicals? A wide range of factors shown in Figure 1 influence a product’s efficacy and residual persistence in a particular environment. By understanding a product’s physical and chemical properties and how they interact with their environment, an educated guess can be made on how a residual product will persist in our production systems.

Figure 1. Factors influencing the persistence of residual chemicals (Source: Richard Holzknecht 2018)

How do we manage plant-back risk? We need to understand what the risks are, and how long we are at risk. The risks that need to be managed with residual herbicides are crop safety, weed control performance and plant-back effects. These risks will vary for each residual herbicide product. Figure 2 highlights the difference between two residual products and how their concentration in the soil declines over time and their effect on risk.

Figure 2. Factors influencing the risks associated with using residual herbicide (Source: Richard Holzknecht and Roger Mandel 2015).

The tolerance of crops to a given product helps sets a maximum rate. The length of weed control provided by a residual product is determined by the product’s relationship between its effective use rate and the rate of product decline in the soil. Once the rate in the soil drops below that which provides sufficient weed control, a period of risk is entered where poor weed control and exposure to low rates increases potential weed resistance and crop damage to the following crop may occur. The length of this risk period varies by product and how it interacts with the environment in which it is used. Over time, the concentration of the product present in soil will decline until a point is reached at which it is safe to plant the next crop. This period of time will set a minimum plant-back period for a given environment. Unfortunately, the persistence of a residual herbicide is highly variable and driven by a wide range of environmental conditions. Therefore, more information is needed on what factors influence how long a residual product will persist in our production systems. The label for a given product provides a set of guidelines, e.g. plant-back periods and environmental conditions which need to be met to minimise plant-back effects.

What guidelines do labels provide to help us manage plant-back risk? A product label is a legal document that outlines the approved use patterns and conditions required to minimise any plant-back effects. They also highlight identified risks that can affect the product’s performance. Common label statements on crop use patterns, plant-back restrictions, incorporation timing, incorporation methods, planting methods, soil type, pH, surface conditions, organic matter, stubble, minimum rainfall amounts and rainfall distribution required before plant-back restrictions are met are identified on labels.

Why do some labels have more label guidelines for use than others? It all relates to a product’s physical and chemical properties (Table 1) and how it interacts in a wide range of environmental conditions. Additional understanding of a product’s physical and chemical properties can help identify what the likelihood for plant-back risk is for each product in any given season.

The primary methods for the residual herbicide breakdown are by microbial and hydrolysis. Both microbial and hydrolysis are driven by soil moisture. Hydrolysis breakdown is the chemical reaction of the product in the presence of water. Products with higher water solubility and lower soil binding are more available for hydrolysis and accessible for microbial breakdown. Microbial breakdown is reliant on the presence of the right soil microbe and soil conditions to enable breakdown. Soil conditions with low soil moisture, low organic matter, low soil pH and cold conditions will reduce the rates of microbial breakdown, increasing the risk of residual herbicide carryover to the next cropping season. Residual herbicides with shorter half-lives (DT50) require less time exposed to these soil conditions, and therefore, have shorter soil persistence.

Conclusion

Understanding the plant-back risks associated with the use of residual herbicides is driven by the product used, and its interaction with the environment and production system in which it is used. Product labels are legal documents which guide the use of these products and seek to set ‘one size fits all’ recommendations to manage plant-back risks. However, a wider range of environmental factors and physical and chemical properties can impact on a product’s persistence in the soil. By understanding the effect these environmental factors have on a product, potential plant-back risks can be identified and a management decision to minimise the impact of these risks on our production systems prior to each season can be implemented.

Useful resources and references

Soil behaviour of pre-emergent herbicides

Acknowledgements

I would like to thank Mark Congreve and John Cameron for their work and report on GRDC Project ICN00016.

Contact details

Richard Holzknecht
Grains Research and Development Corporation
Grower Relations Manager - North
0408773865
Richard.holzknecht@grdc.com.au
@Richardholzkne