Grains Research and Development Corporation (GRDC) Grower Relations Manager North Richard Holzknecht said a range of factors influenced a herbicide’s efficacy and residual persistence in a particular environment. Photo: GRDC

Growers’ guide to reducing plant-back risk this season

Understanding how long herbicides stay in the soil and their potential impact on the environment and future yields is critical for growers and advisors wanting to reduce plant-back risks this season.

Understanding how long herbicides stay in the soil and their potential impact on the environment and future yields is critical for growers and advisors wanting to reduce plant-back risks this season.

So what do growers and advisors need to know?

Grains Research and Development Corporation (GRDC) Grower Relations Manager North Richard Holzknecht said a range of factors influenced a herbicide’s efficacy and residual persistence in a particular environment.

“Herbicides are chemicals, which when applied to the soil and plants in a particular environment interact, just like a chemical reaction,” he explained.

“From the perspective of an agronomist or a grower understanding how our management decisions interact with the following crop and the environmental conditions between and during use can greatly affect the yield produced.

“Understanding a herbicide’s physical and chemical properties and how it interacts with the environment allows us to make informed calculations about its likely persistence in the paddock.”

He said temperature, rainfall, organic matter, soil type, evaporation, wind, light, incorporation, soil moisture and spray conditions all influenced the persistence of residual herbicides.

“To manage plant-back risk, we need to understand what the risks are and how long we are at risk,” Mr Holzknecht said.

“The risks that need to be managed with residual herbicides are crop safety, weed control performance and plant-back effects. These risks vary for each residual herbicide.”

Mr Holzknecht said the first step in managing plant-back risk was to use product labels as a guide to decision-making in the paddock.

Labels generally provide information on crop use patterns, plant-back restrictions, timing of incorporation, 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.

Mr Holzknecht said the tolerance of crops to a given herbicide helped set the maximum rate. However, the length of weed control provided was determined by the herbicide’s relationship between its effective use rate and the rate of product decline in the soil.

“When the herbicide rate in the soil drops below that which provides effective weed control, there is a period of risk where poor weed control and exposure to low herbicide levels increases potential weed resistance and raises the risk of damage to the following crop,” Mr Holzknecht said.

“The length of this risk period varies by herbicide and how it interacts with specific environment in which it is used. Over time, the concentration of the product in soil declines until it is safe to plant the next crop.”

“This period of time gives growers a minimum plant-back period. Unfortunately, the persistence of a residual herbicide is highly variable and driven by a wide range of environmental conditions.”

He said the breakdown of most residual herbicides was primarily dependent on microbial degradation and chemical hydrolysis, which were both primarily driven by soil moisture.

“Low soil moisture, low organic matter, low soil pH and cold conditions will reduce microbial breakdown and increase the risk of residual herbicide carryover,” he said.

Key factors that can slow breakdown of herbicides in soil:

Low soil moisture,

Low organic matter,

Low soil pH, and

Cold conditions.

“Growers and advisors need to follow label recommendations, and they also need to be aware that a range of environmental factors and physical and chemical soil properties can impact a product’s persistence in the soil,” he said.

“By understanding the effect these environmental factors have on a product, potential plant-back risks can be identified and managed to minimise the risks to your next crop.”

Mr Holzknecht will talk more on the topic of reducing plant-back risks at the two-day GRDC Updates at Charles Sturt University in Wagga Wagga on February 13 and 14.

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