Dry summer soils provide an unwelcome haven for herbicide residues

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As growers embrace no-till and stubble retention systems glyphosate is being used more often to control summer weeds. This has inevitably resulted in the evolution of herbicide resistance in summer weed species

KEY POINTS

  • Ensure plantback periods are observed when residual herbicides are applied during summer fallow
  • In addition to the total amount of rainfall required, also pay attention to the pattern of rainfall, especially when conditions are drier than usual
  • Low soil moisture, low organic matter, low soil pH and cold conditions will reduce microbial breakdown and increase the risk of residual herbicide carryover

Predicting how long herbicides will persist in the soil is a real challenge for growers. There are many factors that influence the time needed for herbicides to breakdown. The main factors include soil properties, climatic conditions (in particular temperature and moisture), the chemistry of the herbicide and how it is degraded (for example by microbes or chemical hydrolysis), soil organic matter and in some situations, soil pH (Figure 1).

Soil microbes

Breakdown by soil microbes is the main degradation pathway for many herbicides and is favoured by warm moist soil, oxygen and an adequate level of organic matter. As the majority of microbes are found in the top 10 centimetres of the soil profile, it is often here where most herbicide degradation occurs.

Dry conditions after application of a soil residual herbicide are not conducive for degradation. Microbes are living organisms and without soil moisture their populations decline, resulting in very little microbial breakdown of herbicides under dry conditions.

The plantback period on product labels will often specify both a time period and a rainfall requirement. Think in terms of the number of weeks of moist topsoil, rather than the rainfall total. Regular rain that keeps the topsoil moist for a prolonged period of time provides a far more hospitable environment for microbes to breakdown herbicide residues than an occasional heavy rain storm interspersed by long hot spells.

Recent drought conditions in some areas will mean that residues are likely to persist far longer than they may have in other seasons.

Graphic showing herbicide breakdown and loss pathways
FIGURE 1 Many factors influence the residual herbicide breakdown and loss pathways. SOURCE: Mark Congreve

Other factors

Herbicides that have high water solubility with a low affinity for soil binding could be moved deeper into the soil profile by rainfall events.

Any herbicide that moves below the zone of optimum microbial activity is likely to persist longer. These herbicides include the residual Group I herbicides, such as clopyralid (Lontrel®) and picloram (Tordon®), as well as Group B herbicides such as the sulfonylureas (SUs) and imidazolinones (IMIs).

When there is a soil change, pH change or some other structural limitation that prevents further leaching, it is possible for these herbicides to remain in the soil at depths where degradation will be far slower than on the surface.

Soil type also influences herbicide residues. The Group B imidazolinone (IMI) herbicides bind tighter to acid soils, reducing their availability to be broken down by microbes and leading to longer persistence at low pH.

Conversely, Group B sulfonylurea (SU) and the Group C triazine herbicides persist longer under alkaline conditions. In acid soils these herbicides break down primarily via chemical hydrolysis, while at higher pH levels breakdown is via slower microbial processes.

It can be tempting to think that using a lower rate can improve the plantback time, but due to the way residual herbicides break down in the soil changes in application rate often have minimal impact on the length of persistence. Halving the application rate will not halve the plantback period.

Following labelled plantback periods normally ensures adequate safety, but predicting herbicide persistence can be complex particularly when conditions are much drier than usual. GRDC has recently updated its national reference manual, Soil behaviour of pre-emergent herbicides in Australian farming systems, which provides more detail on how individual herbicides interact with the soil and environment.

More information

Mark Congreve, Independent Consultants Australia Network
0427 209 234
mark@icanrural.com.au

Reference manual: The value of pre-emergent herbicides