WithTheGrain: Understanding herbicide residues in the soil

Author: | Date: 11 Jan 2018

Numerous factors that influence the length of herbicide persistence in soil need to be taken into consideration by growers when using herbicides as part of their farming operation.

Senior consultant with Independent Consultants Australian Network (ICAN) Mark Congreve has been facilitating GRDC workshops on the soil behaviour of pre-emergent herbicides. He says the various factors for herbicide breakdown made predicting their persistence difficult, sometimes causing frustration when it comes to the following season’s planting opportunity.

“Unfortunately, there are not just one or two simple factors that influence the length of herbicide persistence,” he says.

“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 and, in most situations, soil organic matter and pH.”

Soil microbe activity

Soil microbial activity is a major degradation pathway for many of the main herbicides and happens fastest when the conditions are right to support microbial populations.

“Oxygen, warm soil temperature, an adequate level of organic matter, soil pH and good soil moisture are the key requirements,” Mr Congreve says.

“As a result, microbial populations are usually at their highest levels in the top 10 centimetres of soil, with populations rapidly declining below 15cm where conditions are often less suitable to sustain microbial activity.”

Typically, the most limiting factor slowing microbial activity in the topsoil is the lack of soil moisture.

“As a rule of thumb, if the top 10cm of soil is not moist, then little herbicide degradation is occurring, regardless of how many months have passed,” Mr Congreve says.

“Rainfall in summer, when conditions are warm, will lead to much higher microbial populations than rainfall in the colder months.

“In our workshops, we encourage growers to consider how the rainfall has fallen over the summer months since the herbicide application, rather than just looking at the rainfall total. Many labels will have a plantback period specifying the number of months and a rainfall requirement.

“Rather than considering the rainfall in total, it is better to think in terms of the number of weeks of moist topsoil. A single rainfall event with months of dry topsoil is less effective at sustaining microbial populations compared to the same rainfall occurring as a number of events that keep the topsoil wet for longer.”

While rainfall is a major factor, several other factors also substantially impact herbicide persistence.

 

Boomspray

The length of time herbicide residue stays in the soil needs to be a consideration in summer weed spraying strategies.

Location of herbicide residues in the soil profile

Mobile herbicides, or herbicides with higher water solubility and lower affinity for soil binding, can move down through the soil profile after rainfall events. Herbicide that moves below the zone of optimum microbial activity is likely to persist much longer.

“Particular problems often arise with mobile herbicides where there is a physical or chemical change in the subsoil, such as a pH change or some other structural limitation such as an acid, clay, boron, magnesium or sodic layer that causes the herbicide to concentrate at depth and prevents further leaching,” Mr Congreve says.

All Group B herbicides, such as sulfonylureas (SUs) and imidazolinone (IMIs), and the residual Group I herbicides, such as clopyralid (Lontrel®), are highly mobile in soil water and subject to these issues.

The Group C triazine herbicides can also be subject to moving down the profile, however are typically not as soluble as these other groups.

The persistence of the individual herbicide and application rate

Mr Congreve says growers and advisers can get a relative indication of the length of persistence of a herbicide by considering the half-life of the herbicide, which is presented as the DT50, meaning the days of time for 50 per cent of the herbicide to dissipate.

“Residual herbicides with a short DT50, for example less than 20 days, will be lost relatively fast under normal conditions and are less likely to give significant plantback restrictions the following year,” he says.

“Herbicides with a longer DT50 value are likely to persist much longer.”

Table 1 shows both the average DT50 and a range across a number of soils for some of the more common herbicides. Considering the range of DT50 values provides insights into the variability that may be seen with some herbicides.

Table 1: Average DT50 and a range across a number of soils for some of the more common herbicides

Active ingredientExample trade nameDT50 value range (average)
MetazachlorButisan®3-21 (7)
ProsulfocarbArcade®7-13 (10)
Metsulfuron-methylAlly®7-37 (13)
ImazamoxRaptor®(17)
S-metolachlorDual Gold®11-31 (21)
PyroxasulfoneSakura®16-26 (22)
ChlorsulfuronLusta®11-70 (36)
TriasulfuronLogran®16-92 (39)
ClopyralidLontrel®12-70 (40)
Tri-allateAvadex®8-205 (46)
ImazethapyrSpinnaker®7-290 (51)
PropyzamideRustler®18-53 (60)
AtrazineGesaprim®6-108 (60)
SimazineGesatop®(90)
ImazapyrArsenal®25-142 (90)
TrifluralinTreflan®35-375 (170)
ImazapicFlame®31-410 (232)

DT50 values for other residual herbicides used in grain production can be found in the Soil Behaviour of Pre-Emergent Herbicides in Australian Farming Systems manual.

Mr Congreve says due to the way residual herbicides break down in the soil, changes in application rate may often have minimal impact on length of persistence.

“For example, halving the application rate does not mean that plant back periods are halved,” he says.

Other considerations

In addition to the general principles above, there are some other specific examples to note:

  • The Group B imidazolinone (IMI) herbicides bind tighter in acid soils, making them less available for microbial degradation. Therefore, they persist longer in acid soils.
  • In addition to microbial degradation, the Group B sulfonylurea (SU) and the Group C triazine herbicides are also degraded by chemical hydrolysis. However, the speed of this chemical breakdown pathway is strongly influenced by soil pH. Under alkaline conditions the hydrolysis reaction slows or stops and herbicide persistence increases significantly.
  • Residual herbicides that also have post-emergent activity may be applied to a growing crop. If the herbicide absorbed by the crop is not fully metabolised before harvest, there is a risk that herbicide remaining in the stubble after harvest could be released back into the soil as the stubble decomposes. An example of this is clopyralid (Lontrel®), where herbicide residues can be released from wheat or canola stubble as the stubble decomposes, with implications for sensitive pulse crops grown in the following year. This can be an issue in full stubble retained systems that use inter row sowing of pulses between clopyralid treated cereal stubble rows.
  • For some herbicides, years of continual use may lead to elevated levels of specific microbes that can break down that herbicide. Where enhanced microbial degradation occurs, soil persistence of the herbicide will be reduced. This may allow shorter plantbacks to sensitive crops, however length of residual weed control will also be reduced. This has been mainly reported with atrazine and propyzamide, however other herbicides may also be affected.

Mr Congreve says following labelled plant back periods normally ensures adequate safety.

“Predicting herbicide persistence can be complex,” he says.

“When on the edge of the labelled plant back periods, understanding the soil type, the environmental conditions since application and the herbicide properties can give a higher level of confidence as to the likelihood of damaging residue levels remaining.”

The GRDC’s recently appointed crop protection officer for the southern region Aaron Long says understanding the real impacts that persistent herbicides are having on our cropping system is an important area that GRDC are continuing to invest in.

“On top of the recent work already carried out, the GRDC has invested in projects investigating what is real and perceived to be an issue in the southern region,” he says.

“These projects have stemmed from an increased grower demand, as pulse and oilseed crops not previously part of the crop rotation are now expanding into new regions.”

GRDC research code: ICN00016

More information

Mark Congreve, mark@icanrural.com.au 0427 209 234; Aaron Long, aaron.long@grdc.com.au, 0438 647 211

Useful resources

Soil Behaviour of Pre-Emergent Herbicides In Australian Farming Systems, available as ½ day workshop or reference manual

GRDC Project code: ICN00016