Residual herbicides - carryover and behaviour in dry conditions

Page last updated: Tuesday, 9 May 2023 - 11:59am

The term 'residual' applies to a number of herbicides that have a long lasting activity in the soil. These herbicides are often applied directly to the soil prior to planting crops, pre-emergent. Residual herbicides rely on moisture and microbial activity to break down. So one of the consequences of a drier than average growing season is that the herbicides applied may still be active in the following season. In addition, residual herbicides can be applied in advance of dry sown crops and still be expected to have sufficient weed control activity when the season breaks.

Carryover of residual herbicides from season to season

One of the consequences of a growing season being drier than average particularly in the eastern and northern WA wheatbelt, is that herbicides applied in one season could still have significant residues in the soil, and affect crops planted the following year.

Sulfonylurea (SU), imidazolinone (IMI) or triazine herbicides are likely to cause the most concern. And residues, from the previous season may affect crop emergence or even kill sensitive crops or crop cultivars in the next season.

Diflufenican damage symptoms have also been reported following recent dry years on canola crops where products such as diflufenican (for example, Brodal® Options) were applied to lupin crops, or diflufenican/MCPA (for example, Tigrex®) or diflufenican/bromoxynil (for example, Jaguar®) were applied in wheat crops.

Fomesafen (e.g., Reflex®) at rates 1 to 1.5L/ha used on sandy to sandy loam soils, low in organic carbon (OC) (less than 1.25%) and/or non-wetting soils could pose residue issues to succeeding cereals crops. If planning to grow cereals after lupins, don’t use more than 750mL/ha rate of fomesafen in lupins on such soil types to avoid residue issues.  

Clopyralid (e.g. Lontrel™ Advanced) used on canola and cereals could pose a risk of carryover both in soil and plant residues/stubbles for subsequent susceptible legume crops in rotation.

The soil pH will have an impact on which herbicides are more likely to persist. All other things being equal, imidazolinones will be more persistent on acid soils and sulphonyl ureas on alkaline soils. Triazines are very slightly more persistent on alkaline soils.

Dry conditions reduce herbicide degradation

The main factors contributing to residue carryover are poor uptake of herbicides by crop plants and limited microbial or chemical degradation of herbicides in dry soil conditions.

For simazine to be absorbed by plants, the required water content is 5% in sandy soils and 10-15% in heavy soils. Simazine has a half-life of 3-6 months during winter. (Half-life is the length of time it takes for the effectiveness of the herbicide to reduce by half).

In an average year, at least 95% of simazine applied in the crop will be degraded by the end of the season. Any remaining simazine will have no effect on the following crops.

The presence of root diseases in a cereal crop may exacerbate the effects of herbicide residues.

Summer rain can aid in the breakdown of residues but only if the soil surface remains wet long enough for the breakdown processes to occur.

In some instances, where there are large downpours of rain that infiltrate through the soil profile the water may move soluble residues (such as the SU’s) down the profile. This may be an issue of concern on duplex soils, where crop damage symptoms do not appear until the roots reach the zone where the residues have moved to.

Can we test for herbicide residues?

On-farm pot testing

If you are not sure how much of the herbicide is left in the paddock, it is recommended to conduct an on-farm pot test for residues. When pot testing make sure that the pots remain well drained and not too wet as waterlogging will interfere with your ability to assess whether or not there are herbicide residues.

Triazines

Fill three 5L pots with surface soil from suspect paddock, sow 10-15 wheat seeds in each pot, water the pots and keep in a cool, shady but well-lit spot for about 6-8 weeks. Fill another set of three 5L control pots with surface soil from fence line or an area similar to paddock where suspect herbicides were not applied following the preceding procedures.

At the 5-6 leaf stage, triazine symptoms will show up as tip burning or watery blotches in the middle of the leaves. In such cases, there is a danger of wheat yield penalty.

At low levels of triazine residues, there may be only a general paling of the plants and/or a reduction in height and vigour. In such cases, there will probably be no yield penalty because the residue effects will be more severe in the pots than in the fields.

Sulfonylureas:

Prepare as many sets of pots as required using soils treated with sulfonylureas from 0-10 cm and 10-20 cm depths following the same procedures as described above along with a set of control pots filled with untreated soil for each depth.

Use the legume crops of your choice or a more sensitive species.

Sensitivity ranges from field peas (least sensitive)<lupins<chickpeas<lentils (most sensitive). Corn and canola are also good choices to test for SU residues.

Allow the seedlings to grow up to 2-leaf stage, then carefully pull the seedlings and wash the soil from the roots. If sulfonylureas residues are present, the roots will be thickened and shortened with few or no fine root hairs. Reddening of leaf margins is a characteristic

The MyCrop site has further information on how to diagnose the symptoms of herbicide damage in cereal and broadleaf crops.

Planting options for herbicide treated paddocks

If you suspect that there will be an issue with herbicide carryover from the previous year, it may be prudent to plant a crop that is more tolerant to the residual herbicide. Planting options for the common carryover problems are listed in Table 1.  

Table 1 Planting options for soils with suspected herbicide residue problems
 

Previous

crop

Herbicide

residue

Crops

at risk

Crop

options

Comments
1a TT canola or Lupins

Simazine/atrazine

Cereals Lupins Following canola, lupins are the most tolerant crop, but lupins after canola are often poor.  Volunteers are a big problem.  Delay sowing until volunteer canola germinates then control with glyphosate.  Remaining volunteers will compete until Eclipse® can be used.
1b       TT Canola Following lupins, if the soil is suitable and disease not an issue, this is the best option by far.  
1c       Peas/Chickpeas As for lupins.  Delay sowing as above.  Delayed sowing is a better prospect in peas than in lupins.  Use Broadstrike® to control canola volunteers.
1d       Barley

Risky option.  Barley is the most triazine tolerant of the cereals.

Sow as late as possible to allow some breakdown. 

Atrazine will be more damaging than simazine. 4 L/ha in the previous season is unlikely to be tolerated unless sowing is delayed by 6 weeks.

Sow with knife points, putting the seed below the herbicide as much as possible.  Sow after a rain event.  A drying topsoil will help as the plants will tend to produce deep roots and so minimise residue absorption.

Sow the shortest season variety possible.  Not an option if the break is delayed.
1e       Wheat

The riskiest option.

Machete is very sensitive, and should NOT be grown.

Newer varieties have not been tested.

Sowing rules as for barley, 1c.
1f       Pasture Grasses and clover may struggle, but there will be plenty of canola.  Some lupin seed could also be top dressed to fix N2.  Take the opportunity to prevent grass seed set and the paddock will be set up for wheat the following season.
2a

Clearfield ® canola

Imidazolinone tolerant

Imazapyr + Imazapic (OnDuty®)

Cereals Field Peas

Safe option.  Canola volunteers are a problem. 

Chickpeas are not an option, as Broadstrike® will not control volunteers.

Faba beans may be an option.
2b       IT wheat

Safe option.  For sound resistance management do not apply a Group B herbicide in this season.

Use phenoxy and/or diflufenican/pyrasulfotole (e.g. Tigrex®, Jaguar®, Velocity®) based products and mixes for control of canola volunteers.
2c Clearfield® canola (Imidazolinone tolerant) Imazapyr + Imazapic (e.g. Sentry®) Non-Imi tolerant crops IT wheat, IT barley and IT oats

Safe option.  For sound resistance management do not apply a Group B herbicide in this season.

Use phenoxy and/or diflufenican (e.g. Tigrex®, Jaguar®) based products and mixes for control of canola volunteers.
2d Clearfield® canola (Imidazolinone tolerant) Imazamox + Imazapyr (e.g. Intervix®, Intercept®) Non-Imi tolerant crops IT wheat and IT barley

For sound resistance management do not apply a Group B herbicdie in this season.

Use phenoxy and/or diflufenican/pyrasulfotole (e.g. Tigrex®, Jaguar®, Velocity®) based products and mixes for control of canola volunteers.

No registered herbicide options for control of volunteer canola in IT faba beans and IT lentils.

3a Pulses Imazethapyr (e.g. Spinnaker®) Cereals IT wheat As for 2b above.
3b       IT canola

If the soil is suitable and disease not an issue, this is a good option.

Use Lontrel® if volunteers are a problem.
4a Cereal

Sulfonyl ureas –

Chlorsulfuron, triasulfuron, metsulfuron

Legumes Cereal The easy and safe option with wheat being the safest choice.
4b     Non IT canola IT canola

If canola was the planned rotation crop, IT (Clearfield®) varieties will be safe.  The ability to tolerate soil residues of sulfonyl ureas does not imply that these herbicides can be used in-crop.

TT and conventional varieties are not as sensitive to low SU residues as legumes, and may be safe. Do a pot test to check.
5 Cereals and canola Clopyralid (e.g. Lontrel®) Legumes Cereals Use registered herbicides with the appropriate crop species for weed control.

 

 

Further information on safe plant back periods is available on all herbicide labels. Note: the plant back period refers to the period of time, usually in months, that is deemed safe to plant sensitive crops following the application of a residual herbicide.

Decay of pre-emergent herbicides in dry soils

Dry sowing crops has become a popular method of crop establishment in WA. The technique not only relies on having paddocks that are as weed free as possible but also on the longevity of soil applied herbicides. It is the intention that the opening rains will commence soon after the paddocks have been sprayed and sown to optimise the length of the growing season and to minimise the possible decay of applied herbicides.

Dr David Minkey (Western Australian No Tillage Farmers Association, WANTFA) has been studying how the efficacy of residual herbicides change with the length of the dry period following application and exposure to moisture between application and crop emergence.

Sakura®, Boxer gold® and TriflurX® was sprayed onto various soil types, dates and moisture regimes over a two year period.

Results indicate that:

  • Sakura®, Boxer Gold® and TriflurX® decay slowly under dry soil and can be applied early under these conditions
  • Under wet conditions decay of these herbicides was rapid and was highest under warm conditions (earlier application).
  • Sakura® had the slowest decay rates followed by Boxer gold® and then TrilfurX®
  • Soil type had little effect on the decay rates.
  • Do not rely on pre-emergent herbicides to give you adequate control under wet conditions when sowing in April.

Under dry soil conditions the herbicides used in the study all persisted until the 5th or 6th week. A small decline in efficacy was observed but it was concluded that this was due to a slight increase in soil moisture due to humidity and seepage. Under these conditions all pre-emergents used would be deemed suitable for dry seeding purposes.

The risk with early dry seeding wheat is when a rainfall event occurs before dormancy of annual ryegrass is broken and the residual life span is not long enough to last until the major flush of germination occurs – usually mid to late May. To minimise these risks:

  1. Use a longer residual herbicide such as Sakura® (followed by Boxer Gold®, then TriflurX®)
  2. Delay dry seeding until late April or early May with a one week dry period to follow
  3. Ensure weed seed bank is low
  4. Use forecast services and the weed seed wizard to predict when germination over laps with active pre-emergent herbicides
  5. If early rain occurs wait until surface and sub surface has dried out before dry seeding

This research was presented at the 2017 Research Updates, Perth. A full paper on this work is availabe on the GIWA website Decay of pre-emergent herbicides in dry soils.

Dr Minkey has also discussed the behaviour of residual herbicides in a Podcast available on the WeedSmart website.

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