Residual herbicides break down with moisture and microbial activity. A consequence of a drier than average growing season is that herbicides applied may still be active in the following season because conditions have not facilitated the break down.
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.
Testing for residual herbicides can indicate whether you should plant a crop that is more tolerant to the residual herbicide.
Carryover of residual herbicides from season to season
A consequence of a growing season being drier than average, particularly in the eastern and northern Western Australia (WA) grainbelt, 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), and triazine herbicides are likely to cause the most concern. Residues from the previous season may affect crop emergence or even kill sensitive crops or crop cultivars in the next season.
Following dry years, damage symptoms have been reported on canola crops where diflufenican products were applied to prior lupin crops, or diflufenican/MCPA or diflufenican/bromoxynil were applied in wheat crops.
Fomesafen (for example, Reflex®) at the product rates 1 to 1.5 L/ha, used on sandy-to-sandy loam soils that are low in organic carbon (OC) (0.5 to 1.5%), and/or non-wetting soils, could pose residue issues to succeeding cereals crops. If planning to grow cereals after lupins on such soil types, use lower rate of fomesafen at 750 mL/ha in lupins to avoid residue issues. Do not use fomesafen on soils that have less than 0.5% organic carbon.
Clopyralid used on canola and cereals could pose a risk of carryover both in soil and plant residues/stubbles for subsequent susceptible legume crops.
The soil pH will impact 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 to 15% in heavy soils. Simazine has a half-life of 3 to 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 the breakdown of residues if the soil surface remains wet long enough for 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.
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 the pots remain well drained and not too wet, as waterlogging will interfere with your ability to assess whether there are herbicide residues.
Triazines
Fill three 5 L pots with surface soil from a suspect paddock, sow 10 to 15 wheat seeds in each pot, water the pots and keep in a cool, shady but well-lit spot for about 6 to 8 weeks. Fill another set of three 5 L pots with surface soil from the fenceline, or another area like the paddock, where suspect herbicides were not applied. These are the control pots.
At the 5 to 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 likely 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 depths of 0 to 10 cm and 10 to 20 cm, following the same procedures as described above, in addition to a set of control pots filled with untreated soil from each depth.
Use the legume crops of your choice or a more sensitive species.
Sensitivity ranges from field peas as the least sensitive to lupins to chickpeas to lentils being the 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.
If you suspect 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 (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 |
Safe option Following canola, lupin is the most tolerant crop, but lupins after canola are often poor. Volunteers are a problem. Delay sowing until volunteer canola germinates, then control with glyphosate. Remaining volunteers will compete until metosulam (for example, Eclipse®) can be used from 8-leaf to big bud stage of lupins. |
|
1b |
|
|
|
TT Canola |
Safe option Following lupins, if the soil is suitable and disease is not an issue, this is the best option. |
|
1c |
|
|
|
Peas / Chickpeas |
As for lupins. Delay sowing. Delayed sowing is a better prospect in peas than in lupins. Use flumetsulam (for example, 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. Sow after a rain event. A drying topsoil will help, as the plants tend to produce deep roots, minimising residue absorption. Sow the shortest season variety possible. Not an option if the break is delayed. |
|
1e |
|
|
|
Wheat |
Riskiest option Machete variety is very sensitive and should not be grown. Newer varieties have not been tested. Sowing 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 nitrogen. Prevent grass seed set and the paddock will be set up for wheat the following season. |
|
2a |
Clearfield ® canola Imidazolinone tolerant |
Imazapyr + Imazapic (for example, OnDuty® and Sentry®) |
Cereals |
Field peas |
Safe option Canola volunteers are a problem. Chickpeas are not an option, as flumetsulam (e.g., Broadstrike®) will not control volunteers. Faba beans may be an option. |
|
2b |
|
|
Non-Imi tolerant cereals |
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/pyrasulfotole- (for |
|
2c |
Clearfield® canola (Imidazolinone tolerant) |
Imazamox + Imazapyr |
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, IT barley, IT oats |
As for 2b, above. |
|
3b |
|
|
|
IT canola |
If the soil is suitable and disease not an issue, this is a good option. Use clopyralid (for example, 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-Imi tolerant canola |
IT canola |
If canola was the planned rotation crop, IT (Imi-tolerant/Clearfield®) varieties will be safe. The ability to tolerate soil residues of sulfonyl ureas does not mean 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 |
Legumes |
Cereals |
Use registered herbicides with the appropriate crop species for weed control. |
More information on safe plant back periods is available on all herbicide labels. The plant back period refers to the period, usually in months, that is deemed safe to plant sensitive crops following the application of a residual herbicide.
Dry sowing crops has become a popular method of crop establishment in WA. The technique relies on having paddocks that are as weed free as possible and the longevity of soil-applied herbicides. To optimise the length of the growing season and minimise the possible decay of applied herbicides, it is intended that opening rains will commence soon after the paddocks have been sprayed and sown.
Western Australian No Tillage Farmers Association (WANTFA) researched how the efficacy of residual herbicides changes with the length of the dry period following application and exposure to moisture between application and crop emergence.
Over a 2-year trial period, pyroxasulfone (for example, Sakura® ), prosulfocarb + s-metolachlor (for example, Boxer gold®), and trifluralin (for example, TriflurX® ) were sprayed on various soil types, dates, and moisture regimes.
Results indicated that:
- pyroxasulfone, prosulfocarb + s-metolachlor, and trifluralin decay slowly under dry soil and can be applied early under these conditions.
- Under wet conditions, decay of the herbicides was rapid and highest under warm conditions (that is, earlier application).
- pyroxasulfone had the slowest decay rates, followed by prosulfocarb + s-metolachlor and trifluralin.
- Soil type had little effect on the decay rates.
- Growers should not rely on pre-emergent herbicides to provide adequate control under wet conditions when sowing in April.
Under dry soil conditions, the herbicides used in the study all persisted until the fifth or sixth week. A small decline in efficacy was observed but it was determined this was due to a slight increase in soil moisture from humidity and seepage. Under these conditions, all pre-emergent herbicides in the study were 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 lifespan is not long enough to last until the major flush of germination occurs (usually mid to late May). To minimise this risk:
- use a longer residual herbicide, such as pyroxasulfone (followed by prosulfocarb + s-metolachlor, then trifluralin)
- delay dry seeding until late April or early May, followed by a one-week dry period
- ensure weed seed bank is low
- use forecast services and the department’s Weed Seed Wizard online tool to predict when germination overlaps with active pre-emergent herbicides
- if early rain occurs, wait until the surface and sub surface soil has dried out before dry seeding.
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-
Harmohinder DhammuGrains Senior Research Scientist
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Catherine BorgerGrains Principal Research Scientist