Crop weeds: reduce weed seed numbers in the soil

Page last updated: Monday, 11 March 2019 - 11:27am

One method of weed control is to remove weed seeds in the fallow, stubble and pre-sowing phase. This can be achieved by encouraging the germination of weed seeds and then subsequently killing seedlings or destroying seeds and reducing seed viability. This section covers the different methods used to deplete weeds.

Different weed control methods

Burning crop residues

Burning can reduce the surface seed banks of many weeds. All crop residues (canola, wheat, lupin and others) can produce a sufficiently heated burn to kill weed seeds. A narrow windrow will burn at a higher temperature and improve weed seed kill.

Encouraging insect predation of seed

Weed seeds provide a major component of many insect diets (predominantly ants). There are methods to increase populations of insects over the summer/autumn fallow and therefore increase insect consumption of seeds.

Inversion ploughing

Fully inverting the soil will ensure that weed seeds that were on or just below the soil surface are placed at a depth from which they cannot germinate. This can be practiced every 8-10 years, with conservation tillage used in the intervening years. In Western Australia, annual ryegrass seeds failed to establish and eventually died when soil was fully inverted to a depth of greater than 20 centimetres (cm) using a specialist mouldboard plough fitted with skimmers. This single soil inversion event reduced annual ryegrass numbers by over 95% at Katanning and Beverley for a period of two years.

Autumn tickle

This does not destroy seeds but rather buries them to a depth of 1-2cm, enhancing seed germination by increasing contact with soil moisture. This encourages weed seeds to germinate earlier. A delay between the tickle and seeding is necessary to give an opportunity for the weeds to germinate and then be killed using a knockdown herbicide. The delay to seeding will result in a yield penalty for some crops.

Delaying sowing

This allows greater germination of weeds in particularly weedy paddocks, which can then be killed using a knockdown herbicide or cultivation prior to crop sowing. The longer the delay in sowing, the more weeds germinate and the higher the kill. However, a yield penalty is experienced when sowing is delayed.

Burning residues

Burning crop residues can reduce the surface seedbanks of many weeds. All crop residues (canola, wheat, lupin and others) can produce a sufficiently heated burn to kill weed seeds. A narrow windrow will burn at a higher temperature and improve weed seed kill. More information can be found at GRDC IWM hub: Managing the weed seedbank.

Burning stubble residues to destroy weed seeds

 Burning stubble in a paddock

Burning in summer is illegal in Australia, but autumn burns effectively destroy weed seeds. All crop residues can produce a sufficiently heated burn provided that adequate tonnage of residue is present. Hence, higher temperature burns will be obtained by concentrating residue into a narrow windrow. Burning a narrow windrow also reduces the percentage of the paddock that is burnt, thereby reducing the area prone to wind erosion. Seeds close to the soil surface are more likely to be killed than seeds that have been buried.

Benefits

  • Burning destroys weed seeds. In WA, burning windrows of wheat, canola or lupin trash has been found to destroy 75% of wild radish seed and 98% of annual ryegrass seeds.
  • Late autumn burning of crop residues may also kill emerged weed seedlings (including self-sown volunteers such as wheat).
  • Burning can stimulate weed germination of some weed species. Fire can be very effective at stimulating germination of hard or dormant seeds, which allows for subsequent control with another tactic.
  • Burning removes residues and thereby allows more effective incorporation of pre-emergent herbicides.
  • Burning residues makes it easier to sow the subsequent crop, improves disease and pest management and eliminates short-term nitrogen tie-up.

Issues to consider

  • Best success is achieved by a high temperature burn, accounting for seasonal risks.
  • The area to burn should be prepared to ensure best placement of weed seeds. Seeds should be on or just above the soil surface. Grazing should be avoided or reduced to ensure that sufficient residue remains for the burn and that weed seeds are not buried by trampling.
  • Time burning to suit residue conditions and legislative requirements. Burning early in summer gives best weed seed control, but is illegal and also increases the chance of erosion while reducing the efficiency of water conservation.
  • The impact of burning depends on residue placement and quantity. Placing the residue in windrows will result in a slower, hotter burn.
  • Not all weed seed banks can be decreased by effective burning. Some weeds such as wireweed are not affected by burning and others benefit from burning.

Potential disadvantages of burning

  • Environmental concerns about carbon dioxide emissions and pollution from burning crop residues, as well as respiratory health issues like asthma.
  • The risk of soil erosion following burning, especially a total residue burn.
  • Adverse effects on soil fertility, organic matter and soil structure, especially if burning is used frequently.
  • Reduced soil water infiltration and increased evaporation and run-off due to crop residue removal.
  • Reduced numbers of macro and micro-organisms, especially earthworms, and therefore reduced biopores.
  • A shortened sowing window after rain.

Encouraging insect predation of seed

Weed seeds provide a major component of many insect diets this means weed seedbanks can be decreased naturally by encouraging insect (predominantly ant) predation.

Ants removing annual ryegrass seed

Research has found that insect predation of annual ryegrass can significantly reduce seedbank numbers, with removal rates ranging from 0-100% depending on the proximity of the seedbank to ant colonies. Predation by insects was found to be higher for annual ryegrass seed than wild radish seed in a study in the Western Australian wheatbelt. After three months, 81% of the original annual ryegrass seeds had been removed compared to 46% of wild radish.

Maximise insect predation of seed

  • Predation levels tend to be higher in situations close to 'refuge' areas (such as remnant vegetation or fence lines) and decreases with increasing distance from the refuge.
  • Predation can be maximised by avoiding the overuse of broad spectrum insecticides. Prolonged broad spectrum insecticide use will decrease the number of 'friendly' insects in paddocks.
  • Retaining stubble is a double-edged sword. Stubble can provide a refuge for predatory insects, but it also discourages heat-loving ant species which prefer open spaces. Stubble type is also important. Compared to cereal, canola stubble can reduce the numbers of some ant species and consequently the level of wild radish seed pod removal, particularly in early summer.
  • Minimum tillage improves predation of weed seeds. Tillage, especially in heavy clay soil types reduces ant populations. It is thought that a cropping system that employs a minimum amount of soil disturbance is optimal. Additionally, soil disturbance during summer reduces seed predation.

More information can be found at GRDC IWM hub: Managing the weed seedbank.

Inversion ploughing

In suitable soil types, weed seed burial is an effective method of killing weed seeds particularly if herbicide resistant weeds are problematic.

Inversion ploughing to bury weed seed

 Mouldboard plough

Inversion ploughing is used to fully invert the soil to ensure weed seeds that were on or just below the soil surface are placed at a depth where they cannot germinate. This can be practiced every 8-10 years, with conservation tillage used in the intervening years. In WA, annual ryegrass seeds failed to establish and eventually died when soil was fully inverted to a depth greater than 20cm using a specialist mouldboard plough fitted with skimmers. This single soil inversion event reduced annual ryegrass numbers by more than 95% at Katanning and Beverley, Western Australia, for a period of two years.

Benefits

  • In suitable soil types, weed seed burial is an effective method of killing weed seeds.
  • Disease and insect control occur due to the burial of stubble.
  • Non-wetting soils are ameliorated.
  • Nitrogen mineralisation occurs.
  • Nutrient stratification in the soil is removed (that is, mixing nutrients usually concentrated in the surface).
  • There are opportunities for soil ameliorant (for example, lime) application at depth.
  • Inversion ploughing leads to green manuring and incorporation of organic matter into the soil.

Issues to consider

  • Inversion ploughing of windrows will reduce weed numbers with minimal paddock disturbance, but weed seeds will remain in the inter-windrow area. Inversion of windrows is a more effective weed control method than burning residues.
  • Appropriate soil type is needed for effective soil inversion. Soil inversion is limited to soil types where there is sufficient topsoil to allow full inversion. Shallow duplex soils where the clay is less than 15cm deep for example, are unsuitable. It is also difficult to achieve the complete inversion needed for effective weed control in soils with a large number of rocks and/or stumps.
  • In situations where soils exhibit problems at depth (for example, clay, sodicity, salinity, boron, magnesium, manganese), soil inversion should be avoided as it may bring these problems to the surface. Growers should conduct soil tests where problems are suspected.
  • Soil inversion is most effective in reducing weed seedbank numbers with limited dormancy. For those species with dormant seeds, a reinversion in later years may bring viable seeds back to the surface.
  • Careful timing of inversion ploughing will reduce the risk of wind and water erosion.
  • Inversion ploughing is best performed just prior to sowing once the soil profile has become wet.

More information can be found at GRDC IWM hub: Managing the weed seedbank.

Autumn tickle

Autumn tickling (also referred to as an autumn scratch or shallow cultivation) stimulates weed seed germination by placing seed in a better physical position in the soil. At a shallow depth of 1-3cm the seed has better contact with moist soil and is protected from drying out.

Autumn tickle or shallow cultivation

Ploughed paddock

Autumn tickling stimulates weed seed germination by burying weed seeds to a depth of 1-3cm, which ensures the seed has better contact with moist soil and is protected from drying out. An autumn tickle also encourages weed seeds to germinate earlier by changing their position from light to dark conditions or vice versa.

Because weeds that germinate after an autumn tickle can be controlled, such a process will ultimately deplete weed seed reserves. A delay between the tickle and seeding is necessary to give an opportunity for the weeds to germinate and then be killed using a knockdown herbicide. This may cause a yield penalty for some crops.

Benefits and issues to consider include:

  • An autumn tickle can be conducted using a range of equipment, including tyned implements, skim ploughing, heavy harrows, pinwheel (stubble) rakes, dump rakes and disc chains.
  • A well-timed autumn tickle will promote earlier and more uniform germination of some weed species for subsequent control.
  • Autumn tickle can be performed any time in autumn or winter, but post-cultivation erosion risk will be minimised when cultivation occurs closer to sowing.
  • Soil type is critical for a successful autumn tickle. Light textured (sand) soils, non-wetting soils and those where moisture has trouble penetrating the profile are poor candidates for autumn tickling. Where soils wet unevenly, weed seeds may be buried in pockets of dry soil. These pockets may become wet during the season, with seeds subsequently germinating to cause in-crop problems.
  • Use autumn tickling in non-crop situations to stimulate germination of weeds which can then be managed with grazing or a non-selective herbicide.
  • Autumn tickling is a tactic best suited to weeds that are easily released from dormancy such as annual ryegrass.
  • Soil disturbance prior to sowing can reduce soil moisture, placing the sowing operation at risk in a dry season.
  • Soil disturbance prior to sowing can incorporate stubble and, as a result, significant amounts of soil nitrogen will be tied up by microbes that proliferate to degrade the stubble.
  • In the early stages of no-till adoption short-term nitrogen deficiencies are likely if stubble levels are high.

More information can be found at GRDC IWM hub: Managing the weed seedbank.

Delayed sowing

Delaying the sowing of weedy paddocks allows greater weed germination enabling weeds to be killed using a non-selective herbicide or cultivation prior to sowing.

Delayed sowing (seeding) is where sowing occurs beyond the optimum time for yield benefit in order to maximise weed emergence. The longer you delay sowing, the more weeds that will germinate resulting in more being killed. However a yield penalty is experienced when sowing is delayed.

Delayed sowing can reduce early crop/weed competition via management of early germinating weeds prior to sowing. For this tactic to be successful, sowing must be delayed until the first flushes of weeds have germinated and have been controlled. Up to 80% of annual ryegrass emergence occurs within four weeks of opening rain. Providing subsequent control of these seedlings will deplete the weed seedbank.

This tactic is most commonly employed for paddocks that are known to have problematic weed burdens. Tactics for controlling weeds include:

  • Target problem paddocks first. Paddocks with low weed burdens are given priority in the sowing operation, leaving weedy paddocks until later. This allows sufficient delay for the tactic to be beneficial on the problem paddock without interrupting the whole-farm sowing operation.
  • Choose a crop or cultivar with a later optimum sowing time to reduce the risk of yield penalty.
  • Seasonal conditions will influence delayed sowing opportunities. Delays to the start of the season will restrict the opportunity to wait for the first flushes of weed germination and subsequent pre-sowing control. If the season has a late break, consider omitting very weedy paddocks from the cropping program. This will allow for other weed management tactics to be employed in readiness for the following season.
  • Delayed sowing is very effective when used in conjunction with additional weed management tactics. A good weed management benefit is obtained when the autumn tickle is used in conjunction with delayed sowing (see Figure 1 below for the impact of delayed sowing, three weeks after normal sowing time).

Control of annual ryegrass

Contact information