Section 5: Managing the weed seedbank
The weed seedbank is the mature viable seeds that exist in the soil. The seedbank consists of seed of different ages. Some seeds will be able to germinate when conditions are favourable (suitable temperature, adequate water and enough oxygen), while other seeds will be dormant – still viable, but unable to germinate at that time.
In the absence of new weed seed being added, seed dormancy is the main factor that will determine how long the weed seedbank will persist for.
Different weed species have different levels of dormancy. Often a key factor of weed seeds with high levels of dormancy is a thick impervious weed seed coat that slows the uptake of water, e.g. wild radish.
Environmental factors including light, soil conditions such as temperature and moisture, the soil’s gaseous environment and nutrient status, exposure to UV light and the level of change in environmental conditions (i.e. if the seed is on the soil surface) all affect the ability of the seed’s dormancy and ability to germinate.
Weed seedbanks are diminished in two key ways:
1) Weed seed germinates and seedlings are killed.
- Tillage can affect seed germination by redistributing the seed to a different depth in the soil, thus changing moisture, temperature or the amount of light. Autumn tickle (also referred to as an ‘autumn scratch’ or shallow cultivation) stimulates weed seed germination of some weed species by placing seed in a better physical position in the soil. At a shallow depth of 1–3 cm the seed has better contact with moist soil and is better protected from drying than when left on the soil surface. (Note: not applicable to surface germinating weeds). The physical disturbance of tillage can also lead to abrasion of the seed coat – making an otherwise dormant weed seed more likely to take up water and germinate. A well-timed autumn tickle will promote earlier and more uniform germination of some weed species for subsequent control. Tickling often needs to be used in conjunction with delayed sowing. Delayed sowing is the technique of planting the crop beyond the optimum time for yield in order to maximise weed emergence. Weeds that emerge in response to the break in season can then be killed using a knockdown herbicide or cultivation prior to crop sowing.
2) Seed loss other than germination. Most seeds fail to emerge as seedlings.
- If weed seeds are buried at depths too great to permit emergence and are not ‘unburied’ while still viable, many simply lose viability over time and die of old age. After long-term reduced or no tillage, most weed seed is located at or close to the soil surface. Inversion ploughing has been used to fully invert the soil to 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 only be practiced every 10-15 years where no/reduced tillage is used in the intervening years. There is concern that very dormant weed species such as wild radish, may require periods of ~20+ years before the practice could be used again.
- Some may also be eaten or attacked by pathogens. A study in the Western Australian wheat belt found that up to 81% of the original annual ryegrass seed and 46% of wild radish seed was removed by ants where weed seed was left on the soil surface in a no-till situation. (Seed predation)
- Natural mortality rates of weed seed are far higher in no-till systems where weed seed is left on the soil surface than in systems where weed seed is mixed in the top few centimetres. Burying some types of weed seeds can increase seedbank dormancy and slow the rate at which the seedbank is depleted. Wild oats, wild radish, bladder ketmia and cow/peach vine (Ipomoea spp.) are all good examples.
5.2 Burning residues
Chaff dumps can be burnt in autumn killing a high proportion of seeds present (Photo: A. Storrie)
Fire can be used to kill weed seeds on the soil surface if there is sufficient fuel load and the fire is hot enough. Burning over summer poses an unduly high fire hazard and is illegal in most regions. An autumn burn often poses a lower fire hazard and leaves crop residue in place to protect soil from wind and water erosion for a longer period. Maintaining stubble for longer also benefits soil water capture and retention, provided summer weed growth is controlled.
To obtain high levels of control of weeds such as annual ryegrass and wild radish, a hot fire is needed. This is obtained by windrow burning, where crop residues from either cereal, canola or pulse crops is concentrated with weed seed in a narrow windrow and then burnt.
Michael Walsh, (AHRI) explains the subtleties of burning narrow windrows in canola, pulse and cereal crops. How weather conditions, fire temperature and type of crop stubble all contribute to destruction of the weed seed.
Michael Walsh, (AHRI) shows the results of an effective narrow windrow burning operation
5.3 Autumn tickle
Autumn tickling (also referred to as an ‘autumn scratch’ or shallow cultivation) stimulates weed seed germination by improving seed contact with moist soil. At a shallow depth of 1 to 3 cm the seed has better contact with moist soil and is protected from drying. Because weeds that germinate after an autumn tickle can be controlled, such a process can help deplete weed seed reserves.
An autumn tickle can be conducted using a range of equipment including tyned implements, skim ploughs, heavy harrows, pinwheel (stubble) rakes, dump rakes and disc chains.
Tickling can increase the germination of some weed species but has little effect on others.
Tickling needs to be used in conjunction with delayed sowing to allow time for weeds to emerge and be controlled prior to seeding. Insufficient time between soil disturbance and sowing, often results in a much larger weed germination emerging with the crop. This has significant downsides of placing higher selection pressure for resistance on in crop herbicides as well as higher levels of weed competition in the critical early growth phase of the crop.
Additionally, the requirement to delay sowing to enable a weed emergence and subsequent control of these weeds prior to sowing, typically results in a significant loss of yield potential. With winter crops, a two-week delay to sowing, pushes crop emergence closer to the colder winter months, with ensuing significant delays to crop emergence and reduced crop competition.
A preferred approach is to sow a vigorous and competitive winter crop early, with the addition of pre-emergent herbicides. The pre-emergent herbicides significantly reduce weed emergence and protect crop yield, often stressing individual weeds that survive. Warmer early sowing conditions and agronomy set up to enhance crop competition will substantively compete with and suppress seed set of weeds surviving pre-emergent herbicides. However, it is likely that there will still frequently be a requirement for an additional herbicide or non-herbicide tactic to further reduce weed seed set .
5.4 Delayed sowing
Delayed sowing allows use of knockdown herbicides or cultivation to control small weeds prior to sowing and reducing the pressure on selective in-crop herbicides. (Photo: D. Holding)
Delayed sowing (seeding) is the technique of planting the crop beyond the optimum time for yield in order to maximise weed emergence and control prior to sowing. Weeds that emerge in response to the break in season can then be killed using a knockdown herbicide or cultivation prior to crop sowing.
This tactic is most commonly employed for paddocks that are known to have high weed burdens. Paddocks with low weed burdens are given priority in the sowing schedule, 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.
Later sowing of winter crops, pushes crop emergence into colder winter months which decreases crop competitive ability, in addition to substantially reducing yield potential. As a result, other than sowing weedy paddocks last, delaying sowing is not usually a preferred tactic.
Choosing a crop or cultivar with a later optimum sowing time can reduce yield impact of a later sowing date.
5.5 Further information
GRDC fact sheets and other publications
GRDC video links
Mouldboard Ploughs have returned to the landscape but in the modern farming system do they have legitimate place?
Michael Walsh, (AHRI) explains the subtlities of burning narrow windrows in canola, pulse and cereal crops. How weather conditions, fire temperature and type of crop stubble all contribute to destruction of the weed seed.
Michael Walsh, (AHRI) shows the results of an effective narrow windrow burning operation
Specific weeds - information on seedbank ecology is also found under links to specific weeds
RIM (Ryegrass Integrated Management program)
RIM is a computer based decision support tool designed specifically for evaluating various ryegrass management options. It provides information about the economics and seedbank impacts of different weed management tactics in a farming system.
Weed Seed Bank Wizard
A modelling tool to help predict the impact of multiple weed management tactics on weed seed bank decline. (Department of Agriculture and Food, Western Australia)
Weed trimming - a successful non-chemical seed bank reduction technique - 17th Australasian Weeds Conference (2010)
Manipulating the seed bank to manage herbicide-resistant weeds - 17th Australasian Weeds Conference (2010)
Weed seed removal by ants in the crop growing areas of Western Australia - 16th Australasian Weeds Conference (2008)
Weed seed bank response to 12 years of different fertilisation systems - 17th Australasian Weeds Conference (2010)
Depleting weed seed banks within non-crop phases for the benefit of subsequent crops - 16th Australasian Weeds Conference (2008)
Burning snails and weed seeds - YouTube clip of local farmers and experts speaking about the increase in burning paddocks prior to the commencement of the 2012 season. (SANTFA)
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