COMMON SOWTHISTLE AND FLAXLEAF FLEABANE. UNDERSTANDING THE WEEDS LIFECYCLES FOR MANAGEMENT STRATEGIES THAT WORK

GRDC code

DAQ00137 - Improved fleabane control in the NR

Why is a weed’s lifecycle important?

By understanding the lifecycle of a weed we are better able to target the weed for effective control. Annual weeds go through a cycle of life as demonstrated in Figure 1. Both genetic and external driving factors will determine a weeds lifecycle. For example, a weed may genetically be designed to emerge at a temperature of 20°C. However, if there is insufficient water available no germination will occur.

If we have a knowledge of a weed’s lifecycle, we will be able to manipulate and better target weed management practices for improved weed control.


Figure 1. The lifecycle of an annual weed.

Sowthistle lifecycle and management implications

Common sowthistle, also known as milk thistle, is widespread across the grain-growing regions of Queensland and northern New South Wales and is a common weed of reduced till systems. Sowthistle depletes stored fallow moisture and interferes with harvest, adding green matter to harvested grain. There are numerous populations of sowthistle resistant to Group B herbicides such has chlorsulfuron, and the weed is at high risk of developing resistance to group M herbicides (glyphosate) although no such populations are confirmed. Careful consideration of herbicide application history needs to be considered when planning future chemical control events. Don’t place reliance on one herbicide or herbicides from the same mode of action group.

Germination of sowthistle is largely water dependent, but also favoured by light. Seeds are able to germinate between 5 and 35°C but the majority will only germinate at water availability close to field capacity. Hence, germination in the field will usually follow significant rainfall events at any time of the year. In addition, freshly produced seeds possess no innate dormancy and are therefore able to germinate straight away, if the climate is suitable. For both reasons, diligence is therefore required throughout the year for effective control of this weed. Multiple weed management efforts may be required to control subsequent flushes.

Emergence of sowthistle is predominantly from the top 1 cm of soil (Figure 2). A small number can emerge from a depth of 2 cm while none emerge from 5 cm or below. Zero tillage allows the majority of seeds to remain on or near the soil surface where a large number of seedlings will emerge following significant rainfall. Cultivation will bury a large portion of seeds and greatly reduce emergence.


Figure 2. Sowthistle emergence from seed buried at different depths

Sowthistle seed persistence is influenced by the depth at which they are buried, with seed persisting longer as depth of burial increases. A pot study on the Darling Downs showed that after 30 months of burial, less than 1% of the original seeds remained in the top 1 cm, while 3% remained at 2 cm and up to 12% remained at depths of 5 and 10 cm. Seeds that are buried can be brought back to the soil surface by subsequent cultivations, resulting in emergence if conditions are suitable.

A single sowthistle plant in fallow can produce up to 25 000 seeds, all of which possess a pappus that aids in wind dispersal. The majority of seeds fall within 2-3 metres of the parent plant, but a small percentage can travel great distances. It is vitally important that seed production be limited. Survivors of any weed management efforts should be controlled. Sowthistle in adjacent non-cropping areas should also be controlled as these areas can be a source of seed for future infestations in cropping paddocks.

Flaxleaf fleabane lifecycle and management implications

Flaxleaf fleabane is a major weed of cropping in southern Queensland and northern New South Wales. It can be a very difficult weed to control and is thought to have increased for several reasons, namely the:

* increased adoption of no-till farming,
* heavy reliance on glyphosate for fallow weed control,
* reduction in use of chlorsulfuron herbicides (such as Glean®) in wheat, and
* introduction of wide row spacing in sorghum.

Fleabane has a deep tap root that can greatly reduce stored water supplies in fallow and can compete with crops, reducing yields.

The germination of fleabane is largely light and temperature dependent. Fleabane seed requires light and a temperature of between 10-25°C (optimal 20°C) for germination. A larger emergence of fleabane in the field is often associated with crop stubble and parts of the topography that enable increased and longer periods of moisture. Like sowthistle, fleabane seeds posses no innate dormancy. Monitoring for new emergences is important, as young fleabane is much easier to control and there are many more options available.

Seedlings of fleabane will only emerge from the top 1 cm of soil, explaining partly why this weed proliferates in no-till systems. No seedlings emerge from a depth of 2 cm or below. In southern Queensland, fleabane emerges predominantly in autumn, early winter and spring. However, limited emergence can occur if mild conditions occur in winter and early summer.

The majority of fleabane seeds lose their viability within 12-18 months. However a small percentage can persist for several years and the quantity is influenced by burial depth. A pot study on the Darling Downs showed that after 3 years of burial 1%, 10% and 8% of viable seed remained at depths of 0-2, 5 and 10 cm respectively. A tillage trial on the Darling Downs (near Dalby) found that the emergence of fleabane was generally reduced under tillage, but that a light harrow increased emergence, possibly as a result of an increased number of seeds being exposed to light (Figure 3). While seed burial through cultivation may be seen as a possible option for fleabane control, buried seeds can remain viable and be brought back to the soil surface by subsequent tillage events.


Figure 3. Fleabane cumulative emergence following no soil disturbance (NT) or tillage with light harrow (LH), disk plough (DP), and chisel plough (CP).

Fleabane plants that emerge during autumn often grow very slowly above-ground through winter, but their tap root system continues to grow. While such weeds appear small in size, they can be quite old (2 or 3 months) and are therefore very difficult to control. Fleabane that emerge in spring will grow more quickly and their size is a more accurate reflection of their age. Continual monitoring of fleabane emergences will help to understand what age plant you are dealing with. Fleabane is most easily controlled when it is small and young. Once elongation begins, there are few effective herbicide options available.

Fleabane is a prolific seed producer with a single mature plant producing on average 110 000 seeds. Like sowthistle, fleabane seeds possess a pappus, which aids in its dispersal by wind. Stopping seed set on fleabane is important in getting on top of this weed problem and is also important in minimising the risk of resistance.

Both weeds compared

Sowthistle and fleabane have many similar lifecycle characteristics (Table 1). Both species are favoured in zero till systems and germinate from the surface layers of the soil. When buried their seed persist in a similar manner and both produce large quantities of wind dispersed seeds. The main differences in lifecycle are in their time of emergence, due to differences in the temperature range over which they germinate. While sowthistle can germinate all-year-round, fleabane is predominantly a weed of autumn and spring.


Table 1. A comparison between sowthistle and fleabane lifecycles

Management strategies that work

Sowthistle
Since the weed emerges all year, rotation of crops is a less useful tactic than for weeds that grow in winter and summer, and management should focus on all phases of the rotation to keep weed numbers low.

Crop competition greatly impedes common sowthistle growth, particularly in winter cereals (Figure 4). This tactic is important, as it reduces seed production on survivors or escapes from herbicide application. In a trial at Condamine, barley had an 8-fold competitive advantage over wheat in suppressing sowthistle. Even at a wider row spacing (50 cm) and low plant density (50 plants/m2), barley was highly competitive. Wheat was competitive at a narrow row spacing (25 cm) and most competitive when plant density was 75 plants/m2 or above. The quality of barley or wheat was not negatively influenced by the increase in plant density.



Figure 4. Sowthistle biomass in wheat and barley crops of different row spacing and plant density.

For better control with herbicides:
* Spray small seedlings (2-5 leaves).
* Use higher water volumes for atrazine, paraquat and Spray.Seed®, such as 100L.
* Do not mix glyphosate with 2,4-D or metsulfuron (eg Ally®) due to potential antagonism. Other options are in Table 2.
* Control late flushes in winter crops with selective herbicides or pre-harvest sprays instead of waiting for the first fallow spray after harvest.
* In winter cereals mix a Group I with metsufuron or rotate to a Group I.

Since sowthistle is at high risk of glyphosate resistance, it is important to use weed control tactics that take the reliance off this herbicide. Fallow strategies to minimise the risk for Group M resistance include:
* Mix glyphosate with Starane®, Grazon® DS, or Cadence® (Group I) (Table 2).
* Rotate with Spray.Seed® or paraquat (Group L).
* Double knock with glyphosate followed by Spray.Seed® or paraquat on survivors anytime in fallow.
* Apply atrazine (Group C) in late winter or spring fallow prior to sorghum after treatment with a knockdown herbicide. Local research: Atrazine at 3.6L/ha controlled common sowthistle for several months.

Fleabane

The key to getting on top of fleabane is to attack all parts of the weed lifecycle and keep the seedbank low. An IWM strategy that includes chemical and non-chemical tactics for controlling seedlings and preventing seed production on survivors will result in substantially fewer fleabane problems.

The double-knock tactic is the only herbicidal treatment that has been shown to be consistently effective in the control of fleabane. Other treatments can be effective on small, young weeds. However, as fleabane is inherently difficult to control, many herbicides effectiveness seems to depend on numerous weed, application and environmental variables.

The double-knock tactic refers to the sequential application of two different weed control tactics applied in such a way that the second tactic controls any survivors of the first. The most common combination is the application of glyphosate (Group M) followed by paraquat/diquat (Group L) at an interval of between 1 and 14 days.

A trial on the Darling Downs (near Dalby) found that the double knock tactic was effective on fleabane when used on small, young, actively growing plants (Table 3). It is important that robust rates be used for both knocks and that the second knock be applied at a high water rate (100 L/ha) if the contact herbicides Spray.Seed® or paraquat are used. Ideally the second knock should be applied within 5-7 days after the glyphosate application (Figure 5).




Figure 5. Impact of different intervals between the first knock with Roundup CT® and the second knock with paraquat (triangles) and Spray.Seed® (squares) on fleabane control

Fleabane control in winter cereals

Good in-crop control of fleabane is believed to greatly reduce the problem in the following fallow. However, no selective herbicides are currently registered. Numerous herbicide options have been investigated for pre-plant and in-crop control of fleabane (Table 4). Some options for better in-crop control of fleabane include:
* Control fleabane in the autumn fallow prior to sowing the winter crop, using a glyphosate mix, such as glyphosate + 2,4-D amine, on small weeds;
* Sow barley or wheat at a high seeding rate to ensure a competitive crop stand;
* Apply chlorsulfuron (eg Glean®) pre-plant for residual control during the winter season, and follow up with 2,4-D amine or Tordon® 242 on survivors OR apply a post-emergent treatment of metsulfuron-methyl (e.g. Ally®) + Tordon® 242 or follow up with 2,4-D amine.

Fleabane control in winter fallow and spring sorghum

Field trials investigated herbicidal options for the control of fleabane in late winter and early August prior to summer cropping (Table 5). The best strategy was using a double knock followed by atrazine at sowing. Other key points for consideration include:
* Spray small fleabane within 2-3 weeks of emergence in autumn and winter fallows
* Use a robust rate of glyphosate mixed with one or more other effective herbicides;
* Consider using the double knock technique with a paraquat product on surviving weeds, particularly for dense weed populations and/or larger weeds;
* Apply atrazine at 4 L/ha (500g/L product) after using knockdown herbicides to winter fallow if rainfall is imminent OR apply atrazine at 2-4 L/ha at sowing of sorghum, using the higher rate if fleabane and summer grasses are expected to be a problem.

Cost considerations

Many of the approaches considered in this paper will cost more than current practice. When making a decision about which tactics to use, also consider what costs will be reduced by having better weed control. As mentioned earlier, sowthistle reduces harvest quality and fleabane can compete with crops reducing yield.

To minimise the impact of increasing costs, look at your property on a paddock by paddock basis and start with the weediest paddock. Once the populations of fleabane and sowthistle have been reduced, rotate your efforts to another paddock. Remember, both species have short-lived seed-banks (if they remain in the soil surface layers and in the absence of seed replenishment). So you should be able to get on top of a problem paddock within 2-3 years.

Another cost consideration is the cost of preventing versus managing herbicide resistance. With glyphosate resistance becoming more common and with both sowthistle and fleabane at high risk for this resistance, careful consideration needs to be given to what herbicides are being used. Once glyphosate resistance is common in a weed population, alternatives to glyphosate have to be used to control that population all the time and it is therefore a recurring cost. By rotating your weed control tactics to include alternative herbicides from different modes of action, you are less likely to get glyphosate resistance and therefore it will still be effective on that population. While the alternative herbicides will cost more, this isn’t a recurring cost as glyphosate will remain an important tactic that works.

Contact details

Michael Widderick
Queensland Department of Primary Industries and Fisheries
Leslie Research Centre
13 Holberton Street Toowoomba Qld 4350
Ph: 07 4639 8856
Fx: 07 4639 8800
Email: Michael.widderick@dpi.qld.gov.au




Table 2. Control of sowthistle in winter fallow on the Darling Downs

Glyphosate treatments were applied in 60L/ha and Spray.Seed in 100L/ha. For the double knock treatment, glyphosate was applied to weeds with 3 leaves, and Spray.Seed was applied 1 week later when untreated weeds had 7 leaves.

Table 3. Control of fleabane 6-10 leaf (7-8 cm wide) with the double knock tactic under good growing conditions on the Darling Downs

Table 4. Fleabane control in wheat: two experiments comparing various pre-plant treatments with Glean® and Ally®

Table 5. Fleabane (and summer grass) control: 3 experiments in sorghum comparing various treatments with Atrazine and Glyphosate

Atrazine and dicamba rates are for 500g/L products, Glyphosate 450 g/L product, Spray.Seed was applied 1 week after the glyphosate treatment.