2017 Green bridge - are you getting more than you bargained for this season?
It has been a very wet start to 2017 with widespread rain across Western Australia. Since 1 January, the recorded rainfall has been between approximately 100mm at Yuna in the north of the cropping region to nearly 300mm in the central region and the eastern south coast (most places received between 100-200mm)
While much of that rainfall fell in two major fronts in February this year, there has also been over 30mm recorded in March in many places.
This is great news for the WA cropping region, setting up the season with abundant stored soil moisture (see Figure 1).
However, this has also resulted in the widespread germination and continued growth of summer weeds and volunteer crops causing a ‘green bridge’.
This green bridge might impact on this year’s crop by competing for water and nutrients, providing a habitat and encouraging the build-up of insect pests and providing a host for pathogens.
Summer weeds
A range of weed species emerge during summer following adequate rain, grow and utilise water and nutrients stored in the soil profile. In addition to having an economic cost for control during the summer period the impact of summer weeds can extend into the cycle of winter cropping.
Some weeds that are labelled as ‘summer weeds’ do not only germinate in the summer period. Weeds like sowthistle and fleabane can germinate in August and grow unnoticed in the maturing crop. These weeds will still produce seeds to replenish the soil seed bank and may also cause harvesting issues if they remain green at grain maturity.
Summer weeds can reduce wheat grain yield and protein
Heavy weed (and volunteer) growth over summer can have a major effect on the yield of the subsequent crop by reducing the amount of available nitrogen and moisture. By using a combination of modelling and real data, the GRDC-funded Water Use Efficiency (WUE) Initiative (which ended in 2013), found that controlling summer weeds saves water and N in equal measure and increases WUE by an average of 60%.
In contrast, a national study funded by CRC Australian Weed Management found that the greatest impact tended to be on soil nitrogen. Summer weeds move soil nitrogen to the surface rendering it unavailable to following crops and consume the free nitrogen that is mineralised during the fallow period. Research conducted by Abul Hashem (DAFWA) from 2003-2005 in Merredin found that summer weeds can reduce cumulative soil N (to a 90cm depth) by 12-66% compared to weed-free conditions (Figure 1)
In the same study, uncontrolled summer weeds significantly reduced the subsequent wheat grain yield and protein, (particularly in those years with a high potential yield). In 2003, where the site was irrigated (to simulate summer rain), wheat yield and grain protein was reduced by 40% and 20% respectively (Table 1).
| Treatments | Wheat grain yield (t/ha) 2003 | Wheat grain yield (t/ha) 2004 | Wheat grain yield (t/ha) 2005 | Wheat grain protein (%) 2003 | Wheat grain protein (%) 2004 | Wheat grain protein (%) 2005 |
| Irrigated (clean) | 2.9 | 1.5 | 1.4 | 9.2 | 8.2 | 9.0 |
| Irrigated (weedy) | 1.8 | 1.4 | 1.4 | 7.5 | 8.1 | 8.4 |
| LSD (p<0.05) | 0.20 | ns | ns | 0.25 | ns | ns |
The analysis of aboveground tissues of summer weeds (data not presented) showed that 1t/ha dry biomass of summer weed species such as caltrop could tie up about 15kg N/ha. This nitrogen will not be available to wheat plants, especially during the vegetative growth stages (first 6-8 weeks) when the demand for N is high.
Some summer weed species reduce crop emergence
The species of weed species present can impact on the emergence of the subsequent crop. In the Merredin trial, the dominant species was caltrop (Tribulus terrestris) in 2003 and Afghan melon (Citrullus lanatus) in the other two years. Wheat emergence was significantly reduced in 2003. One explanation is that allelochemicals being released by the weeds are inhibiting wheat emergence. In other words, the caltrop (interesting fact – caltrop is an ingredient in a “natural” libido-increasing preparation and is called horny goat weed) plants might have a stronger allelopathic effect on wheat emergence than Afghan melon.
Recent survey results
Historically, fleabane was primarily a weed of roadsides but is now a common summer weed within paddocks across most of the grain growing region of WA. Surveying roadsides gives us an indication of the major weed issues in different agronomic regions and provides direction for future research on emerging weed species.
A roadside weed survey was conducted over all main roads in the WA wheatbelt during February to April 2015 and 2016. Sites were selected approximately every 10km, where weeds were visible on the roadside. Weed density (seed head or tillers for grass weeds and plant numbers for broadleaf weeds) was determined by visual assessment.
- The most common roadside summer weed species were African love grass (Eragrostis curvula), windmill grass (Chloris truncata), fleabane (Conyza bonariensis), wild radish (Raphanus raphanistrum) and sowthistle (Sonchus spp.).
- The four most common species in the northern and central agricultural regions varied between years, although the four most common species in the southern agricultural region remained constant.
- Summer weed density increased in 2016 compared to 2015, due to higher December-January rainfall
| Year | Region | Number of sites | Most common species (and percentage frequency) | Second most common species (and percentage frequency) | Third most common species (and percentage frequency) | Fourth most common species (and percentage frequency) |
|---|---|---|---|---|---|---|
| 2015 | Northern agricultural region | 49 | Fleabane (65%) | African lovegrass (47%) | Couch (35%) | Wild radish (29%) |
| 2015 | Central agricultural region | 140 | African lovegrass (43%) | Windmill grass (30%) | Wild radish (29%) | Fleabane (29%) |
| 2015 | Southern agricultural region | 54 | African lovegrass (65%) | Fleabane (46%) | Sowthistle (41%) | Windmill grass (39%) |
| 2016 | Northern agricultural region | 16 | Windmill grass (63%) | African lovegrass (50%) | Wild radish (44%) | Mulla mulla (44%) |
| 2016 | Central agricultural region | 89 | African lovegrass (70%) | Windmill grass (49%) | Wild radish (48%) | Roly poly (33%) |
| 2016 | Southern agricultural region | 33 | African lovegrass (79%) | Windmill grass (67%) | Sowthistle (67%) | Fleabane (64%) |
| Total | Total wheatbelt | 246 | African lovegrass (87%) | Windmill grass (61%) | Fleabane (60%) | Wild radish (52%) |
New work in summer and emerging weeds
The two years of survey work highlighted the variability of summer weeds. It is not surprising that the high summer rainfall in the summer of 2015/2016 increased the number of species present at high densities. However, it was interesting that the most common weeds found in the northern and central agricultural regions should vary between years.
Wild radish, fleabane, windmill grass, sowthistle and roly poly (Salsola australis) were identified as problematic weeds in prior surveys. As a result, they are the subject of current integrated weed management projects in WA, along with Afghan melon, button grass, wireweed and caltrop (GRDC supported projects UA00149 and UA00156). Further, stinking lovegrass (Eragrostis cilianensis and E. minor), feathertop Rhodes grass (Chloris virgata), mallow (Malva parviflora) and matricaria (Oncosiphon spp.) have been highlighted as locally important weeds to research in the GRDC supported project DAW00257 led by Alex Douglas, DAFWA, Katanning.
Many summer weeds have significant seed dormancy and/or traits that prevent their rapid germination when the conditions are right. This contributes to the variable appearance of this group of weeds from year to year. Understanding the dormancy and persistence of each weed allows growers to plan their management, most importantly to stop seed set.
In the GRDC supported UA00156 Emerging Weeds project, DAFWA staff have been busy identifying the dormancy and persistence of a range of summer weeds including sowthistle, tarvine (Boerhavia coccinea), fleabane and button grass (Dactyloctenium radulans).
- Fleabane and sowthistle have limited seed dormancy and usually germinate when the conditions are right. Tarvine and button grass do not germinate readily and requires further treatment to stimulate germination.
- In laboratory conditions of 35/25°C day/night temperature in a germination cabinet under dark conditions for 14 days, approximately 90% sowthistle germinated while only 12% of tarvine did.
- Soaking tarvine seeds in bleach for 20 minutes increased its’ germination percentage to nearly 70%. It is likely that bleach removed some of the germination impediments including thick seed coat or alkaloids on the seed coat that may limit the absorption of water by seed embryo to initiate the germination process.
- Button grass seed needs to be scarified in order to germinate suggesting that mechanical impacts such as tillage may improve its germinability.
- The germination of all four weed species was greater under light conditions than dark conditions, suggesting that these species are likely to have improved establishment when they remain on the soil surface, such as in no-till systems, than where they are buried in a full disturbance system.
Control options
From work over the last few years a standard summer mix of glyphosate and 2,4-D will control most of the common summer weeds and those mentioned in this article. In some cases a follow-up (or double-knock) application of paraquat or paraquat + diquat may be required to give complete control.
One thing to keep in mind when planning a spray program for summer weeds is the potential for the development of herbicide resistance, particularly to glyphosate. Glyphosate is an important component in the majority of summer spray mixes, it is also the chief component in the mixes used to treat roadsides.
Some of our common summer weeds may already have developed resistance to glyphosate but have yet to be confirmed. Sowthistle populations collected during the roadside surveys (in 2015) have been tested and four populations have survived 1080g/ha glyphosate. South Australia has confirmed glyphosate resistant populations of feathertop Rhodes grass and fleabane on their roadsides with the other states recording resistant populations of these species plus sowthistle on roadsides and within paddocks.
Visit the DAFWA website for more information on summer weeds, their control and individual weed pages.
Green bridge - diseases and pests
The green bridge will act as host for many different insects and diseases and will result in population build-ups. Growers and consultants are urged to survey weeds and plant volunteers to identify what insects and diseases are present.
Subscribe to the DAFWA PestFax newsletter to receive weekly news updates, risk alerts and management advice throughout the growing season on pests and diseases threatening crops and pastures throughout the grain belt of WA. The first issue for the season will be distributed soon. The PestFax service is an interactive reporting service that also provides a map of reported pests and disease, a PestFax Reporter app and free insect diagnoses. For more information visit the PestFax webpage on the DAFWA website.
Cereal rust and mildew
The presence of volunteer crop hosts can provide opportunity for cereal rusts and mildews to establish infection and provide early season sources of airborne inoculum. Cereal rusts can only survive on living plants so in seasons where volunteer hosts die before new crops are established outbreaks are more sporadic and less intense.
In 2015, it was wet in the northern cropping areas with a green bridge. This was associated with high levels of powdery mildew and leaf rust plus the first significant amount of stripe rust for several years. With significant summer rainfall across most of the grainbelt in 2017, it’s expected that these diseases could again be problematic. The frosts of 2016 could also have implications where there is more spilled grain than usual resulting in more regrowth.
Soil borne pathogens
Summer rainfall can have significant impact on the level of soilborne pathogens (root lesion nematodes, rhizoctonia and crown rot) in a paddock, with the common theme that control of green bridge volunteer hosts well before the start of the season providing best management of disease carryover and build-up.
Insects, slugs and snails
Aphids (and some mites) need a living host to survive and when present in very high numbers they can inflict considerable feeding damage if they are not effectively managed and disperse onto emerging crops.
Insects, primarily aphids in WA crops, also transmit viruses to crop plants. Grass weeds and cereal volunteers provide a reservoir for barley yellow dwarf virus (BYDV) and wheat steak mosaic virus (WSMV) (vectored by the wheat curl mite), while wild radish, other broadleaf weeds and volunteer canola host beet western yellows virus (BWYV) and turnip mosaic virus (TuMV).
This year's cool, wet conditions and an abundance of stubble, weeds and volunteer crop growth have provided ideal conditions for slugs. They are actively moving and feeding right now.
It is recommended to monitor regularly, ideally before seeding as there are more control options available, including burning, grazing and tillage. Before seeding is a good time to put out slug traps, for example hessian bags or carpet squares. This applies not only on the heavy country but also on the lighter soil types especially in long term no till paddocks. If one or more slugs are found under the hessian sacks they can damage emerging canola crops leading to re-seeding becoming necessary.
Other insect pests to be on the lookout for in a green bridge include pest mites such as balaustium mite and bryobia (clover) mite and diamond back moth which may all impact on this year’s crop.
Disease and insect surveys by DAFWA
DAFWA is currently undertaking (or will be undertaking later this year) a number of surveys to provide information on a range of pests.
- DAFWA’s new Regional Research Agronomy team (funding supported by GRDC and Royalties for Regions) is currently conducting surveys of existing green bridge vegetation across the grainbelt and are sending samples to be tested for diseases, aphid viruses and soil borne pathogens.
- DAFWA’s virology team is also currently conducting surveillance work by sampling over-summering hosts and trapping of migrant aphids. This work (funded by Royalties for Regions) is being used to validate new loop-mediated isothermal amplification (LAMP) technology that allows rapid on-farm diagnosis of plant pathogens. DAFWA Research Officer Benjamin Congdon who is working in the virology project says they are developing the protocol using the turnip yellows virus/canola pathosystem. This project and a similar activity at the Centre for Crop and Disease Management (CCDM) at Curtin University are exploring how this technology could be extended to a range of crop pathogens. Detecting pathogens with the user-friendly LAMP machine can be a relatively simple and rapid procedure meaning, once developed, an agronomist could have a diagnosis for the grower in a single meeting in under an hour.
- DAFWA plant virologist Roger Jones advises that a damaging new resistance-breaking strain of TuMV is spreading in canola crops in eastern Australian and the WA oilseeds industry needs to be on the lookout for it because it overcomes all four TuMV resistance genes that currently suppress spread of this virus in Australian canola cultivars. Wild radish and volunteer canola are reservoirs of this virus from which many aphid species can spread it to canola crops. The team is conducting surveillance to find whether this new strain of TuMV has reached WA yet from the eastern states. It is mostly symptomless in wild radish, but this virus can cause yield losses up to 80% in canola.
- DAFWA will also conduct an insecticide resistance testing survey for red-legged earth mites (RLEM) this year through the GRDC supported project Insecticide resistance management in RLEM and chemical sensitivities of other grain pests, given the rise in resistant populations discovered in WA over the past decade.
- DAFWA is part of a national GRDC supported project Biology and management of snails and slugs in grain crops (DAS00160), looking to model what the micro climatic conditions are that initiate slug and snail movement and egg laying, so baits can be timed to kill the pest before crops are damaged. Remote cameras and weather stations have been set up in trials at Mount Barker, Gairdner and Esperance.
More information
You can also search the DAFWA website for more information on individual weeds pests and diseases, or use the MyCrop tool to help with diagnostics in your paddock.