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.
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|
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.
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.