Investigating the role of competitive wheat against three northern region weeds
Turnipweed, Mexican poppy and common sowthistle are important weeds in the Northern grains region of Australia.
Field trials in 2016 and 2017 investigated the impact of each weed on a competitive wheat crop. The researchers observed ecological characteristics in each weed species that could be used to develop an integrated weed management program to reduce the impact of these weeds in northern farming systems.
Practical tips:
- Turnipweed and common sowthistle are highly competitive weeds in wheat.
- Wheat has a suppressive effect on Mexican poppy seed production.
- Turnipweed is a likely candidate for harvest weed seed control due to 100 per cent seed retention at crop harvest.
- A competitive wheat crop would be a good option for a field with a heavy infestation of Mexican poppy.
- Stubble retention from a high-yielding wheat crop or strategic tillage may be effective strategies to counter a common sowthistle infestation.
The WeedSmart Big 6 integrated weed management program includes herbicide and non-herbicide tactics to manage the weed seed bank. Understanding the target weed’s ecology is critical when designing an effective weed control program.
Experimental design features
Field studies of these three weeds, turnipweed (Rapistrum rugosum), Mexican poppy (Argemone mexicana) and common sowthistle (Sonchus oleraceus) were conducted from May to October in 2016 and 2017 at the Research Farm of the University of Queensland, Gatton.
To ensure a clean seed bed, the field was cultivated two to three times using a rotary cultivator before planting Spitfire wheat in a competitive configuration on 18 cm row spacing at a seeding rate of 60 kg/ha.
Turnipweed and Mexican poppy seeds were collected from a chickpea field near St George, and common sowthistle seeds were collected from a wheat field near Gatton.
The trial used a randomised block design with three replications and plots measuring 5.0 m by 2.3 m. The three weeds were established at four densities – weed-free, low, medium and high – on the same day the wheat was sown.
In both years, the crop yield in the control plots exceeded 5 t/ha, representing a high-yielding wheat crop for the region.
Detailed findings
Turnipweed
Turnipweed is widely distributed in Australian agricultural environments and is becoming increasingly common in the northern grain-growing regions of NSW and Queensland. This weed produces a large quantity of seed (up to 13,000 seeds/plant) that has extended dormancy due to the seed pod. It can establish in a variety of environments and at different stages of the cropping season. Although it is usually considered a winter weed, turnipweed is often found in summer crops and fallows.
Turnipweed is highly competitive in wheat. In this field trial, 18 to 24 plants/m2 caused a yield reduction of 50 per cent in a competitive wheat crop in 2016 and 2017, respectively. The presence of turnipweed reduced the number of wheat panicles per unit area and the number of grains per panicle.
The high level of seed production enables turnipweed to establish a large seed bank quickly. Resistance to the acetolactate synthase (ALS)-inhibiting herbicides, such as chlorsulfuron and tribenuron-methyl, has been observed in turnipweed in Australia and overseas. The risk of herbicide resistance in turnipweed is likely to be reduced in diverse cropping programs that include winter and summer crops.
Turnipweed had 100 per cent seed retention, and maturity coincided with crop harvest time in wheat, suggesting that harvest weed seed control tactics could be effective against turnipweed in wheat crops.
Common sowthistle
Common sowthistle is increasing in prevalence across the northern grains region due to its ability to thrive in varying environments and management regimes. Previously considered a winter weed, it is now commonly present in summer crops and fallows.
Common sowthistle thrives in cropping systems that use conservation tillage and include glyphosate-tolerant crops. Evidence suggests that some common sowthistle biotypes have evolved resistance to glyphosate and acetolactate synthase–inhibiting herbicides.
In this field trial, 43 and 52 common sowthistle plants per m2 growing in a competitive wheat crop caused a grain yield reduction of 50 per cent in 2016 and 2017, respectively.
High seed production and wind dispersal underpin the invasive success of common sowthistle. There is very low seed retention on common sowthistle plants at the time of wheat harvest, so this weed is not a good candidate for harvest weed seed control.
Most germination occurs at or close to the soil surface, and only 2 per cent of common sowthistle seeds remain viable at the soil surface after six months. Cultivation, stubble cover, haymaking and herbicides (post-emergent and residuals) can help manage common sowthistle populations.
Mexican poppy
Mexican poppy is rapidly emerging as a problematic weed in chickpea crops and fallows in the northern region. It is poisonous to humans and livestock.
This weed flourishes under moist conditions, producing about 10,000 seeds per plant, which can persist in the soil for several years if seed set and dispersal is not controlled.
Although Mexican poppy plants established on the border of the trial site grew well and produced seed, those growing within the competitive wheat crop did not thrive or set seed. High-density weed pressure (73 and 94 plants/m2) reduced crop yield by 17 per cent and 23 per cent in 2016 and 2017, respectively.
The ability of a competitive wheat crop to suppress seed production in Mexican poppy indicates that including wheat in the crop rotation can effectively drive down the weed seed bank in fields infested with Mexican poppy.
For this strategy to be effective, the wheat crop must be sown in fields with adequate moisture and nutrition, at high density, on narrow rows and have no gaps in the plant stand.
Research papers
The research outlined in this article was conducted by former Honorary Associate Professor, Sudheesh Manalil1, Professor, Bhagirath Chauhan1 and Assistant Professor, Hafiz Ali2.
- Queensland Alliance for Agriculture and Food Innovation (QAAFI)
- University of Sargodha, Pakistan
The research work was supported by a grant (project UA00156) from Grains Research Development Corporation, a WeedSmart platinum sponsor.
Read the full papers:
- Manalil S and Chauhan BS (2019) Interference of turnipweed (Rapistrum rugosum) and Mexican pricklepoppy (Argemone mexicana) in wheat. Weed Sci. 67: 666–672. doi: doi.org/10.1017/wsc.2019.42
- Manalil S, Ali HH, Chauhan BS (2020) Interference of annual sowthistle (Sonchus oleraceus) in wheat. Weed Sci. 68: 98–103. doi: doi.org/10.1017/wsc.2019.69
