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Double down on the double break in the high rainfall zone

In high rainfall zones it can be easier to overlook the impact of weeds like annual ryegrass, as the effect on crop yield is often less visually dramatic than in lower rainfall zones.

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Through a long-term trial at Lake Bolac, Victoria, researchers at The University of Adelaide and Southern Farming Systems set out to quantify the impact ryegrass has on crop yield, and the integrated weed management strategies that can be used to control this competitive weed in high rainfall zone cropping systems.

Practical tips

  • Annual ryegrass responds well to high rainfall and long growing season conditions and can suppress yield to a similar extent to that experienced in other climatic zones.
  • The control by pre-emergent herbicides often declines quickly in the high rainfall zone, particularly for products that have short persistence in the environment.
  • Annual ryegrass plants can produce more seed in the high rainfall zone than in other growing regions. Tactics aimed at stopping weed seed set are essential to avoid blow-out situations.
  • Double break cropping, crop competition and robust combinations of pre and post emergent herbicides, including mixtures, pays off in the long-term.

Comparison of high and low density ryegrass in wheat crop as a result of differences in herbicide strategy.

The high rainfall zones present serious challenges to weed control and particularly to pre-emergent herbicides. The combination of long seasons, high rainfall and waterlogging, high weed density and herbicide resistance can compromise the efficacy of these herbicides.

Consequently, growers need to devise long-term, integrated weed management strategies such as the WeedSmart Big 6, to keep ahead of aggressive weeds like annual ryegrass.

Experimental design features

A unique, 9-year integrated weed management trial was conducted at Lake Bolac from 2012 to 2020 to investigate the effects of cultural and herbicide control strategies across a rotation on annual ryegrass populations in a high rainfall environment.

The pre-sowing cultural tactics applied to the main plots were retained stubble, burning stubble, incorporated stubble and mouldboard ploughing followed by retained stubble.

Three herbicide strategies were then applied to 20 m wide by 60 m long sub-plots (not randomised) for the nine-year duration of the trial. The herbicide programs differed in cost (‘low cost’, ‘medium cost’, ‘high cost’ being $39/ha/yr, $56/ha/yr, $89/ha/yr respectively) and varied according to the crop grown across the whole site each year. There were four replicates of each treatment.

The crop rotation at the field site started with wheat (2012) then barley, RT canola, wheat, faba beans, TT canola, wheat, TT canola and finally wheat in 2020. The double break in 2016 and 2017 was done in response to a blow-out in the ryegrass population.

In the last two years of the trial, the research team included a crop competition treatment through a sub-optimal canola plant density of 20 plants/m2 compared to the optimal density for hybrid TT canola of 45 plants/m2, applied across the three herbicide treatments. Similarly, in 2020, the wheat crop was sown to achieve plant densities of 150 plants/m2 and 200 plants/m2, in plots matched to the 2019 canola sowing treatments.

Detailed findings

Manage the weed seed bank

The mouldboard plough operation reduced establishment of annual ryegrass by more than 95% in the year that it was implemented. After two years, there was no difference in annual ryegrass populations between the pre-sowing cultural treatments and so the yield and weed seed data were averaged across the pre-sowing cultural treatments to focus on the effect of the three herbicide regimes.

In the early years of the trial (2012 to 2016), annual ryegrass seed head numbers increased under all three herbicide regimes, although the more intensive (high cost) management did suppress weed seed head numbers more than the medium and low cost strategies.

In the high rainfall zone, the long growing season challenges pre-emergent and post-emergent herbicides to provide season-long annual ryegrass control in cereals. Consequently, weed populations increase over time unless strategies that stop weed seed set are used in the integrated weed management program. Such tactics include harvest weed seed control, cutting for hay and crop-topping in break crops. The long growing season often results in greater weed seed shedding before harvest so tactics such as windrowing can bring harvest forward and maximise the benefit of harvest weed seed control.

Reset using the double break tactic

The crop-topping of the 2016 faba bean crop curbed the ryegrass population for all three herbicide treatments, however in 2017 ryegrass seed head numbers were much higher in the low cost, compared to the other two herbicide treatments.

The research team decided to implement a double break tactic at the site, sowing TT canola in 2017 after the faba bean crop. The ryegrass population was much reduced as a result of the double break, with the weed seed head numbers in the low-cost herbicide treatment reduced from 2399 to 35 heads/m2 by the end of 2017. The effect of the double break persisted after 2017, showing that it had degraded the weed seedbank significantly.

Graph of ryegrass seed production showing very high seed numbers were reduced to very low levels using a double break crop tactic.

Investing in herbicide mixtures pays off

For the period 2012 to 2017, the crops grown with the medium and high cost herbicide treatments were 6 and 12 per cent higher yielding than crops grown with the low cost herbicide treatment. This demonstrates the extent that annual ryegrass impacts on crop yield in the high rainfall zone.

After all herbicide costs were paid, the cumulative benefit of the medium cost and high cost herbicide treatments was $1288 and $1613 per ha respectively. The medium and high cost herbicide treatments typically included more mixtures, rather than single mode of action, pre-emergent treatments.

Crop competition supports herbicide use

The effect of crop competition was measured in low and high seeding rate plots within the trial in 2019 (TT canola) and in 2020 (wheat). The lower plant density treatment in the 2019 TT canola crop resulted in an average yield reduction of 0.4 t/ha and higher annual ryegrass seed head production across the three herbicide strategies. The medium and high cost herbicide strategies in the 2020 wheat crop were able to maintain the lower weed numbers achieved in the canola crop.   

Maintaining high crop competition through sowing early, optimal sowing rates and competitive cultivars maximises yield potential in the current year and can also protect the yield potential for future years. Crop competition also complemented all herbicide strategies for weed control in both years.

Graphs showing that high seeding rate is an effective way to reduce ryegrass seed production for all levels of herbicide use.

The Lake Bolac trial shows the value of adopting an integrated weed management strategy like the WeedSmart Big 6, that includes a diverse crop rotation, competitive crops, robust herbicide packages and cultural practices that stop weed seed set, to profitably maintain low density populations of annual ryegrass.

This research was conducted by researchers at The University of Adelaide (a WeedSmart scientific partner) and Southern Farming Systems, with investment from the GRDC, a WeedSmart platinum sponsor. Neil and Graham Vallance hosted the 9-year trial at Lake Bolac site, and the unique dataset is available to the industry thanks to their remarkably faithful collaboration.

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