New research has advanced the tactical art of weed control targeting two of the country's most competitive cropping weeds: annual ryegrass (Lolium rigidum) and brome grass (Bromus diandrus and B. rigidus).

These grasses are ranked first and fourth respectively in the Australian roll call of damaging crop weeds. Annual ryegrass and brome grass are among a range of cropping weeds that cause total national yield revenue losses, estimated at $745 million a year.

Helping to curb this weed toll, Associate Professor Gurjeet Gill from the University of Adelaide examines the effectiveness of three integrated weed control tactics: time of sowing, crop seeding rate and pre-emergent herbicide use.

Dr Gill says the research with GRDC-investment showed that time of sowing, in particular delayed sowing, resulted in yield penalties that far outweighed its weed control benefits as part of integrated management strategies.

The yield penalties stem from reduced use of resources important to crop development and productivity: water, light and nutrients, he says.

Yield penalty

Gurjeet Gill discusses seeding rate manipulation as a weed control tactic at the 2019 Birchip Cropping Group field day in Victoria's Mallee region. Photo: University of Adelaide

For instance, a 2019 trial at Washpool, South Australia, showed a three-week delay in wheat sowing caused a one tonne per hectare yield penalty.

In 2018, trials at Minnipa and Marrabel, SA, delayed sowing also caused yield penalties ranging from 25 to 43 per cent in wheat and 26 to 29 per cent in barley.

"Delaying sowing until June resulted in a significant yield penalty across all three trials," Dr Gill says. "A decision to delay sowing to manage weeds needs to be considered carefully against the potential for yield losses."

The SA trials also showed that the effectiveness of delayed sowing as a weed control tactic was variable and limited.

For example, late sowing provided two main benefits for control of annual ryegrass in wheat at Minnipa and brome grass in barley at Marrabel: reduced weed densities and decreased weed seed production.

However, these weed control benefits did not necessarily lead to crop yield gains.

For instance, delayed sowing saw "a large reduction" in the density of brome grass plants in barley at Marrabel, but the surviving weeds were more vigorous which, in effect, compensated for their reduced density, Dr Gill says.

Delayed sowing even has the potential to be counter-productive as a weed control tactic. The trial at Washpool showed a three-week delay in wheat sowing increased the density of ryegrass heads and, ultimately, weed seed production. Plus there was no reduction in the density of annual ryegrass plants in the late-sown crop.

"Late-sown wheat at Washpool had more than double the number of ryegrass heads compared with the early sown crop," Dr Gill says.

But he adds that across all three trial sites, the efficacy of delayed sowing as a weed control measure was influenced by weather conditions, particularly growing-season rainfall, and the dormancy of weed seed populations.

Weed seed dormancy

Weed control integrating crop seeding rate, hybrid vigour and herbicide use to suppress annual ryegrass in canola trials at Lake Bolac in Victoria's western districts.
Photo: University of Adelaide

"The impact of delayed sowing on weed density is influenced not just by the weather conditions, but also the seed dormancy of weed populations," Dr Gill says.

This is because weed populations with low seed dormancy tend to germinate and establish quickly following rainfall events, meaning growers have the opportunity to manage them with pre-emergent herbicides where sowing is delayed for short periods (one to two weeks).

In contrast, weed populations with high seed dormancy take longer to emerge after rainfall events, so longer delays in sowing (more than four weeks), likely to result in yield penalties, are needed to manage these weed populations.

Highlighting the twin influence of growing-season rainfall and seed dormancy on weed control efficacy was the Minnipa trial, where wheat was late-sown on 25 June.

"A six-week delay in wheat sowing reduced establishment of ryegrass at this site," Dr Gill says.

This was particularly evident in the untreated control (non-selective herbicide knockdown only) that saw weed density decrease by 123 plants per square metre when the crop was late-sown instead of early-sown.

Where wheat was late-sown on 25 June, the in-crop weed density was 139 plants/m, but where it was early sown on 11 May the weed density was 262 plants/m.

He says it is likely that the ryegrass population at the Washpool trial site, where a three week-sowing delay was ineffective for weed control, has a high level of seed dormancy.

"Much longer delays in sowing would be needed to reduce infestations of highly dormant weed populations," Dr Gill says. "Dry surface soil conditions during the three-week sowing delay might also have been responsible for the lack of ryegrass control."

Seeding rates

Trials looking at integrated approaches to brome grass management in barley at Kinnabulla, north-west Victoria.
Photo: University of Adelaide

The trials found increased crop seeding rates were consistently effective as a weed control practice, especially where cereals were late-sown.

For example, the trial at Minnipa showed that wheat yields increased in parallel with seeding rates. Wheat sown at a low seeding rate (100 seeds/square metre) yielded 1.25t/ha, a medium seeding rate (150 seeds/sqm) yielded 1.41t/ha, and a high seeding rate (200 seeds/sqm) yielded 1.44t/ha.

"The overall increase in wheat yield response to seed rate was 13 per cent," Dr Gill says.

"There was no negative effect of crop seed rate on grain screening content, which ranged from four per cent in the low seed rate to three per cent in the medium and high seed rates," he says.

Seed set prevention

Another important finding of the research was that applying Intervix herbicide (after a pre-emergent herbicide application combining Treflan and Avadex Xtra) completely prevented weed seed set of brome grass populations in barley at Marrabel, irrespective of sowing time.

To achieve this result, Intervix herbicide was applied at a rate of 750 millilitres/ha, at crop growth stage 14, following a pre-emergent herbicide application combining Treflan applied at 2 litres/ha and Avadex Xtra at 2L/ha.

GRDC Research Code 9175134

More information: Gurjeet Gill, 0428 592 825, gurjeet.gill@adelaide.edu.au