Can harvest weed-seed control work in the north?

Photo of specialist weeds researcher Dr Michael Walsh

Specialist weeds researcher Dr Michael Walsh believes northern grain growers will start to take up harvest weed-seed
control because of the benefit it offers in retaining herbicide effectiveness.

Many northern grain growers have been “a little sceptical” about introducing harvest weed-seed control (HWSC) as another tool for combating herbicide resistance. Department of Agriculture and Fisheries (DAF) principal weed science researcher Dr Michael Widderick says few growers in Queensland or New South Wales incorporate HWSC into their management practices, but, like other leading researchers, he believes this will change.

“Many northern growers thought weed seeds were shed before harvest, so there was little point in using HWSC tactics,” Dr Widderick says.

But he says GRDC-supported research has found excellent seed retention at harvest of several key weed species in northern winter crops.

“Our work has found seed retention and the height of weed seeds in winter crops is well suited to HWSC. However, there are not as many opportunities in our summer crops,” Dr Widderick says.

“But HWSC nationally has proved to reduce the weed seedbank and if we can make that work in the northern region it is a major positive for weed control in what is an increasing herbicide-resistant environment.”

The northern battle against herbicide resistance has been boosted this year with the arrival of former Western Australian-based weed specialist Michael Walsh.

Internationally recognised for his work in this field, Dr Walsh has moved from Perth, where he worked with the Australian Herbicide Resistance Initiative (AHRI) at the University of Western Australia.

His new role as director of weeds research at the University of Sydney was created with the GRDC in response to the escalating problem of herbicide resistance in the northern region.

Dr Walsh says weed control is easy when herbicides work, but when chemical resistance starts, an integrated approach incorporating herbicide and non-herbicide tools is critical.

The associate professor has been integral in developing HWSC processes, and says his research shows regardless of method (Harrington Seed Destructor, chaff cart behind the header, direct baling harvest residues or narrow windrow burning) the key is to capture and destroy weed seeds.

“Annual weeds such as ryegrass, wild radish, brome grass and wild oats have adapted to cropping systems, growing to similar heights as cereals and maturing at the same time as annual crops,” Dr Walsh says.

“There is some weed-seed shedding at maturity, which may make some growers a little sceptical about how much seed is captured at harvest, but research shows a high percentage of total weed-seed production is retained on plants at a height that ensures collection during harvesting.

“We know at the start of harvest that high proportions of weed seeds are retained above a low harvest height (15 centimetres) for annual ryegrass (88 per cent), wild radish (99 per cent), brome grass (73 per cent), and wild oats (85 per cent), and by harvesting at this height these weed seeds are captured by the header and can be dealt with from there.”

Which northern weeds suit HWSC?

Dr Widderick says DAF research has shown some northern weeds are candidates for HWSC.

  • Definitely in – turnip weed and African turnip weed are potentially very good candidates for HWSC, although these species are not yet affected by resistance issues.
  • Definitely in (winter crops) – annual ryegrass and wild oats. “We know wild oats shed seed at about two per cent per day compared to ryegrass at one per cent a day, but there is still a good opportunity for HWSC at the start of harvest,” Dr Widderick says.
  • Possibly in (winter crops) – barnyard grass and feathertop Rhodes grass are known to shed their seed in summer crops, but where they germinate in spring in winter crops they may be suitable for HWSC.
  • Possibly in (summer crops) – feathertop Rhodes grass provides an opportunity for HWSC in summer crops where there is a high percentage of seed retention at the start of harvest.

Why HWSC in winter crops?

“Our research shows weeds in winter crops retain more of their seed at harvest than weeds in summer crops, like sorghum.

There are a couple of reasons for this,” Dr Widderick says.

“Firstly, the growing conditions in summer allow for weeds such as barnyard grass and feathertop Rhodes grass to germinate with the crop, so by the time harvest comes around they have matured, set seed and these seeds then have the opportunity to shed.

“In contrast, sorghum is planted on wide rows (one metre) giving the weeds opportunity to grow with low levels of competition. This may reduce the height of the weeds at harvest.”

He says weeds such as barnyard grass and feathertop Rhodes grass need warm soil to germinate. These weeds can germinate in spring in winter crops and, as a result, may retain their seed at winter crop harvest.

How to start?

Dr Walsh says winter crops are the obvious starting point as seed retention is highest here. The national weeds survey found 15 per cent of northern region growers are already diverting the weed-bearing chaff fraction onto permanent wheel tracks in controlled-traffic farming (CTF) systems. Growers already practising CTF can do this at low cost.

“Narrow windrow burning in chickpea crops is the other easy place to make a start on HWSC,” he says. “Seed retention was excellent in chickpea crops and it is simple to burn chickpea windrows without burning the entire paddock. Burning cereal windrows is also possible, but more challenging given the higher yields and dry matter.”

More information:

Dr Michael Walsh, director weed research, Plant Breeding Institute, Narrabri,
0448 847 272,

Dr Michael Widderick, Queensland DAF, Leslie Research Station, Toowoomba,
07 4639 8856,

Useful resources:

HWSC in the north – GRDC Hot Topic

Control the seeds to control the weeds’ – Ground Cover September–October 2014

Weed seed control tactics help growers fight the herbicide resistance war

End of Ground Cover issue #124 northern edition

Ground Cover Supplement will be back in the November–December issue.

Why not check out the latest news from other regions?
South: Hot narrow burn keeps weed seeds down
West: Stubble tactics in frost plan at Tincurrin

GRDC Project Code US00084, UQ00062

Region North, South