Burn, bury or eat?
[Photo: Brookton wheat at Beverley in 2004: The right plot was treated with inversion by a mouldboard plough and the left was untreated. Photo - Susan Hall]
Burn them, bury them or have them eaten - three options for graingrowers in dealing with troublesome weed seeds.
Incineration by burning windrows, inversion with a mouldboard plow to a depth where seeds cannot emerge from the soil, and ingestion by ants can all be used to tackle weed seeds. Each has advantages and disadvantages, allowing growers to decide what is best for their particular farming system.
Incineration is widely practised, although weed seed removal is only effective when the burning conditions are right, according to researcher Dr Michael Walsh of the GRDC-supported WA Herbicide Resistance Initiative.
"Burning temperatures and durations of 400°C for 10 seconds are required to kill annual ryegrass seeds and 500°C for 10 seconds for wild radish, meaning burning narrow windrows will be more effective than burning stubble because of the concentration of fuel in the windrows," he says. "Obviously, seed not collected in the windrow due to shattering before harvest will not be targeted by windrow burning.
"This tactic is variable and depends on seasonal conditions, but provides between 80 and 90 percent control of weed seeds present in the windrow."
Another researcher, Dr Sally Peltzer from the WA Department of Agriculture, says inversion, or burial, can be an extremely effective method of weed seed removal on suitable soil types.
Trials show a one-off soil inversion using a mouldboard plough can reduce ryegrass numbers by more than 95 percent if full inversion at greater than 10 centimetres is achieved, resulting in substantially higher grain yields. The yield benefits can continue into later years.
"While this method is expensive, at more than $50 a hectare, the cost would be spread over eight to 10 years for whole paddock inversion, or 10 percent a year where only the windrow is buried," says Dr Peltzer. "Soil inversion is a disruption to conservation tillage practices, which should be implemented in the years between inversions."
Dr Peltzer says re-inversion would not bring up any viable ryegrass seed after about eight to 10 years, but other species, such as wild radish and wild oats, may still be viable, although this has not been confirmed.
The other weapon against weeds that is being extensively studied in WA is the role of ants as biological control agents. The department’s David Minkey says ants can eat their way through thousands of seeds from the soil surface. However, trials show that while this method costs nothing, control rates can range from zero to 100 percent.
Mr Minkey notes that although it is a noninvasive, non-chemical method, biological control by ants may limit the use of some other farm practices such as insecticide use and tillage. Weed seed predation is usually higher on lighter soils and more effective on lupins, barley or pasture, compared to canola and wheat residues, he says.
GRDC Research Codes: UWA266, DAW613, DAW492.