Is crop rotation an economic option for managing weeds?
with Tony Swan, farming systems researcher, CSIRO
While wheat is the dominant commodity in Australia’s grain production systems, sowing consecutive wheat crops results in reduced production and profitability due to the effects of diseases, pests, weeds and declining nutrition.
Adding a break crop to the cropping sequence helps keep wheat profitable in a sustainable cropping system.
Tony Swan, CSIRO says their 5-year GRDC-funded project illustrated that adding at least one break crop, and preferably two, to the crop rotation was beneficial for weed control and nitrogen management, and can be as profitable or more profitable than continuous wheat. A series of experiments were established to challenge the idea that break crops are risky and not profitable.
“Many farmers in south-east Australia are sceptical about growing break crops such as pulses and canola,” says Tony. “The problem is, once high populations of herbicide-resistant annual ryegrass become apparent, the profitability of continuous wheat significantly reduces.”
“Rotations that include a break crop in paddocks with high populations of resistant annual ryegrass were more profitable than continuous wheat and had significantly lower ryegrass numbers after three years, provided all the available tactics were used to reduce germination and prevent seed set,” he says. “Our experiments demonstrated that it is cheaper and more effective to control ryegrass using one of the many break crop options than attempting to achieve control in wheat using expensive herbicides.”
What break crop options did you trial?
The break crops included in the trial were RR canola, TT canola, lupins for grain, field peas for brown manure, fallow and wheat cut for hay.
The combination of a fallow or break crop in year 1, followed by a second break crop in year 2, resulted in the greatest reduction in annual ryegrass seed bank population and panicle number after 3 years. This sequence was significantly more profitable than continuous wheat, but not as profitable as a RR canola–wheat (high input) –wheat rotation.
What was the most effective option in a weedy situation?
A 2-year break crop option was the most effective tactic against weeds.
The double break rotations of lupins grown for grain, followed by RR canola, or RR canola followed by wheat cut for hay provided a very high level of weed control while also generating high average annual 3-year gross margins of $790/ha/yr and $834/ha/yr, respectively.
This was comparable to the most profitable 3-year sequence of RR canola followed by wheat (high input) / wheat of $883/ha/year. However, this sequence did not achieve the same reduction in annual ryegrass and the cost of grass herbicides in the wheat crops exceeded $140/ha.
Sequences that included fallow or brown manures followed by RR canola were extremely effective at reducing the annual ryegrass seed bank, but were not as profitable as continuous cropping.
Where herbicide-resistant annual ryegrass is a major problem, an alternate 3-year sequence of wheat-hay (sprayed afterwards) in year 1, pulse-grain (spray topped) in year 2, and RR canola in year 3 can be profitable and also reduce the seed bank to extremely low levels.
What is the key recommendation from this trial work for annual ryegrass control?
The key recommendation is: Two consecutive years of total annual ryegrass control using break crops and all available weed seed control options.
Break crops are an effective weed control tactic and can also be profitable. Two or more years of effective ryegrass control using break crops and other management options, including strongly competitive crops, rotating herbicide groups, using pre- and post-emergent herbicides and preventing weed seed set using crop topping, hay making and brown manuring, along with fallow management and harvest weed seed control.
Other resources
Science backs double-breaks to beat annual ryegrass
First published September 2015, updated April 2026
