Break crops hit weeds

16 Sep, 2015 02:00 AM
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It is cheaper and more effective to control ryegrass using one of the many break crop options

IS crop rotation an economic option for managing weeds?

Tony Swan, Senior Experimental Scientist, CSIRO Agriculture, says 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 broadleaf break crop to the cropping sequence helps keep wheat profitable in a sustainable cropping system, he says.

Mr Swan says their five-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,” Mr Swan says.

“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 less ryegrass numbers after three years, provided all the available tactics were used to reduce germination and prevent seed set.

“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?

Short answer:RR canola, TT canola, lupins for grain, field peas for brown manure, fallow and wheat cut for hay.

Longer answer: The combination of a fallow or break crop in year one followed by a second break crop in year two resulted in the greatest reduction in annual ryegrass seed bank population and panicle number after three 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?

Short answer: A two-year break crop option.

Longer answer: 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 three-year gross margins of $790/ha/year and $834/ha/year, respectively.

This compared to the most profitable three-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 grass herbicides cost over $140/ha in the wheat crops.

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 three year sequence of wheat-hay (sprayed afterwards) in year one, pulse-grain (spray topped) in year two, and RR canola in year three 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?

Short answer: Two consecutive years of total annual ryegrass control using break crops and implementing all available weed seed control options.

Longer answer: Break crops work and can be profitable. Two or more years of effective ryegrass control using break crops and other management options including strongly competitive crops, rotating herbicide groups, pre and post emergent timing and prevention of seed set using crop topping, hay making and brown manuring along with fallow management and harvest weed seed control such as narrow windrow burning.

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READER COMMENTS

Mike Tancock
16/09/2015 9:26:09 AM

This concept was achieved thousands of years ago hence "Crop Rotation" with a year of extremely clean fallow included. It was the time of chemical free farming not by choice but by circumstance. Whilst farmers are generally a mean, greedy and hungry lot we will continue to see the results of over use in chemicals eventually being severely injurious to the health of the general populace, not just land dwellers.
John Newton
18/09/2015 6:48:33 AM

Beats glyphosate and doesn't engender resistance. Funny how the old methods often turn out to be the best methods. Great Granpa was no dunce.
Zero till
18/09/2015 10:54:07 AM

They are talking break crops and chemical fallow not ploughing. Granpa would have used roundup if it was invented. Ploughing the paddock 6 times per year may be a wholesome recreation for old timers and fruitcake organic zealots but its a step backward for farmers who value soil quality and topsoil and not watch your years work of ploughing blow away.

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