LONG-term planning of crop rotations is the only way to maintain a profitable system in WA as growers contend with growing seed banks and depleting soil nutrition.
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A study by CSIRO principal research scientist Roger Lawes has debunked the profitability myth of short-term rotations.
Dr Lawes presented his findings at the Grains Research and Development Corporation's Grains Research Updates in Perth last week, warning continuous cropping was only profitable in the short-term.
His modelling with Kojonup growers has shown it effectively hides the costs of dealing with weeds and diseases.
"Looking at crops rotations around Australia it became apparent people were focusing on what was happening next year," Dr Lawes said.
"So there's this idea of 'OK what problems have I got right now? And how will I deal with them next year?
"But we need to be asking 'is there any need to think beyond that extra year? Should we be looking at a planning horizon of three, five and 10 years?"
Dr Lawes developed a modelling system on actual farming results which uses varying timelines, cropping rotations, commodity prices and yield to illustrate the importance of thinking beyond the next season and was surprised with some of the results.
"In the short-term, you need extraordinarily high seed bank numbers to force a change, about 600-800 seeds per square metre, to force a rotation change," he said.
"But as you stretch out beyond five and 10 years, the number of seeds you need in that seed bank to force an economically sensible change, almost regardless of commodity price, is much much lower. It is more like 100-250 seeds/m2.
"Once you get that sort of number, if you don't do something now you're going to pay for it might later on.
"This is where short-term and long-term thinking is vital.
"On paper it may look like you can get away with pushing cereals really hard, but it's important long-term to keep an eye on what those problems are because you will pay for them."
In an optimisation test, Dr Lawes outlined several scenarios to session attendees and highlighted that continuous wheat cropping performed solidly given a two-three year outlook, as did a wheat, canola, wheat rotation, but beyond five years its profitability dropped sharply.
In fact, several systems showed minimal profitability or losses beyond the five-year horizon.
"You need to reset the system to get it back on track," Dr Lawes said.
"As the planning horizon increases, and the price of wheat decreases, the number of weed seeds necessary to trigger a break crop or pasture decrease.
"Therefore, as profit margins tighten, the need to successfully manage the weed seed bank through the rotation increases.
"In the short-term, it is more profitable to grow the most profitable crop.
"But at some point, usually after three years, a break crop will need to be grown.''
Dr Lawes said a one-year break made a significant difference to the system's profitability.
Rachael Oxborrow
Grains writer for Farm Weekly.
Grains writer for Farm Weekly.
