The effect of rotation is clear – the canola/wheat/wheat rotation (left) had 31 kilograms of nitrogen per hectare applied, while the vetch/canola/wheat rotation (right) had 9kg of N/ha applied.
PHOTO: Michael Moodie
Rotations in the Mallee that have a break crop for two consecutive years are proving more profitable than rotations with only one break crop. This is the finding so far of the GRDC Low-Rainfall Crop Sequencing Project, conducted by the South Australian Research and Development Institute (SARDI) and low-rainfall farming groups including Mallee Sustainable Farming.
The trial’s aim is to identify the effects of different break crops and rotations on Mallee farming systems in long-term cereal paddocks where agronomic constraints such as high brome grass levels have emerged.
Mallee Sustainable Farming’s Michael Moodie says the benefits of a break crop were large: an increase of 0.5 to 1.25 tonnes per hectare was achieved following a two-year, non-cereal break phase compared with continuous wheat.
“Growers often comment that they will only grow a break crop if it is profitable. But in our trial, eight of the most profitable crop sequences over the three-year period had two-year breaks,” he says.
“The most profitable rotations had at least one break phase and a highly profitable wheat crop as a result of the break crop benefits.”
The trial site was 30 kilometres west of Mildura on low-fertility sandy soil, where cereals had been cropped 10 years in a row and brome grass was becoming hard to control.
In 2011, nine different break crops were sown and compared with continuous wheat. In 2012, a second phase of break crops were sown to give a two-year break, or the rotation was returned to wheat to measure the effects of a one-year break.
In 2013, all rotations were sown to either the conventional wheat variety Shield or, if brome grass levels were rising, to the Clearfield® wheat Grenade CL Plus. Plots were sown again to wheat in 2014 to monitor the ongoing effects of rotations on cereal production.
FIGURE 1 0 to 60cm soil nitrate (kg/ha) measured prior to seeding in 2012 and 2013.
Click the image above to enlarge
Wheat yields in the 2013 phase were about 1 to 1.5t/ha for crops following a one-year break from continuous wheat, or 2 to 2.6t/ha after a two-year break. The benefit of the two-year break had little to do with the phases chosen for those two breaks.
Mr Moodie says the trial has shown the benefit of a one-year, non-cereal break crop only lasts a single season, partly due to the rapid re-establishment of brome grass in the following year. High brome grass numbers re-emerged two years after a one-year break.
The trial has also measured soil nitrogen, soil disease and plant-available water.
“By quantifying the agronomic benefits that break crops can provide in Mallee cropping rotations, growers can be confident of the long-term benefits of more diverse crop sequences,” Mr Moodie says.
The trial found that including legume crops and pastures in the rotation increased soil nitrogen even two years after the legume was grown.
In 2012, 0 to 60 centimetre soil nitrate levels were highest under vetch and in 2013, the four rotations where brown manure vetch had been included also had the highest soil nitrate levels.
Rhizoctonia was the most prevalent cereal root disease at the trial site, with inoculum levels increasing where cereals were grown in 2011 and 2012.
“Rhizoctonia inoculum levels are highest following cereal crops and high rhizoctonia levels correspond with poor root health,” Mr Moodie says.
“In the trial, where wheat in 2013 was grown on wheat in 2012, the soil inoculum levels pointed to a high risk of rhizoctonia infection, with possible yield losses of 10 to 50 per cent.”
Crop sequences had a small and variable impact on plant-available soil water. In 2012, there were no major differences between any of the treatments because high summer rainfall had filled the soil profile. But in 2013, the fallow treatment stored about 28mm more water than land under continuous wheat.
Table 1 lists the top 10 of the 21 most profitable rotations in the trial, which ranged from the top cumulative gross margin at $765/ha down to fallow/fallow/wheat at $303/ha.
Table 1: Wheat yield and grass weed burden.
2011 gross margin ($/ha)
||2012 gross margin ($/ha)
||2013 gross margin ($/ha)
||Cumulative gross margin ($/ha)
| Medic pasture/medic pasture/wheat
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Full trial results are available at: http://msfp.org.au/resources/research/2013compendium
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