A four-year study into mixed pasture and cropping rotations has found that pasture breaks can significantly reduce annual ryegrass populations.
The 'Integrated farming systems in the medium-rainfall zone'study has been conducted through a strategic joint investment between GRDC and Primary Industries and Regions South Australia's research division, the South Australian Research and Development Institute (SARDI).
SARDI senior research officer, Amanda Pearce, says results from 2019 indicate that not only can pasture breaks lead to significant reductions in annual ryegrass numbers, but they can also improve the yield of subsequent wheat crops and boost soil nitrogen.
"In 2018, plots at both Sherwood and Bordertown (in SA) had higher ryegrass plant numbers following cereal crops," Ms Pearce says.
"We counted 34 ryegrass plants per square metre at Bordertown and 225 plants per square metre at Sherwood in cereals following cereals."
"But plots that followed a pasture break had only 23 ryegrass plants per square metre at Bordertown and 47 ryegrass plants per square metre at Sherwood."
In comparison, the trial plots planted with a single break of canola, beans or lentils and lupins the previous season had an average of 63 ryegrass plants per square metre at the Bordertown site and 128 plants per square metre at Sherwood.
This was replicated in 2019 - where continuous wheat rotations had ryegrass populations of 72 plants per square metre at Bordertown and 209 plants per square metre at Sherwood.
Double break results
Double breaks of subterranean clover and balansa clover pasture were incorporated into the research in 2019.
Plots planted to these double breaks had an average ryegrass population of only two plants per square metre at the Bordertown site and 26 plants per square metre at Sherwood.
"We have really been able to knock those ryegrass populations back with double breaks," Ms Pearce says.
"Even comparing a double pasture break to our single pasture breaks, we found ryegrass numbers were reduced by a further 20 plants per square metre at Bordertown and 125 plants per square metre at Sherwood with a double break of pasture."
The 2019 harvest repeated the increase in wheat yields that was observed at Sherwood in 2018 following pasture breaks.
"The 2018 wheat yield following one balansa clover rotation was 26 per cent higher than the yield from a wheat on wheat rotation at Sherwood," Ms Pearce says.
"We were able to show a very similar - 24 per cent - improvement in 2019 when wheat followed a subterranean clover break at Bordertown, compared to the wheat-on-wheat rotation."
Wheat achieved H1 quality with higher protein following a balansa clover, burr medic or faba bean rotation and H2 when following canola, oats, barley, lentils or lupins.
The 2018 wheat yield following one balansa clover rotation was 26 per cent higher than the yield from a wheat on wheat rotation at Sherwood.
Ms Pearce says the study is measuring a wide range of parameters, including:
- soil-borne diseases using PREDICTA® B profiling;
- soil moisture; and
- soil nitrogen levels.
"So far, the results are showing that pasture legumes are just as effective as legume crops for returning nitrogen to the soil," she says.
Initial analysis shows that balansa clover and faba bean rotations delivered the highest levels of residual soil nitrogen at Bordertown. Subterranean clover out-performed all other crops at Sherwood.
Soil nitrogen research continues
Research into residual soil nitrogen levels following dual pasture breaks and nitrogen persistence in following seasons is ongoing.
"We are also tracking grain yields, grain quality and plant biomass value for pasture and hay," Ms Pearce says.
"This will inform our final sensitivity analysis so we can present the economic impacts of incorporating pasture breaks in an integrated farming system."
The final season of the study will also provide more data about the soil benefits of double pasture breaks and the flow-on effects for second-season crops.
The outcomes of this project are being delivered to farmers via the strategic research partnership between GRDC and SARDI.
This partnership has facilitated a range of projects which provide innovative research outcomes relevant to SA's cropping zones.
More information: Amanda Pearce, 08 8762 9105, 0407 400 939, firstname.lastname@example.org