Pasture breaks found to 'knock' annual ryegrass populations

The 'Integrated farming systems in the medium-rainfall zone' study has been conducted through a joint investment between the GRDC and 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 following wheat crops and boost soil nitrogen.

"In 2018, plots at both Sherwood and Bordertown had higher ryegrass plant numbers following cereal crops," she said.

"We counted 34 ryegrass plants per square metre at Bordertown and 225 plants/sqm at Sherwood in cereal following cereals. But plots that followed a pasture break had only 23 ryegrass plants/sqm at Bordertown and 47 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/sqm at the Bordertown site and 128 plants/sqm at Sherwood.

This was replicated in 2019 with continuous wheat rotations having ryegrass populations of 72 plants/sqm at Bordertown and 209 plants/sqm at Sherwood.

Double breaks of subterranean clover (sub 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/sqm at the Bordertown site and 26 plants/sqm at Sherwood.

"We've really been able to knock those ryegrass populations back with double breaks," Ms Pearce said.

"Even comparing a double pasture break to our single pasture breaks, we found ryegrass numbers were reduced by a further 20 plants/sqm at Bordertown and 125 plants/sqm 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 said.

"We were able to show a very similar 24pc improvement in 2019 when wheat followed a subclover 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.

Ms Pearce said the study was measuring a wide range of parameters, including soilborne diseases using PredictaB profiling, soil moisture and soil N levels.

"So far the results are showing that pasture legumes are just as effective as legume crops for returning N to the soil," she said.

Initial analysis shows balansa clover and faba bean rotations delivered the highest levels of residual soil N at Bordertown, while sub clover out-performed all other crops at Sherwood.