THE decay of problematic weed seeds such as annual ryegrass in chaff lines has been found to be highly dependent on summer and autumn rainfall tallies.
In a joint trial between Minnipa Agricultural Centre, SARDI and University of Adelaide weed researcher Gurjeet Gill, weed seedbank decline in nine chaff-lining systems across SA in 2018-19 was investigated.
The trial demonstrated that the practice, which has been widely adopted by growers, was beneficial to help manage annual ryegrass, brome grass and Indian hedge mustard, but seeds did not decay and therefore, further weed seed management strategies were needed.
Trial sites were located at Minnipa, Kimba, Cowell, Port Lincoln, Kadina, Maitland, Clare, Pinery and Adelaide.
Random sampling of four chaff lines at each site, as well as sub-samples at 0.5 metres, was undertaken.
Associate Professor Gill said sampling was undertaken from December to April and material was also put in germination trays in the first week May and watered close to field capacity.
"Weed seedlings were counted to determine weed seed decay in the summer-autumn period," he said.
"But it was a very dry period and the weed seeds did not decay - moisture is needed so microbes become active and rot the weed seeds.
"So in this trial, the benefit of decaying seeds did not happen but we have now realised growers must have a plan for chaff lines in place."
Across the nine sites, there were no consistent trends that demonstrated the susceptibility of annual ryegrass to decay in chaff lines.
But, there was a slight reduction in annual ryegrass at two Mid North sites and one on the Eyre Peninsula.
But Associate Prof Gill said this was most likely because of natural variability at the sites.
"Further research found it was not significant," he said.
"The trials still showed that the weed seed capture tactics could have an important role in reducing weed seedbanks."
The trial plots also showed that high residual annual ryegrass, brome grass and Indian hedge mustard seedbanks were established in the paddock because the species did not breakdown over time.
He also said that annual ryegrass seedbank decline in particular was independent of the chaff line configuration and chaff density.
But despite finding out new information about the behaviour of weed seeds, further research to narrow down how to better-manage chaff-lines during dry years was needed.
"Burning can be difficult but it is better than weed seeds spreading across a paddock, but mowing the chaff lines could also be an option to stop seed set," Associate Prof Gill said.
MORE WEED DECAY RESEARCH NEEDED
GROWERS are encouraged to undertake a significant evaluation of weed seed capture in chaff lines before sowing and advised to expect variability in the effectiveness of this tactic between seasons.
The University of Adelaide's Gurjeet Gill was a project leader for an SA trial to examine seedbank decline of weed species in chaff lines made at harvest.
Associate Professor Gill said many growers had adopted the practice because it was not costly to modify a harvester to confine the chaff into a narrow row or into dedicated wheel tracks in controlled traffic farming systems.
"Failure to control weed species that persist through cropping phases aids establishment of weed seedbanks," he said.
"Implementation of targeted control of these chaff lines is, however, very necessary to mitigate seedbank replenishment."
The trial held in 2018-19 assessed the rate of weed seed decay before the next sowing but results consistently recorded "plenty" of ryegrass germinating within chaff lines.
A key take-home message from the trial, was that a very dry summer will most likely impact weed seed decay.
"If it is a wet summer, we would anticipate a different outcome but growers need to be prepared for chaff lines to be inconsistent for control until we are able to do more research," Associate Prof Gill said.
"Funding for a three- to five-year trial would help determine this."
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