HARVEST weed seed control can still impact the seed bank of notorious early-shedders, such as great brome grass and barley grass, according to the Department of Primary Industries and Regional Development (DPIRD).
Many weed species shed seed before the grain crop is ready to harvest, but despite that, harvest weed seed control (HWSC) can be effective against such weeds.
DPIRD weeds researcher Catherine Borger said they studied several populations of great brome grass and barley grass in Western Australia and South Australia.
"What we found is that firstly there is a lot of variability in how these weeds behave in different seasons," Dr Borger said.
"Secondly, even relatively low levels of weed seed capture at harvest can make a big difference to reducing the weed seed bank."
The research being conducted by DPIRD is part of a national Grains Research and Development Corporation (GRDC) investment in better understanding the ecology of key weed species in each region.
"Great brome grass and barley grass cost farmers in the southern and western cropping regions about $22 million and $2m annually respectively, in control costs and lost production," Dr Borger said.
"Great brome grass and barley grass are problematic weeds on 1.4 million hectares and 235,000ha of farming land respectively across these two regions.
"Consequently, farmers are spending more than $3m a year on additional herbicide costs to manage herbicide resistant great brome grass."
The weed ecology work on these two species showed that an integrated control program can effectively run down the seed bank for both species in three or four years.
It also showed that staggered emergence, particularly in brome grass, means that end-of-season control tactics must be included in the strategy.
In terms of seed retention at harvest for the two weeds, in some years it is quite low, but in other years a large proportion of the seed is still in the seed heads at harvest and beyond.
Dr Borger said seed shedding was not well understood and was driven by a complex combination of genetic and environmental factors.
"Harvest date obviously has a large bearing on the amount of weed seed still on the plants at harvest," she said.
"In paddocks with high or increasing weed numbers it may be worth harvesting as early as possible to maximise the benefit of HWSC.
"In 2016 to 2018 great brome seed retention at the Wongan Hills site in WA was between 40 and 70 per cent at crop maturity (around mid-November), however a later harvest date in 2016 resulted in almost no seed being present on the plants at harvest."
It was a similar story for barley grass, in 2016 all the seed shed by harvest in December and in 2017 and 2018 seed was still on the plants well into summer.
HWSC can be a useful control tactic, particularly for the highly competitive great brome grass and even capturing 20 to 40pc of the weed seed produced can make a big difference to future weed pressure.
Dr Borger said it was difficult to control weeds that exhibited staggered germinations during the cropping season with herbicides alone.
"Both these weeds can be difficult to get into the harvester, great brome can bend forward and slip under the cutter bar, while barley grass seed heads are often held very close to the ground," she said.
"While barley grass might be almost impossible to get into the header it is also much less competitive in the crop than great brome.
"Modelling with the Weed Seed Wizard decision support tool showed that if the header is able to capture just 20pc of the great brome grass seed produced, the seed bank can be halved over a six-year rotation."
The research conducted by DPIRD showed that consistently collecting and destroying 60pc of the great brome seed each year could reduce the weed seed bank from almost 11,000 seeds to just 86 at the end of a six-year rotation.
Any herbicide tactic applied early in the season that only achieved 20pc control would be considered a waste of time, which highlights the value of late season weed control tactics such as HWSC.
According to Dr Borger, great brome grass can be brought under control in three or four years if an integrated weed management plan is implemented.
"Under irrigation, about 40pc of the seed germinated in the first year and almost all the seed had germinated by the end of the third year," she said.
"In field conditions a similar pattern was recorded for both great brome and barley grass.
"If control tactics are used to stop seed set then it is possible to reduce weed numbers within a few years."
Great brome populations in SA were found to exhibit more delayed emergence traits than populations in WA.
This could be due to the longer history of pre-emergent herbicide use in SA that has resulted in the evolution of delayed emergence to avoid early herbicide application.
However, in both WA and SA, barley grass populations exhibited staggered emergence.
"Great brome is highly competitive and is a costly weed for growers, particularly in low crop yield seasons," Dr Borger said.
"However, when moisture is not a limiting factor, crops can often produce good yield even when high weed numbers are present."