COVER crops are being trialled to suppress weeds in fallow fields in southern Queensland.
Department of Agriculture and Fisheries researcher Dr Annie Ruttledge has been running experiments at Kingaroy to investigate the benefits of bringing crop competition into the fallow phase of cropping systems.
Dr Ruttledge said weeds generally did not compete well with vigorous crops, but in the fallow they could rapidly take advantage of the lack of competition for resources.
With investment from GRDC the cover crop project, led by Charles Sturt University, is investigating the weed-suppressive power of cover crop species suited to either summer or winter fallows at three locations in the northern grain growing region, Kingaroy, Narrabri and Wagga Wagga.
At the Kingaroy site, both winter and summer-growing cover crops were shown to suppress weeds by over 85 per cent and up to 95 per cent in some cases, compared to an untreated fallow where the sown weeds were not inhibited by a cover crop.
A chemical or non-chemical tool was then used to terminate the cover crop and kill any survivor weeds.
Dr Ruttledge said grazing oats and tillage radish were the best cover crops for weed control in Kingaroy in winter.
"These species provided early season ground cover and suppressed our mimic annual grass weed, Italian ryegrass, by up to 94 per cent relative to the weeds-only fallow," she said.
"None of the cover crop species we tried were able to suppress the quick-growing mimic broadleaf weed, Oriental mustard."
In summer, the best cover crop options for Kingaroy were white French millet, Japanese millet, forage sorghum and buckwheat.
Again, early-season biomass and ground cover was the key to suppression of both grass and broadleaf weed mimics by up to 95 per cent when compared to the weeds-only fallow.
Dr Ruttledge said so far in the trial, there has been no measurable weed suppression benefit in sowing mixed species cover crops rather than monocultures.
However, a mixed species cover crop may be preferred if a grower wanted to achieve multiple outcomes, like selecting grazing oats as a fast growing and highly competitive species, teamed with a less competitive legume to boost soil nitrogen stores.
"Cover crops also provide many other services to the farming system and so the grower could select a cover crop species, or mix of species, that would also provide a break from disease or insect pressure, increase moisture infiltration, build up organic matter or break down compaction."
Dr Ruttledge said light interception was a critical force in the effectiveness of cover cropping for weed control.
"In selecting cover crops for weed suppression, choose species that grow well in your locality and that restrict light penetration to the soil through strong early growth and the development of a dense canopy," she said.
"For greatest benefit, terminate cover crops at maximum biomass, which should coincide with the beginning of flowering; however, earlier termination may be required if soil moisture is limiting."
