Depriving weeds of sunlight and space has been proven to work as a complementary tactic to take the pressure off herbicides as the ‘heavy lifters’ of in-crop weed control.
As part of a five-year Grains Research and Development Corporation (GRDC) investment project, crop competition factors are being assessed in a bid to boost weed control options for grain growers and agronomists in the northern region.
The Queensland Department of Agriculture and Fisheries (DAF) evaluated the effect of crop row spacing and density on the presence and growth of common sowthistle in faba beans and chickpea, and on early emerging awnless barnyard grass in wheat.
DAF program leader, Dr Michael Widderick (pictured) said by using crop competition alongside knockdown and in-crop residual herbicides, growers could delay herbicide resistance and limit future weed populations by reducing seed set.
Both awnless barnyard grass and common sowthistle have glyphosate resistant populations and common sowthistle resistance to chlorsulfuron in Queensland is also present.
“Awnless barnyard grass is increasingly emerging earlier and affecting winter crops when residual chemistries are no longer persistent in the soil and when in-crop herbicides are either unavailable or can no longer be applied,” Dr Widderick said.
“Results across faba beans, chickpea and wheat showed that growing these crops at a narrow row spacing and increased crop density reduced weed biomass and seed production, and we generally saw a yield benefit at narrow row spacing and consistently at high crop density.
“In widely spaced crop rows, weeds have a better chance to flourish due to plenty of available space for capturing light and other resources.”
Researchers found that:
• Growing a competitive crop can significantly reduce weed numbers, biomass and seed production in-crop while providing increases in grain yield.
• Growing faba beans and chickpea at a narrow row spacing of 25cm and a high plant density of 70 and 80 plants/m2 respectively significantly reduced common sowthistle biomass and seed production while increasing crop yield. While the crop density was well above industry practice, these results show the potential benefits this can have on competition against weeds.
• Growing wheat at a narrow row spacing of 25cm and a high density of 120 plants/m2 significantly reduced awnless barnyard grass density, biomass and seed production while increasing crop yield.
These trials were carried out at The Hermitage Research Station in southeast Queensland but Dr Widderick said the impact of row spacing and density on weed control and crop yield will differ with the growing environment.
To take into account factors such as soil type, climate and weed density, multi‐site crop competition trials are also being conducted in Wagga Wagga (NSW DPI) and Narrabri (University of Sydney).
Future multi-site trials will examine other agronomic factors that can influence the competitiveness of the crops, including variety choice, fertiliser placement and precision planting.