Fifty years ago, many farmers and researchers expected herbicides to be the panacea for weed control.
The reality has been that no weed species has been eradicated through the use of herbicides alone, and in fact many weeds have increased in population in the presence of herbicide application in farming systems.
This is what motivates Queensland Alliance for Agriculture and Food Innovation principal research fellow Dr Bhagirath Chauhan to study how crops could manage their own weed control with the application of cultural practices.
He said his agronomic trials in summer legume (mungbean and soybean) crops were consistently showing that early canopy closure resulted in lower weed biomass and higher crop yield.
‘‘We have demonstrated that narrower row spacing in summer legumes such as mungbean and soybean will take the pressure off in-crop herbicide applications, provided the crop is sown into clean paddocks and weeds are controlled for at least three weeks after planting,’’ he said.
‘‘Increased yield is a significant benefit and will support growers’ decisions to set up their planting gear to suit farming on narrower row spacings.’’
In the 2015 and 2016 mungbean seasons, 30kg/ha of Jade-AU was grown at 25, 50, and 75cm row spacing and Rhodes grass was sown to provide a known weed density in all plots.
Rhodes grass was spread at 300 seeds/sqm to create even competition across the site, which is generally not possible if researchers rely on the natural weed seed bank of the experimental site providing the competition.
‘‘In plots where the mungbean crop and the weed emerged together, it didn’t matter which row spacing was used — the weed biomass was high and the grain yield was low, less than 360kg/ha,’’ Dr Chauhan said.
‘‘If the crop is kept weed-free for the first three weeks after planting then the narrower row spacings of 25 and 50cm saw a reduction in weed biomass.
‘‘We know from other studies that weed biomass correlates well with weed seed production, so reducing biomass can be expected to also reduce seed production in the weed.’’
In both seasons, the combination of keeping the crop weed-free for at least the first three weeks and planting on the narrower rows (25 or 50cm rather than 75cm) generated a yield increase of 159 to 197 per cent in 2015 and 198 to 223 per cent in 2016.
Even in a completely weed-free growing environment the two narrower spacings generated higher yield than the 75cm rows.
‘‘This trial demonstrates the value of reducing row spacing to 50cm in mungbeans and keeping crops weed-free for the first six weeks after planting,’’ Dr Chauhan said.
‘‘After this point the crop has the competitive edge and any later germinating weeds struggle to get established.’’
■For more information about improving pulse crop competitiveness, visit the WeedSmart website: www.weedsmart.org.au