Home gardeners know the benefits of mulch in suppressing weeds, but researchers are also encouraging broadacre grain producers to deprive weeds of sunlight and space and take the pressure off herbicides as the ‘heavy lifters’ of in-crop weed control.

While defeating weeds through crop competition isn’t new, Queensland trials have shown that crop density and narrow row spacing have significantly suppressed the prevalent weeds common sowthistle and fleabane in the northern region.

The Queensland Department of Agriculture and Fisheries (QDAF) and the University of Queensland (UQ) are currently testing other non-chemical controls such as varietal differences and fertiliser placement for their impact on weed growth and seed production.

QDAF project leader Dr Michael Widderick says narrow row spacing has so far shown the clearest benefit in limiting weeds.

The tactic suppressed sowthistle and fleabane quite dramatically, but in trials testing control of feathertop Rhodes grass and awnless barnyard grass, the feathertop was quick growing and competed very well against sorghum and mung beans which didn’t suppress it as much as barnyard grass, which isn’t as robust a plant.

Both awnless barnyard grass and common sowthistle no have glyphosate resistant populations and sowthistle is becoming an all year round weed issue.

The trials showed 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 sowthistle biomass and seed production while increasing crop yield.

Growing wheat at 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.

Narrow row spacing (25cm) in mung bean and soybean can reduce weed growth and increased crop yield. The critical time of weed removal in mung bean and soybean occurs at later stages of crop growth in narrow (25cm) compared to wide rows (75cm).

Michael says these key weeds are increasingly emerging later and affecting winter crops at a growth stage when residual chemistries are no longer persistent in the soil and when in‐crop herbicides are either unavailable or can no longer be applied.

But by purposefully using crop competition alongside in-crop residual and knockdown herbicides, producers can delay herbicide resistance and limit future weed populations by reducing seed set.

Further information:

Dr Michael Widderick

Leslie Research Facility

Queensland Department of Agriculture & Fisheries Toowoomba, Qld

07 4529 1325

[email protected]

GRDC project code: US00084 https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2018/03/deprive-weeds-of-sunlight-and-space-crop-competition-research

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