The higher rainfall across many grain growing regions in 2016 is providing farmers with more opportunities for summer cropping. Weeds also stand to gain from the additional soil moisture putting additional pressure on summer fallow spraying programs.

Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation (QAAFI) says agronomic trials measuring the effect of early canopy closure in summer crops are consistently resulting in lower weed biomass and higher crop yield.

“Our research in mungbean, cotton and soybean have shown that more even plant spacing across the paddock is more important for weed suppression than increased seeding rate, and this is best achieved through narrower row spacing,” he says.

“In cotton, we demonstrated a clear benefit in planting at 50 cm row spacing rather than the conventional 100 cm spacing however, the limitation for growers is the inflexibility of the current harvesting equipment.”

One configuration that has shown promise internationally but not yet fully investigated in Australia is the ultra-narrow row (UNR) concept where the beds remain at 100 cm spacing to suit the harvester but two rows of cotton are planted either side of the bed, effectively shading the inter-row earlier than a single row planting. The ultra-narrow rows are planted 19 to 38 cm apart on the bed and seeding rate is usually increased slightly.

With limited options to increase crop competition in cotton, and the widespread adoption of Roundup-Ready (RR) technology in the industry, there is now a focus on finding alternative herbicide chemistry to manage the risk of glyphosate resistant weeds in cotton systems.

Dr Chauhan says growers are achieving good weed control with the pre-emergence herbicides registered for use in cotton. “The biggest challenge with these herbicides is getting the application right, taking into account the effect of rainfall, irrigation type and timing and the soil type,” he says. “There are emerging weeds such as feathertop Rhodes grass, sesbania and amaranth that are challenging the Roundup-Ready cropping system and so growers need to have other weed management tactics in place early.”

The cotton industry is promoting the adoption of the 2 + 2 + 0 weed management system to protect glyphosate and the Round-Up Ready hybrids. This entails the use of two non-glyphosate herbicide options, two non-herbicide tactics and zero weed survivors. Re-introducing the use of pre-emergent herbicides in cotton farming is an important part of this weed management program. Increasing crop competition is also worth further investigation given the potential weed control and crop yield benefits to be gained if the limitations of current harvesting equipment can be addressed.

To maintain yield in cotton it is important to restrict all weed management operations to the early stages of crop growth, a distinct advantage of using pre-emergent herbicides to minimise weed growth prior to crop canopy closure.

Peter Newman, communication lead with Australian Herbicide Resistance Initiative, has long been an enthusiastic advocate of crop competition in cereals. He says the findings in summer crops provides growers with a valuable non-herbicide tool they can use to help suppress weeds.

“Over and over we are seeing results come from crop competition trials showing suppression of weed biomass in competitive crops, and usually a yield benefit,” he says. “This is a win win for growers and needs to become standard practice in all crops – not only regarding row width but all agronomic practices that boost early crop growth and result in early canopy closure.”