HALVING sorghum row spacing can halve weed seed production in both feathertop Rhodes grass and awnless barnyard grass, according to four years of research at Narrabri in New South Wales and Hermitage in Queensland.
With investment from the Grains Research and Development Corporation (GRDC), researchers from the University of Sydney and Queensland Department of Agriculture and Fisheries (DAF) have conducted a range of field trials to identify ways to increase the competitiveness of sorghum and summer pulses.
DAF principal research scientist Michael Widderick said the findings from these field trials have shown that a change to narrower row spacing for sorghum greatly suppressed weed growth and seed production, without reducing crop yield.
"This is very significant for sorghum growers who have struggled with controlling these grass weeds in wide-row configurations," Dr Widderick said.
"These weeds are difficult to control with herbicides and there are few chemical options available to growers to control grasses in a grass crop.
"Any non-chemical strategies that reduce seedbank replenishment are very valuable to growers."
Sorghum is often grown on one metre row spacing with an expectation that the crop will have access to more soil moisture, however a considerable downside to planting on the wider row configuration is that canopy closure does not occur, allowing weeds to proliferate in the inter-row.
With soil moisture at a premium, there is nothing spare to waste on growing summer weeds.
For a sorghum crop to be competitive against weeds it requires adequate stored soil moisture (or access to irrigation) to establish the crop and achieve canopy closure as quickly as possible.
This is most reliably done at a row spacing of 50 centimetres and the trials demonstrated that row spacing did not significantly impact crop yield within a season.
Two of the most difficult to control summer weeds, feathertop Rhodes grass (FTR) and awnless barnyard grass (ABG) can produce 40,000 and 42,000 seeds per plant respectively.
Other studies have found these numbers could be even higher, so every effort to reduce seed production is worthwhile.
Both these species have populations confirmed as resistant to glyphosate and recently a population of FTR was confirmed to be resistant to haloxyfop (Group A).
Including a poorly competitive sorghum crop in the crop rotation provides a weak link in any strategy to reduce the weed seedbank for these weeds and potentially allows a blow-out in herbicide resistant biotypes, making future control in other crops or summer fallows very difficult.
Dr Widderick said sorghum competitiveness across all seasons and both sites was increased with narrow row spacing (50cm) and a plant density of 10 to 15 plants per metre squared.
"In the 2017/18 season at Hermitage, the researchers demonstrated that planting sorghum at a density of 10 to 15 plants/m2 reduced seed production of both weed species by more than 50 per cent compared to the seed production at the low crop density of fiveplants/m2," he said.
"In the same season, cultivar choice, sorghum density (5, 10, 15 plants/m2) and row spacing (50, 75 and 100cm) had no statistically significant effect on crop yield."
Armed with this information, the 2018/19 sorghum trial at Hermitage was sown at a crop density of 10 plants/m2 and the effect of row spacing (50 cm and 100 cm) on weed production was measured.
"Biomass and seed production of ABG was reduced by 55 per cent and 65pc, respectively when the sorghum was sown at the narrower spacing," Dr Widderick said.
"Similarly for FTR, the 50cm row spacing reduced biomass and seed production by 48pc and 56pc, respectively.
"There is now solid evidence that growers can maintain crop yield and reduce summer grass seed production by planting sorghum crops at a density of 10 plants/m2 and a row spacing of 50cm."