WeedSmart suggests faba beans can sub common sowthistle

According to information gathered by WeedSmart, faba beans are more competitive than other winter pulse crops, such as chickpea, and can play a useful role in reducing sowthistle seed bank replenishment.

Department of Agriculture and Fisheries, Queensland, principal research scientist (weeds) Michael Widderick said trials conducted in 2019 showed the right faba bean variety grown at high density and on narrow row spacing could reduce sowthistle seed production by up to 48 per cent.

According to Dr Widderick, in the trials - which were held at Narrabri, New South Wales and Hermitage, Queensland - narrow row spacing suppressed sowthistle biomass by as much as 68pc, resulting in a faba bean grain yield increase of up to 20pc

Common sowthistle is a challenging weed for many growers as its light, fluffy seed is easily spread, carrying any genes for herbicide resistance with it.

Widely known as a weed that readily takes advantage of gaps in a crop, sowthistle struggles to thrive when faced with stiff crop competition.

The seed does not persist for long on the soil surface, so any reduction in seed production will make a difference to future weed pressure.

"In broadleaf crops such as faba bean, there are few in-crop herbicide options, and it is not easy to collect and destroy sowthistle seed at harvest, so other tactics are needed to attack the weed seed bank," Dr Widderick said.

"Crop competition is a key component in the WeedSmart Big 6 integrated weed management program.

"Trials repeatedly show increased crop competition can reduce weed seed production and increase grain yield, and faba beans are no exception."

In the trials, narrowing faba bean planting to 25 centimetres row spacing reduced sowthistle seed production by 48pc at Narrabri and 17pc at Hermitage.

At the Narrabri site, sowthistle seed production was reduced by 48pc when row spacing was 25cm compared to 50cm, averaged across the two plant densities and three cultivars, while at the Hermitage site, narrowing the row spacing to 25cm reduced sowthistle seed production by 17pc.

Faba bean grain yield, averaged across the three cultivars, was 13pc and 20pc higher in plots sown at 25cm row spacing at Narrabri and Hermitage respectively.

On top of that, higher plant density - 30 plants per square metre - reduced sowthistle seed production by 29pc at the Hermitage site.

However, at the Narrabri site, faba bean crop density did not affect sowthistle seed production or crop yield.

"At the Hermitage site, increasing faba bean density from 20 to 30 plants/m2 caused a 29pc decrease in sowthistle seed production," Dr Widderick said.

"Increased crop density reduced weed seed production in all cultivars.

"Faba bean yield, averaged across the three cultivars, was 11pc or 0.2 tonnes per hectare higher where the crop density increased from 20 to 30 plants/m2."

Overall, PBA Nasma (Narrabri) and PBA Nanu (Hermitage) were the most weed suppressive cultivars.

According to Dr Widderick, cultivar choice was the easiest and cheapest lever to pull to increase crop competitiveness.

However, the effect was less reliable than other crop competition tactics and depended upon the prevailing environment.

Dr Widderick said at Narrabri, PBA Nasma suppressed weed seed production by 38pc more than PBA Warda and PBA Marne.

"PBA Nasma also yielded 9pc higher than PBA Marne and PBA Warda when grown in the presence of 10 sowthistle plants/m2," he said.

"At Hermitage, PBA Nanu was more weed suppressive than PBA Warda and PBA Nasma.

"With narrow row spacing and high crop density, it reduced sowthistle seed production by 45pc compared to low crop density configurations, but that trend was also seen in PBA Nasma and PBA Warda."

Overall, PBA Warda was poorly competitive on wide rows and low crop density, allowing an average 51pc higher sowthistle seed production than the other two varieties.

However, at wide row spacing, increasing plant density in PBA Warda reduced sowthistle seed production by 44pc

At Hermitage, all three faba bean cultivars grown on narrow row spacing yielded on average 23pc more grain at the high crop density than at the low-density configuration.

Want weekly news highlights delivered to your inbox? Sign up to the Farm Weekly newsletter.