Key points
- Increasing crop competition by using narrower row spacings and/or increasing crop density can reduce weed burdens
- Over six years, trials across the northern region looked at mungbean, sorghum, faba beans and chickpeas
- The weeds included were awnless barnyard grass, feathertop Rhodes grass and sowthistle
- Lead researcher DAF principal research scientist Dr Michael Widderick says the
overall message is to consider growing a competitive crop
At both ends of the weed spectrum, increasing crop competition by using narrower row spacing and/or increasing crop density has great potential to reduce both weed pressure and herbicide use.
Explaining results from six years of research, Queensland Department of Agriculture and Fisheries (DAF) principal research scientist Dr Michael Widderick says that crop competition can fit in a problematic paddock.
“That might be where you have a large density of weeds or maybe herbicide-resistant weeds. If you grow your competitive crop in combination with your herbicides, you should get an additive effect.
“You’re very rarely going to get 100 per cent control with herbicides. But if you combine them with a competitive crop, you should be able to then suppress weed growth and weed seed production,” he says.
At the other end of the scale is low weed density. “You may actually be able to get away without applying herbicides to the crop by growing a competitive crop.”
However, an integrated weed management approach incorporating more than a single tactic will give the most consistent control.
Dr Widderick and researchers from the University of Sydney and the New South Wales Department of Primary Industries have investigated the impact of crop competition on weeds. With investment from GRDC, the trials at Narrabri, Wagga Wagga and Hermitage pitted mungbean and sorghum against awnless barnyard grass and feathertop Rhodes grass, and faba beans and chickpeas against sowthistle.
The effect of crop row spacing, crop density, cultivar and a combination of row spacing and crop density was measured against weed growth, weed seed production and crop yield. No herbicides were used.
The results varied. For example, narrowing mungbean row spacing and increasing crop density led to a significant reduction in awnless barnyard grass seed production in all environments. In 94 per cent of test environments this approach either increased or had no effect on mungbean yield.
Similarly, a narrow row spacing in sorghum resulted in a significant reduction in awnless barnyard grass seed production in 27 per cent of environments with no impact on sorghum yield. Adding increased crop density caused a reduction in weed seed production across all environments while maintaining or increasing sorghum yield.
In most cases, where a more competitive crop was grown, crop yield was maintained and, in some cases, there were yield benefits. In a small number of environments there was a reduction in yield at the more competitive crop configurations, and this was more likely for faba bean and chickpea when the crops were grown both narrow row spacing and increased density.
Dr Widderick says the overall message is this: consider growing a competitive crop. “What we have seen across these different crops is that reducing row spacing and increasing crop density, either by itself or in combination, has generally resulted in a reduction in weed growth and production.
“There is an added benefit that a more competitive configuration mostly maintained crop yields, while in some cases had yield benefits. But be aware that in some cases crop competition has had a negative impact on yield.”
Table 1. Effects of narrow row spacing, increased crop density or a combination of both in mungbean on awnless barnyard grass (BYG) biomass, seed production and yield. (A reduction or increase is statistically significant.)
Agronomic factor | BYG biomass | BYG seed production | Mungbean yield |
|---|---|---|---|
Narrow row spacing | Reduction in 22% | Reduction across | Increase in 16% of environments by 18 – 53%. |
Increased crop density | Reduction in 18% | Reduction across | Increase at 6% of environments by 32 – 51%. |
Narrow row spacing × increased crop density 25cm × 30/35 plants/m2 vs 50cm × 20 plants/m2 | Reduction across | Reduction across | Increase at 13% of environments by 20 – 21%. |
Table 2. Effects of narrow row spacing, increased crop density or a combination of both in sorghum on awnless barnyard grass (BYG) biomass, seed production and yield. (A reduction or increase is statistically significant.)
Agronomic factor | BYG biomass | BYG seed production | Sorghum yield |
|---|---|---|---|
Narrow row spacing | Reduction in 35% | Reduction in 27% | No effect. Yield maintained. |
Increased crop density | Reduction across | Reduction across | Increase in 18% of environments by 17 – 51%. |
Narrow row spacing × increased crop density | Reduction in 44% | Reduction across | Increase in 35% of environments by 42 – 78%. |
Challenges
Competitive crops will need more resources – in particular water – and this should be factored into planning.
“Rain is out of our control. So, you don’t have to grow a competitive crop in every paddock or every season. If next season is favourable forecast-wise or you have a better than expected, stored soil water profile, you might give it a go in combination with herbicides in a problematic paddock, or in a paddock that’s pretty clean.”
Dr Widderick says that if changing row spacing is inconvenient, consider changing crop density. “A lot of growers are already set up for specific row spacing and may not want to alter machinery. That’s okay.
“Our research has shown an increased crop density, which doesn’t require machinery change, can provide competitive advantages against weeds that equal the effects of narrowing row spacing. However, combining a narrow row spacing with an increased crop density provided the greatest weed suppression advantages.”
Dr Widderick presented results from the winter pulse research at the GRDC updates.
More information: Michael.Widderick@daf.qld.gov.au, GRDC Update Paper: Crop competition effects on weeds and crops