Lift sowing to outcompete weeds

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There is a widespread belief that lifting sowing rates results in increased screenings, but GRDC-funded research in Western Australia shows higher crop densities can significantly lower weed burdens without any impact on wheat quality

Photo of machinery

Increasing the sowing density of wide row wheat crops by using a stiletto boot (pictured) to create paired crop rows can halve ryegrass seed-set through enhanced crop competition.

PHOTO: Peter Newman

Crop competition is the unsung hero of herbicide resistance management.

Department of Agriculture and Food, Western Australia (DAFWA) research in the northern WA wheatbelt shows that increasing the wheat sowing rate can decrease seed-set of annual ryegrass by nearly 80 per cent.In addition to outcompeting ryegrass, increasing the wheat density lifted the efficacy of the pre-emergent herbicide Sakura® from a 95 to 99 per cent seed-set reduction of ryegrass.

The concept of crop competition in weed management is not new, but in recent years many growers in southern Australia have inadvertently undermined the competitive capacity of their crops by increasing row spacing, lowering sowing rates and sowing uncompetitive cultivars.

While the shift to wider rows and lower sowing densities has been based on valid reasoning, the question remains of how much these practices are costing growers in terms of crop yield and weed control.

With widespread multiple herbicide resistance now the norm in WA, growers need all the non-herbicide tools available to them to help control the number of weeds.

Lifting density in wide rows

In the DAFWA research, crop density was increased using stiletto boots to generate paired cropping rows 75 millimetres apart, effectively reducing row spacing by the same amount without having to add extra tynes to the seeder.

Paired-row sowing doubles the length of the cropping row without significantly affecting row spacing. The paired rows result in less in-row competition between crop plants than the same sowing density in single rows, enabling more seeds to germinate and establish at higher sowing rates. 

Existing sowing machinery can be modified for paired-row sowing so that the tyne spacing that is best for stubble handling and sowing speed can remain the same.

Even in the absence of a pre-sowing grass-selective herbicide, ryegrass seed-set was reduced by as much as 65 per cent with an increase in crop density from 40 to 160 kilograms per hectare.

FIGURE 1 Impact of wheat sowing density on ryegrass
seed production.

Green bar graph showing ryegrass seeds/m2 compared to seeding rate/wheat density
SOURCE: Department of Agriculture and Food, WA

Combined with the pre-emergent grass-selective Sakura®, crop competition was further enhanced and ryegrass seed-set reduced from 811 seeds per square metre at the 40kg/ha sowing rate to 178 seeds/m2 at the 120kg/ha seed rate (Figure 1).

The density research found what many crop competition trials have repeatedly shown: as crop density increases, crop biomass increases and weed growth and seed-set decrease. The challenge is to do this in a practical and cost-effective way. The stiletto boot system requires only a minor adaptation to existing machinery and the paired rows do not interfere with stubble management or time of operation.

The big question surrounds economics and how viable it is to increase the sowing rate to reduce weed seed-set. The research showed that spending $15/ha on increasing the sowing rate could halve ryegrass seed-set. While this is unlikely to increase crop yield in the short term, with very few non-herbicide weed-control tools available enhancing crop competition against weeds could prove very valuable in the future.

Investing in non-herbicide tools, such as chaff carts and crop competition, enables the paddock to remain in crop by keeping the number of weeds low.

Future research may show there is no need to go to extremes of sowing rates, but there is little doubt that there is significant room for improvement in the area of crop density and control of herbicide-resistant weeds.

More information:

Peter Newman, AHRI,
08 9964 1170,

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GRDC Project Code DAW00196, DAW00218

Region West