Keeping weed numbers low pays off

Lead researcher and data analyst, CSIRO’s Jackie Ouzman, says growers’ investment in weed control is minimising yield losses from residual weeds growing in crops and fallows.

“From an economic perspective, the optimal level of weed control occurs when the marginal benefit of investing in an additional unit of weed control no longer outweighs the marginal cost,” she says. “When both the current-season yield benefits and the long-term advantages of reducing the weed seed bank are taken into account, the investment in keeping weed density low usually pays off.”

This latest report on the impact of weeds in broadacre cropping systems, supported by the Grains Research and Development Corporation (GRDC) and the Cotton Research and Development Corporation (CRDC), confirms that yield losses due to weed pressure on Australian grain and cotton growers are relatively small. This indicates that growers are ‘seedbank aware’ as they implement long-term, economically optimal strategies that maintain low in-crop weed densities.

“Growers are using a wide array of herbicide and non-herbicide tools to keep weed numbers low and maintain their flexibility in profitable farming systems,” says Jackie. “The average annual expenditure on weed control is $176/ha in grain and $387/ha in cotton production.”

“Weed pressure from weeds in grain crops reduces yields by an estimated three per cent, or $24/ha. The revenue loss is split between residual weeds in crop, which cost growers $12/ha, and weeds in fallow, which cost an additional $12/ha.”

“For cotton growers, yield losses due to weeds are around 0.4 per cent of production, attributable mainly to the cotton industry’s near 100 per cent adoption of seed technologies, worth a conservative $24 million per year, or $75/ha, in weed control.”

Weed control is one of the most expensive input costs in broadacre grain production systems, accounting for around 20 per cent of the average cash expenditure on grain farms.

In this new report, expenditure on weed control included the costs associated with using herbicides and seed technology and implementing a range of additional integrated weed management practices (IWM).

“Expenditure on in-season knockdown, pre- and post-emergent herbicides represents the largest weed control cost, accounting for 55 per cent of the total weed control spend for grain production, or $97/ha,” says Jackie. “In addition, grain growers invest an average of $51/ha in other in-crop weed management practices and $29/ha to control weeds on fallow fields, nationally.”

The total amount grain growers spent on weed control was similar for the northern, western and southern growing regions ($174/ha to $178/ha).

Jackie says that the costs associated with in-season and fallow herbicide programs are relatively straightforward to track on-farm, and the full cost can be attributed to weed control. In contrast, other tactics used within an integrated weed management program are more complex to cost because they are often applied over small areas, and some have multiple benefits aside from weed control.

This analysis considered both the area of land receiving each weed control practice and the cost of implementation, while also accounting for additional factors such as the level of attribution to weed control and the yield loss associated with some practices.

When considering the implementation of these strategies, keep in mind that the following practices are not applied to every hectare every year. The report provides estimated implementation costs and the percentage attributed to weed control for the following practices:

• Tillage prior to seeding costs $40 in grains and $50/ha in cotton, with 80 per cent of the cost attributed to weed control.
• In grains, delayed seeding with knockdown costs $30/ha to implement and incurs a yield reduction of 7 per cent. This tactic is 100 per cent attributable to weed control.
• Double knockdown for weed control costs $25/ha to implement in grains and $30/ha in cotton. This cost is 100 per cent attributable to weed control, and could be reduced if the second pass is done using weed detection spot-spraying technology.
• Harvest weed seed control in grain crops costs $5–41/ha depending on the method used, and is 100 per cent attributable to weed control.
• Crop topping in suitable grain crops costs $14/ha in cereals and $16/ha in broadleaf crops to implement and incurs a yield reduction of 2–5 per cent. These costs are 100 per cent attributable to weed control.
• Stubble burning in grain systems costs $1/ha, and is 90 per cent attributable to weed control.
• Green or brown manure crops cost an estimated $213/ha to implement. This accounts for both revenue loss and improved yield in future crops. This practice has multiple benefits, including weed control.
• Break crops managed primarily for weed control costs $80/ha, based on foregone revenue on more profitable crop choices, and is 100 per cent attributable to weed control.
• Competitive crop seeding in grain crops costs $15/ha to implement and is 100 per cent attributable to weed control.
• Chipping costs $5/ha in cotton crops, and is 100 per cent attributable to weed control.

The 2025 ‘Impact of weeds on Australian Grain and Cotton Production’ report does not include analysis of other weed control tactics used in broadacre cropping, such as grazing, hay making, soil amelioration, cover crops and inter-row cropping (in cotton crops), because these weed management practices are challenging to cost, due to their low adoption rates, highly variable costs, or the multiple agronomic benefits they provide.

The model also did not include fallow and in-crop weed detection and spot spraying technologies, due to the lack of available data for each analysis zone. When used in fallow (green-on-brown), this technology significantly reduces herbicide costs in regions where the practice is well-established. In-crop weed detection and spot-spraying (green-on-green) is not yet widely available but has the potential to reduce herbicide use and conserve yield by reducing the area of crop being treated with herbicide.

The analysis shows that Australian broadacre growers are using the integrated weed management strategies within the WeedSmart Big 6 to achieve low residual weed densities despite increasing herbicide resistance and major shifts in cropping systems.

Read the full report here: Impact of weeds on Australian grain and cotton production

Co-authors of the CSIRO’s ‘Impact of weeds on Australian Grain and Cotton Production’ report release in July 2025, (left to right) Masood Azeem, Jackie Ouzman and Rick Llewellyn. (Photo: CSIRO)