Keeping weed numbers low pays off in cotton

Lead researcher and data analyst, CSIRO’s Jackie Ouzman, says cotton growers’ investment in weed control is minimising yield losses from 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 farms 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.

“Cotton growers are keeping weed numbers low, mainly through effective herbicide use,” says Jackie. “Weeds cost cotton growers an average $445/ha annually, primarily through an average annual expenditure on weed control of $387/ha.”

“For cotton growers, yield losses due to weeds are very low at around 0.4 per cent of production, or $58/ha, 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 benefit.”

A near-zero tolerance approach to in-crop weeds in cotton has resulted in no measurable yield losses due to weeds. Revenue losses in cotton are made up of yield loss from
fallow weeds ($13/ha) and yield damage associated with off-target herbicide application ($45/ha).

Most fallow weeds occur at low densities, and the most extensive fallow weeds are
sow thistle, wild turnip and wild oats. The costliest fallow weeds are heliotrope/potato weed, wild turnip and windmill grass.

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, including application, represents the largest weed control cost, accounting for 53 per cent of the total weed control spend for cotton production, or $203/ha,” says Jackie. “In addition, cotton growers invest an average of $28/ha in other in-crop weed management practices, $75/ha in seed technology and $80/ha to control weeds on fallow fields, nationally.”

The total amount cotton growers spent on in-crop herbicide applications varied somewhat between growing regions ($182/ha to $223/ha). The expenditure on knockdown herbicides was the main in-crop cost in each growing area at an average of $113/ha (plus application costs). The use of pre- and post-emergent herbicides varied between regions but averaged out at a similar cost of $45/ha (plus application costs).

Cotton growers who grow rotational grain crops are able to implement a wider range of non-herbicide tactics to control weeds and manage herbicide resistance risk, including diverse planting times, crop competition, break crops, green and brown manure crops, harvest weed seed control, crop-topping and stubble burning.

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 $50/ha in cotton, with 80 per cent of the cost attributed to weed control.
• Double knockdown for weed control costs $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.
• 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 systems, 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 manage the weed seedbank despite increasing herbicide resistance and major shifts in cropping systems.

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