Using paraquat and diquat wisely and well

Paraquat and diquat are key herbicides used primarily for pre-harvest weed control and crop desiccation, the control of fallow weeds to conserve soil moisture for crops, protect soils from erosion and establish a weed-free environment for competitive crop establishment. 

WeedSmart extension agronomist Greg Condon says that the recent review of ‘quat’ herbicides by the Australian Pesticide and Veterinary Medicines Authority (APVMA) will drive further investigations into their effective use and innovation in alternative weed control strategies in broadacre grain and cotton cropping systems.

“The APVMA review of paraquat and diquat takes immediate effect for products manufactured from 23 June, 2026,” he says. “For broadacre grain and cotton production systems there are significant reductions to the maximum application rates of active constituent per hectare (ac/ha) and many use patterns will be phased out as the new product labels take effect.”

To maintain efficacy across a broad spectrum of weed species, growers will need to be very strategic in their use of these products in the double-knock strategy for managing herbicide resistance. Many weed species have been removed from the ‘weeds controlled’ claims on the product labels as the new, lower rates will not control these weeds.

“The WeedSmart Big 6 ‘spray efficacy – make every drop count’, ‘mix and rotate’ and ‘stop seed set’ pillars work together to minimise the risk of paraquat resistance. We recommend growers continue to use premium formulations of paraquat and diquat at the full label rates, include the correct adjuvants and appropriate spike products, use optical spot sprayers where possible, target weeds when they are small and take additional precautions to ensure that no survivors are allowed to set seed.”

The main use of paraquat and diquat in broadacre cropping systems is to maintain weed-free fallows, usually as the second pass following glyphosate in a double-knock tactic. This use pattern remains open to grain and cotton growers only when using optical spot spraying, and at a significant reduction in application rate (max. 770 g ac/ha, down from max. 2250 g ac/ha), in paddocks with up to 30 per cent weed coverage. This use pattern is for the control of bladder ketmia, barnyard grass and yellow vine (caltrop) only. Volunteer cotton, ratoon cotton and hard-to-kill weeds, including fleabane, sowthistle, turnip weeds and Australian bindweed, are not included on the new paraquat label.

For boomspray applications as an aid to cultivation (fallow, crop or pasture establishment) the new maximum legal rates of paraquat are 321 g ac/ha (for wild oats 2-5 leaf only) or 251 g ac/ha for paraquat+diquat mixes for a reduced selection of weed species mainly at seedling stage. Diquat can be used post-emergence in wheat and oats (3–4 leaf to early tillering) to control small capeweed seedlings through a boomspray at up to 122 g ac/ha per season.

Researchers have found that a 3-way mix of paraquat, Group 14 spikes and a pre-emergent herbicide is a useful second-knock tactic to follow glyphosate ahead of sowing.

At least 50 populations of paraquat-resistant ryegrass have been confirmed in Australia, most of which are also resistant to glyphosate. These samples have been tested against the ‘pre-review’ rates of paraquat, so many more individual plants would be expected to survive the ‘post-review’ product rates.

Glufosinate is the only other registered knockdown herbicide currently available to broadacre growers. Glufosinate is less robust than glyphosate or paraquat, it controls a different weed spectrum, and particular application requirements must be met for it to provide effective weed control. It will not be the best option in every situation but will have an ongoing fit to complement glyphosate and paraquat in the knockdown space.

Crop protection companies are reviewing their product range in search of existing chemistries that may enhance paraquat efficacy or provide an alternative double-knock partner. The industry will look to emerging herbicides to provide alternatives for resistant weeds during fallow and prior to sowing.

Spray topping (or croptopping) is a valuable late-season tactic to stop weed seed set in annual ryegrass. This continues to be a supported use of paraquat in field peas, lupins, lentils, chickpeas and faba beans with no change to the label rate. The harvest withholding period is now extended from 7 days to 14 days, which may incur grain losses prior to harvest, particularly in windy conditions. Spray topping in pastures is not permitted on the new labels. The new diquat label does not allow for crop desiccation as an aid to harvest in any crop (except poppies in Tasmania).

Paddock borders breed herbicide resistance – don’t use glyphosate or paraquat as a standalone weed control tactic along borders or non-agricultural areas. Slashing, tillage, baling paddock borders for hay and using long-residual herbicides are other options.

Paraquat and diquat are dangerous (Schedule 7) poisons and must be used and handled with care. The review has mandated several additional safety measures for individuals handling these chemicals, including strengthened personal protective equipment (PPE) requirements, the use of closed mixing and loading systems, enclosed cab requirements for broadacre boom application, revised re-entry intervals and updated label statements. Backpack sprayer use is no longer permitted for the application of products containing paraquat or diquat.

The review has introduced buffer zones to protect bystanders, aquatic and vegetation from potential spray drift from boomspray applications. The new labels include instructions for mandatory buffer zones, boom height and spray droplet size must not be less than Medium spray droplet size category.

Since the application rates have reduced significantly, there is no change to the domestic maximum residue limits (MRL) for paraquat. Export destination markets can and do have MRLs that differ from the Australian MRL for paraquat.