Figure 1. Onion hook-leaf stage
Critical issues for using EC formulations of oxyfluorfen herbicide in onions and other alliums
- There is a Wettable Powder (WP) formulation of oxyfluorfen that has a different concentration of the active ingredient, and a different use pattern to the Emulsifiable Concentrate (EC) formulations. Onion growers wishing to use this WP formulation should read the specific label instructions.
- Check the registered herbicide label for onions and obtain a copy of a relevant off-label permit for other alliums (only leeks at time of publishing) from the Australian Pesticides & Veterinary Medicines Authority (APVMA) permit search or local reseller and read thoroughly.
- Check the crop regularly during emergence, as proper spray timing is vital.
- If weeds are present, spray oxyfluorfen as soon as onions are at the hook-leaf stage (see Figure 1) or other alliums have developed a true leaf.
- Spray oxyfluorfen again as new weeds emerge, increasing application rates as crops become older and more tolerant of herbicide applications. Only two sprays per crop are permitted for leeks.
- Apply oxyfluorfen in at least 100 L/ha of water (preferably 200-300 L/ha) and at spray pressures not exceeding 150 kPa. Low pressure, flat-fan nozzles are best.
- Do not spray in high humidity or where crops are stressed.
- Do not add wetting agents or adjuvants to the spray mixture.
- The total dose for the season cannot exceed 2 L/ha for onions. Total doses for other alliums are detailed on the respective permits.
- Oxyfluorfen is highly toxic to wildlife and fish. To safeguard the health of spray applicators and the local environment, follow label directions for personal protective equipment and chemical use.
What is oxyfluorfen?
Current herbicides with 240 g/L of oxyfluorfen as the sole active ingredient include a range of trade names. These herbicides kill broadleaf weeds by destroying cell membranes within leaves and shoots. At low rates, oxyfluorfen acts as a contact herbicide, though it has good pre-emergence activity at higher rates. The 400 g/kg wettable powder formulation has little contact activity, and must be applied to a weed free soil.
Why use oxyfluorfen?
In Queensland onions, the EC formulations of oxyfluorfen can be safely sprayed much sooner after crop sowing than the other post-emergence broadleaf herbicides ioxynil, linuron, or methabenzthiazuron. EC formulations of oxyfluorfen can be applied once onions reach the hook-leaf stage (see Figure 1), while the other herbicides cannot be used before the crop has 2-3 true leaves. Early application of oxyfluorfen means weeds are not too large to be successfully managed by the time other herbicides can be safely applied.
EC formulations of oxyfluorfen are used at very low rates early in the crop and are relatively inexpensive. For example, spraying 50 mL/ha of Goal® at the hook-leaf stage would cost about $12/ha, (including boom-spray costs but excluding labour). Even with a probable requirement for multiple applications, the total costs for the maximum seasonal dose of 2 L/ha would be less than $70/ha.
The development of herbicide-resistant weeds is an important issue in integrated weed management. One way to minimise the development of resistance is rotate chemicals with different modes of action (hence the herbicide groupings shown on herbicide labels). As a Group G herbicide, oxyfluorfen can be used in rotation with the other post-emergence herbicides ioxynil, linuron and methabenzthiazuron, all of which belong to Group C. This is sensible weed management.
How to use EC formulations of oxyfluorfen effectively
Oxyfluorfen is most effective on small weeds, ideally with fewer than two true leaves. Onion growers need to monitor their crops regularly, noting new weed flushes and responding accordingly.
Oxyfluorfen will kill onion seedlings as they emerge, so the first spray should not be applied before the onion crop is at the hook-leaf stage (see Figure 1). Onion seedlings still coming through the ground when oxyfluorfen is applied will be killed. The initial application at the hook-leaf stage should be at 50-100 mL/ha. Under normal growing conditions, this should kill most emerging susceptible weeds with a reasonable crop safety margin.
As each new weed flush emerges, another oxyfluorfen application is required. The rate used depends on weed age, older weeds require higher rates if the crop can tolerate the dose. Onion plants become more tolerant of oxyfluorfen as they age. A general rule is that for each clearly visible true leaf (i.e. 3 cm apparent), application rates can increase by 100 mL/ha to a maximum of 500 mL/ha. For example, an onion crop with three true leaves could withstand 350 mL/ha of herbicide under normal conditions. The registrations indicate the total dose of oxyfluorfen formulation applied in onions must not exceed 2 L/ha per season.
Many broadleaf weeds are controlled by oxyfluorfen, as indicated in the registrations and off-label permits. Table 1 outlines these weeds.
|Common name||Scientific name|
|Blackberry nightshade||Solanum nigrum|
|Common cotula||Cotula australis|
|Common groundsel||Senecio vulgaris|
|Potato weed||Galinsoga parviflora|
Experimental evidence suggests at least some activity against species such as fat hen (Chenopodium album). Weed mortality/suppression will depend on weed age compared to oxyfluorfen application rate. Do not rely on oxyfluorfen to control emerged grasses.
Reducing the risks of EC formulations of oxyfluorfen damage
Onions are not completely resistant to oxyfluorfen. They can be damaged or killed by this herbicide. Crop phytotoxicity symptoms include necrotic flecking, twisting and stunting of plants. These symptoms may be confused with other disease and pest damage, however if they occur within two days of herbicide spraying they are probably due to oxyfluorfen phytotoxicity. Some minor flecking may follow spraying under normal conditions, this damage is usually transient and does not affect yields.
Onion tolerance of oxyfluorfen is a result of a lower uptake of herbicide compared to weeds. This reduced uptake is principally due to the vertical leaf structure of onions which enhances herbicide flow off the plant, and development of a waxy leaf coating which reduces herbicide penetration. To minimise crop damage, it is vitally important that the onion plants take up as little herbicide as possible.
Practical steps to minimise EC formulations of oxyfluorfen uptake
- Create large spray droplets by increasing water volumes and minimising spray pressures. Water volumes of more than 100 L/ha (preferably 200-300 L/ha) with spray pressure less than 150 kPa are effective. Low pressure, flat-fan nozzles are best.
- Do not use wetting agents or other adjuvants that disrupt the waxy cuticle and increase herbicide uptake. Do not spray oxyfluorfen in a mixture with other pesticides.
- Do not spray onion plants that are stressed, as they will be more susceptible to damage.
- Do not spray in conditions of very high humidity where oxyfluorfen concentrations around the plant remain high with increased leaf penetration.
Use of the Wettable Powder formulation of oxyfluorfen
The WP formulation (has a higher concentration of active ingredient, 400 g/kg. Compared with the EC formulations, it has little contact activity, so any emerged weeds must be controlled by some other method before spraying with this WP formulation. The other major difference is that the WP formulation cannot be sprayed before the onions have two true leaves (excluding the hook or flag leaf). The application rates vary from 150-300 g/ha of the formulated product - the higher rate is not to be used before the crop has three true leaves. According to the label, no more than 1.2 kg/ha of formulated product can be applied per crop.
Responsible use of oxyfluorfen
As with all herbicides, it is important to read instructions before using the product. Onion growers wishing to spray oxyfluorfen should read the relevant herbicide labels.
Although many current oxyfluorfen herbicide formulations are less dangerous than previous formulations, they should still be stored, handled and used appropriately. This means following the directions for personal protective equipment, environmental precautions and disposal instructions on the label.
To improve weed control and reduce build up of problem weeds, alternate oxyfluorfen with other post-emergence herbicides (ioxynil, methabenzthiazuron or linuron), within and between seasons.
Be aware that even small doses of oxyfluorfen can potentially result in significant plantback periods of 60-180 days for following sensitive, small-seeded crops. Refer to the labels for specific plantback information.
Use of oxyfluorfen in alliums other than bulb onions
There may be APVMA off-label permits for use of oxyfluorfen in allium crops other than bulb onions, such as leeks. Much of the above information for onions may be relevant for those other alliums, though the specific requirements of the permits must be followed. Note that the availability of other herbicides for rotating with oxyfluorfen should also be checked for each different allium crop. Use the APVMA permit search site to check for permits.
Because oxyfluorfen can damage alliums, growers using this herbicide on new crops should be very careful and treat only small areas on the first few occasions until they are confident in their system.
The uses and legal status of oxyfluorfen are likely to continue to vary over the next few years. There are now registrations for several herbicides containing oxyfluorfen for use in onions. There are also changing uses for oxyfluorfen in other alliums as permits are approved or removed. The information provided in these guidelines was current at the time of publication, but is subject to change.