Key messages...

• Resistance remains for many years, until all resistant weed seeds are gone from the soil seed-bank.
• Resistance evolves more rapidly in paddocks with frequent use of the same herbicide group, especially if no other control options are used.
• Aim for maximum effectiveness of control tactics, as resistance is unlikely to develop in paddocks with low weed numbers.
• Do not rely on the same mode of action group.
• Monitor weed control regularly.
• Stop seed set on survivors.

Why be concerned?

Herbicide resistance is an increasing threat for growers and agronomists across Australia´s northern grain region. Already 10 cropping weeds have been confirmed as herbicide resistant in various parts of this region, and more have been identified at risk of developing resistance, particularly to glyphosate.

In southern Queensland (SQ), seven weeds are confirmed resistant to Group A, B or C herbicides (see Table 1). Central Queensland (CQ) has no confirmed resistant weeds, although a small random survey by the University of Queensland identified six weed populations as having the potential to develop resistance.

As well, barnyard grass, liverseed grass, sweet summer grass, common sowthistle and wild oats are at risk of developing resistance to Group M (glyphosate) herbicides (see Table 2). While no glyphosate resistant weeds have been identified at this stage in Queensland, about 20 paddocks in the Liverpool Plains area of northern New South Wales have glyphosate-resistant annual ryegrass.

Other broadleaf and grass weeds are also at risk of developing resistance, depending on weed numbers and management practices used. More information on preventing herbicide resistance in specific 'at risk' weeds.

Resistance is a costly problem. If resistance develops, growers will have to use other control measures or different herbicides, and these may be more expensive or less effective. In some cases, growers will not be able to grow certain crops, or may have to change their farming system to include more tillage. The impact will be greater in the more marginal cropping areas with lower and less reliable returns.

It is particularly important to give priority to preventing resistance to glyphosate, as this is likely to have the most adverse impact on cropping viability and sustainability. The measures needed to prevent or slow the development of herbicide resistance could be equated to the payment of an insurance premium. The cost of this premium is proportional to the risk level to be avoided.

How does resistance start?

Resistance starts in a paddock in several ways. Some rare mutations can occur naturally in weeds already in the paddock, with the likelihood varying from one plant in 10,000 to one in a billion plants, depending on the weed and herbicide. A grower may also import weed seed with the herbicide resistant gene in contaminated feed, seed or machinery. Resistance may also be introduced by natural seed spread by wind and water or by pollen, which may blow short distances from a contaminated paddock.

How does resistance then become a problem?

Once a few resistant plants are in a paddock, they will only become a widespread problem if the grower relies totally on those herbicides for weed control. Frequent applications of the same herbicide or same herbicide mode of action group will kill the susceptible part of the weed population, and eventually allow the rare plants with the resistance gene to increase and dominate. Higher risk management options that will lead to resistant weeds becoming a problem are presented in Table 3.

How long does resistance last?

Herbicide resistance will remain for many years until all resistant weed seeds are gone from the soil seed-bank. Local research has shown that a small percentage of seeds of annual grasses and common sowthistle persist for 3-4 years in the absence of any new seed replenishment (see Table 4). However a significant portion of weed seeds with hard seed coat, such as bladder ketmia, climbing buckwheat and turnip weed, will persist for four years and possibly longer.

Data supplied by Hanwen Wu^(A), Bruce Wilson^(B), Michael Widderick^(C), Ian Taylor^(D), Steve Walker^(E)

General principles to avoid resistance

Herbicides have a limited life before resistance develops, if they are used repeatedly and exclusively as the sole means of weed control, particularly in zero and minimum tilled systems. Resistance can develop within 4-8 years for Group A and B herbicides and after 15 years for Group L and M herbicides (see Table 5). This can be avoided by keeping weed numbers low, changing herbicide groups, using tillage, and rotating crops and agronomic practices. Information on a computer simulation of the effectiveness of integrated weed management strategy components can be found here.

Strategies to prevent or minimise the risk of resistance developing are based on integrated weed management (IWM) principles as outlined below.

• Ensure survivors do not set seed and replenish the soil seed-bank.
• Keep accurate paddock records of herbicide application and levels of control. Monitor weeds closely for low levels of resistance, especially in paddocks with a history of repeated use of the same herbicide group.
• Rotate between the different herbicide groups, and/or tank mix with an effective herbicide from another mode of action group. It is important to use effective ´stand-alone´ rates for both herbicides in the mix.
• Aim for maximum effectiveness to keep weed numbers low. The primary aim of weed control is to minimise their impact on productivity, and resistance is much less likely to develop in paddocks with fewer weeds than in heavily infested paddocks.
• Use a wide range of cultural weed control tools in your weed management plan. Sowing different crops and cultivars provides opportunities to use different weed management options on key weeds. Tillage is useful when it targets a major weed flush and minimises soil inversion, as buried weed seed generally persists longer than on the soil surface. Competitive crops will reduce seed production on weed survivors.
• Avoid introduction or spread of weeds by contaminated seed, grain, hay or machinery. Also, manage weeds in surrounding non-crop areas to minimise risk of seed and pollen moving into adjacent paddocks.

Specific guidelines for reducing the risk of glyphosate resistance are outlined in Table 6. Aim to include as many as possible of the risk decreasing factors in your crop and weed management plans.

Know your herbicide groups

Despite the large number of herbicides marketed to control weeds, there are relatively few mechanisms of herbicide toxicity, which means that many different herbicides may have the same actions on plants (see Table 7). More information is available on the Croplife Australia website (see related information below).

It is important to rotate between the different herbicide modes of action groups. All herbicides sold in Australia carry a letter code designating their mode of action (mechanism of herbicide toxicity of that herbicide).

What to do if you think you have resistant weeds

When resistance is first suspected, we recommend that growers contact their local agronomist.

The following steps are then recommended:

1. Consider the possibility of other common causes of herbicide failure by asking:
   • Was the herbicide applied in conditions and at a rate that should kill the target weed?
   • Did the suspect plants miss herbicide contact or emerge after the herbicide application?
   • Does the pattern of surviving plants suggest a spray miss or other application problem?
2. Has the same herbicide or herbicides with the same mode of action been used in the same field or in the general area for several years?
3. Has the uncontrolled species been successfully controlled in the past by the herbicide in question or by the current treatment?
4. Has a decline in the control been noticed in recent years?
5. Is the level of weed control generally good on the other susceptible species?

If resistance is still suspected:

1. Contact one of the researchers below for advice on sampling suspect plants for testing of resistance status.
2. Ensure all suspect plants do not set any seed.
3. If resistance is confirmed, develop a management plan for future years to reduce the impact of resistance and likelihood of further spread.