How do the Australians manage herbicide resistance?

HESTIA NIENABER, ARC-SMALL GRAIN INSTITUTE

 

Australian producers and researchers firmly believe that managing herbicide resistance starts with managing the weed seedbank. For them, exhausting the seedbank is the key to integrated weed management strategies.

What is integrated weed management?

The management of weed seed-set offers the most practical long-term management of hard to control weeds, like annual ryegrass, wild oats and wild radish. Control strategies include destroying or burying seeds, encouraging germination, strategic herbicide use and crop agronomy.

No single integrated weed management programme is ideal for all conditions. The tactics chosen depends on the soil type, rainfall pattern, crop rotation, available equipment, budget and producer preference.

There are several steps involved in an effective integrated weed management strategy:

• Identify the problem weeds and develop a multi-year approach for their management;
• Control weeds that survive early weed control or germinate incrop and set seed, thus reinvesting the seedbank; and
• Once the seedbank has been reduced, use crop competition to combat weed germination and seed-set, particularly for annual ryegrass.

Strong crop competition, combined with rotating herbicide modes of action and the use of appropriate agronomy for crop nutrition are the best methods of keeping seedbanks low. Where weed populations are high or seedbank life is long, multiple years of seed-set control are required to drive populations down.

Preventing weed seed-set

When managing weed seed-set the best results are obtained if control occurs before the point where mature weed seeds are formed. Monitoring and managing regrowth is essential. The choice of control tactics needs to consider the weed species, population size and onfarm management options in relation to rotation, expenditure, machinery and labour availability, market opportunities and future plans for the field. Tactics can be divided in three sections: prevention of weed seed-set, destroying weed seeds and encouraging germination for timely control.

Prior to, or at seeding
Although a fallow period between two crops or a crop and a pasture phase, is designed to conserve soil moisture and nutrients, it also makes an ideal time for repeated and focused weed control.

The lack of crop competition during the fallow means large numbers of weed seeds germinate. If left unchecked, these weeds can set high numbers of seeds per plant. Non-selective herbicides and cultivation are the most relied on control methods; however, the use of any single method will lead to a shift in weed species adapted to that control method. Rotation of chemical and physical control methods through the fallow period is essential.

Crop choice, seeding rate and row spacing all influence a crop’s ability to compete with weeds. Species with rapid establishment and good early vigour, such as barley or oats as a pasture/hay crop, are the most effective at suppressing weed growth, and durum is the least competitive.

During the growing season
Multiple mowings, silage or hay combined with a follow-up non-selective herbicide or heavy grazing to remove any late germinating plants, provide excellent levels of control for several weed species. Timing is essential and cutting when grass weeds are flowering prevents viable weed seeds returning to the field or entering the hay.

At harvest
Ryegrass and wild radish both reach maturity at a similar time to wheat. As the majority of seeds are retained on the plant, they enter the harvester. In research done by Australians, up to 80% of wild radish seeds have been found to be collected in the grain sample, while more than 95% of annual ryegrass seed that enters the harvester, has been found to exit with the chaff.

Seed cleaning and chaff carts (a cart that is pulled behind the harvester and collects the chaff) or direct baling of chaff offer alternative methods to reduce the amount of weed seeds entering the seedbank.

Destroying weed seeds

Destroying weed seeds by burning requires exposure to temperatures of 400oC for ryegrass and 500oC for wild radish for ten seconds.

Concentrating stubble and weed seeds into windrows increases the effective biomass. Burning windrows is more effective for wild radish, but it can be time consuming. It however results in only about 10% of the field being burnt, reducing the risk of soil erosion.

The level of soil disturbance has been shown to affect the percentage of the seedbank that emerged in the following crop. Small-seeded weeds emerge in much greater numbers in no-till seeding compared to two passes with a wide shear.

Under minimum tillage, larger seeded species, such as wild radish and annual ryegrass show greater germination. Low disturbance discs leaves more weed seeds on the surface, while other single pass seeding equipment will bury seeds in the inter-row, resulting in delayed germination in the crop.

Another advantage of minimising seed burial, is that the seed may be removed by insects. Reduced tillage, stubble retention and minimal use of broad spectrum insecticides can help encourage populations of insect predators.

Encouraging germination

Tillage can have a negative and positive effect on weed germination. This is because most weed species have a narrow depth preference for successful emergence. A shallow cultivation, using a range of equipment including tined implements, heavy harrows, pinwheel rakes and disc chains can stimulate some weeds to germinate by placing them in contact with moist soil. Deep seed burial using inversion ploughing can prevent weed germination and has been used for herbicide resistant populations. Cultivation can be used as a non-herbicide component of a double-knock.

Early seeding has been recognised as a key component in securing yield in cereal crops. The yield penalty from delaying sowing by up to two weeks after the opening rains may be justified by improved weed control.

Delayed seeding not only provides time for more weeds to germinate, but also creates an opportunity to use two non-selective herbicides (glyphosate and paraquat), known as a double-knock, ensuring control of glyphosate survivors. Best results have been recorded when paraquat is applied one to five days after the application of glyphosate.

Electronic tools

In Australia there are two electronic tools available for use. The Ryegrass Integrated Management computer package allows many different combinations of weed treatments and their predicted impacts on ryegrass populations, crop yields and long-term economic outcomes to be observed. A wide variety of chemical and non-chemical weed treatment options are included.

The Weed Seed Wizard is an interactive computer-based system that provides an insight into the hidden weed seedbank and helps in the co-ordinated long-term management of weeds. This programme is currently under development.

Conclusion

No single management technique provides 100% weed seed control. A combination of techniques needs to be employed throughout the year. Decisions about which tool to use should be based on the weed species, rotation, farming system, budget and market opportunities. As some seeds can last in the soil for several years, back-to-back years of weed control therefore needs to be employed to drive numbers down.

For further information, contact Hestia Nienaber at (058) 307 3420 or email deweth@arc.agric.za.