Wild oats pose a significant and on-going challenge to grain growers across New South Wales and Queensland with recent surveys identifying it as one of the most common grass weed species found in northern region cropping paddocks.

The argument for why strategic control programs should be developed and implemented is compelling.

Wild oats is highly competitive, producing up to 225 seeds per plant, is easily spread and acts as a host for a number of cereal diseases and pests. It is also at risk of developing resistance to herbicides.

Impacting around 600,000 hectares across the north, it is estimated the revenue loss due to wild oats equates to $4.5 million annually.

The Grains Research and Development Corporation (GRDC) is investing in research to improve wild oats management through crop competition, herbicides and mechanical strategies as part of a collaborative Crop Weed Control program overseen by Dr Michael Walsh, Director Weed Research from the University of Sydney.

A knowledge of the biology and ecology of wild oats can assist growers in developing improved integrated management practices. Some key points to note include:

• Two main species – Avena fatua and Avena sterilis subsp. ludoviciana can be present. A. fatua tends to germinate earlier and retains less seed at crop maturity.
• On average 225 seeds are produced per plant, so even if only a small number of plants are allowed to survive and set seed, the seedbank can be quickly replenished.
• Tends to grow in discrete patches at low to moderate densities (but up to 100 plants/m² are possible).
• Seedling leaves are twisted anticlockwise, the opposite direction to wheat and barley. Wild oats have a large ligule with no auricles and the leaves tend to be hairy with a slight bluish hue. The emerging leaf is rolled.
• Main flush of germination occurs in autumn/early winter (~40 per cent of seedbank in the soil), late germinations (~10-30 per cent) replenish the seedbank.
• Germinates from a variety of depths.
• Approximately 75 per cent of the seed will germinate in 12 months. Requires 2-3 years with no seedbank replenishment to deplete the seedbank.
• Wild oats is highly competitive and can inhibit the growth of other plants. If left uncontrolled, wild oats can cause yield losses of up to 80 per cent in wheat.

Herbicide resistance in wild oats

Herbicide resistance in wild oats tends to develop at a slower rate compared to other weed species such as annual ryegrass.

However, significant levels of resistance are present in a number of populations and this an issue that needs to be managed through the strategic use of both herbicides and non-herbicide tactics.

Data from Charles Sturt University herbicide resistance testing shows high levels of resistance to the Group A ‘fop’ herbicides (70-80 per cent), significant levels of resistance to Group Z herbicides (Mataven), concerning levels present in a number of populations to the Group A ‘den’ herbicides, and some levels of resistance in the Group A ‘dim’ herbicides and Group B herbicides. It is estimated herbicide resistant wild oats are costing growers in the northern region an additional $2.3M in herbicides to control.

Testing

The GRDC’s Integrated Weed Management Hub provides information on testing to understand the herbicide resistance status of weeds.

Testing can be conducted either by carrying out test strips in the paddock or by sending plants or seeds to a commercial testing service.

There are two types of commercial resistance tests:

1. Seed test - seed is collected and sent to the testing service. Results may take up to four to five months. Suitable for pre-emergent and post-emergent herbicides.

2. Quick-test® - live plant seedlings are collected and sent to the testing services. Results within six weeks. Unsuitable for pre-emergent herbicides.

Growers are encouraged to test any paddock where herbicide resistance is suspected as the cause of a spray failure. When testing, assess the spectrum of products that could be used in the future to see what is still working. The cost of a resistance test pales in comparison to the cost of a spray failure or an overly conservative change to crop rotation based on a bad assumption.

While plants can be sampled before a herbicide is applied (e.g. break of season), the ability to detect resistance is much better when testing occurs after herbicides have been applied and poor control is noticed.

Sample patches of weed escapes from across the paddock as this is normally where resistance appears in the early stages and provides information as to where the rest of the paddock is likely to be heading. As wild oats are self-pollinating, each patch of resistant weeds is likely to have been derived from a single weed survivor. It is quite possible that different types of cross resistance could exist in different weed patches in the same paddock.

Consult the testing service for more details on sampling.

Management

Effective management requires an integrated approach to drive down the weed seedbank such as:

Crop rotation: Where summer crops are an option, a highly effective strategy is to rotate to a summer crop for a couple of years and drive down seedbank numbers during the winter fallow phase by the use of non-selective herbicides, green or brown-manure crops or cultivation.

Crop competition: Increasing the competitive ability of crops such as wheat has a major role to play in the management of wild oats as well as other weed species. According to Dr Walsh, a recent pot study observed a 40 per cent reduction in wild oat seed production by increasing wheat plant density from 60 to 120 plants/m². The same study found that wheat crop competition increased the proportion of seed retained higher in the canopy.

Harvest weed seed control: Harvest weed seed control (HWSC) can play a part in wild oats management when used in addition to current control tactics however it is not likely to be robust enough to be able to be relied on as a stand-alone option, Dr Walsh said. Although high proportions of seed retention have been recorded for populations of wild oats maturing in wheat crops, seed shedding begins at plant maturity and continues throughout the harvest period so the opportunity to target wild oats with HWSC may be brief.

Herbicides: Herbicides are currently still the most effective tool for controlling wild oats in northern region cropping systems despite existing resistance to some chemistries. Several in-crop selective herbicide options are available for wild oats control while additional pre-emergence herbicide options have become available over the last few years. There are also later season herbicide options (pinoxaden and flamprop) available for specifically targeting the seed production of wild oat plants. However, it is fool hardy to rely on herbicides alone. Some non-herbicide tactics are needed in all farming systems on all weeds or we will simply select herbicides into history.

Contact Details

Dr Michael Walsh, University of Sydney
m.j.walsh@sydney.edu.au
02 67992201 or 0448847272

Useful Resources

ICAN webinar on the ecology and management of wild oats via the GRDC Resources and Publications section of the website.

GRDC Update paper The challenge of managing wild oats in northern region cropping.

Further detail about wild oats including integrated weed management tactics can be found in the section on problem weeds in the GRDC’s Integrated Weed Management Manual.