Weed Management in Retained Stubble Systems Under Irrigation

Key messagesICC Logo with Slogan

  • Controlling the weed seedbank requires several strategies over a number of years, such as using herbicides from a broad range of groups, stubble management such as cutting and baling and rotational options.
  • High stubble loads and/or watering up reduce the pre-emergent herbicide options available and reliance on in-crop herbicides has placed selection pressure for herbicide resistance
  • Controlling weeds early in the year (ie August/September) along fence lines, channels and check banks reduces a source of re-infestation

The proportion of irrigators with resistant weeds is an increasing problem for growers in the irrigation district. Many of the pre-emergent cereal herbicides rely on being applied to the soil surface and then incorporated. High stubble loads commonly generated by irrigated crops intercept these herbicides that can bind to varying degrees to the stubble, rendering them ineffective. For example, Trifluralin will more readily bind to stubbles than Sakura® which can be washed off by rainfall. A lack of herbicide efficiency in high stubble loads has placed greater importance on the in-crop selective herbicides. As a result, many growers are now having to deal with ryegrass, and to a lesser extent wild oats, that are resistant to the Group A and B herbicides. Quite often a poor result is blamed on the conditions when the herbicide was applied. However, to be sure resistance is not to blame, testing should be done. The most appropriate way to determine if resistance is present and at what level is to get seed or plants tested. When choosing the herbicides to test, also look at testing for the alternatives that you would use if resistance to your current herbicide options are confirmed.

The practice of watering up crops also works against using a pre-emergent herbicide due to the large volumes of water applied (typically 100 – 200 mm) that simply dilute the herbicide or transport it below the seed and render control unsuccessful.

Many strategies to apply herbicides and manage resistance in irrigated cropping systems can be adopted from dryland farming but may have variable success in irrigation. A simple strategy is to make sure water rates when applying herbicide are around 100L/ha to ensure good coverage or penetration of the stubble.

Southern Farming Systems in the high rainfall zone has researched many aspects of herbicide use in stubble retained systems that may have application in the irrigation area as both areas have to deal with similar stubble loads following harvest.

Windrow burning can be a successful harvest weed seed control option in irrigation, particularly with canola, as long as the weed seeds are harvested by the header front and not simply pass underneath the cutter bar. Windrow burning in irrigated wheat or barley stubble is likely to see the whole paddock catch fire. So in these instances windrow burning is often not a viable option.

The Irrigated Cropping Council (ICC) Experience

ICC has a block established for trials under irrigation. Due to continuous cropping for 17 years, and the high use of post emergent herbicides, a high level of Group A resistant ryegrass has become established in the block. The following strategies are those that ICC have adopted to manage this resistance.

Smart cropping rotations

The general rotation for the non-trial areas of the block is faba beans – canola – wheat – barley/oaten hay. Apart from the N management aspect of this rotation, it gives two years of broadleaf crops to control grass weeds followed by two years of cereal crops to manage our broadleaf weeds. Another benefit of this rotation is the ability of the next crop, Along with planned herbicide options to manage the stubble loads from the preceding crop. For example wheat after canola means the stubble residue rarely excludes the use of the pre-emergent herbicides. The only “difficult” year is barley into wheat, and there are options such as baling, or even burning, to overcome stubble management issues.

Herbicide Tolerant Varieties

The herbicide tolerance traits of canola have also been used as a method to provide more effective herbicide options. Each tolerance group has its own positives and negatives. Triazine Tolerant (TT) varieties have seen mixed results depending upon post-swing rainfall, however these varieties, due to the TT trait, have a yield penalty of up to 15%. Newer and recently released Triazine Tolerant varieties have reduced this yield gap in comparison to other herbicide tolerant canola varieties.

Roundup Ready has been the simplest to use but late germinating ryegrass plants post green bud formation have not been controlled through herbicide applications. By using canola after fabas, there is little stubble to intercept the herbicides. Clearfield varieties (Imi’s) still rely on grass selectives for good control of grass weeds so tend to add to the pressure on selection for group A resistance. The choice of herbicide tolerant canola will be determined by weed spectrum and rotations as part of the entire system.

Alternative End Use

Hay has been effective in reducing the weed seed bank but relies on timely post baling knockdown herbicide application to gain the most advantage. The benefit of cutting for hay is that weed seeds are generally removed before they have time to mature. Therefore, priority needs to be made to spraying the weedy paddocks post bailing to ensure that weeds in these paddocks are controlled. Alternatively hay paddocks can be sprayed pre-mowing but be sure to use the appropriate registered herbicide and any restrictions end-users may have.

Post-harvest cutting and baling of straw has removed extra weed seeds plus reduced stubble height making the use of pre-emergent herbicides more effective.

Pre- Irrigation

Pre-irrigation is vital to our weed management. Irrigation approximately three to four weeks before sowing results in a good opportunity to control weeds with a knockdown herbicide. In some seasons, a double knock can be achieved. A factor that can reduce the effectiveness of pre-irrigation as a tool is the chaff lines from harvest. Ideally the chaff and straw would be evenly spread across the header front width, but not all headers are so equipped. Consider modifying your header or using a contractor if this will be economically viable.

Reduce the weed seedbank

While difficult to achieve on a broadacre scale, eliminating weeds that survive a herbicide application does prevent the escapees from adding more resistant seed to the seedbank. Our trials are hand weeded to minimise these escapees.  Similar to a fence line, channel or check bank, any weeds that survive in the trials provide a source of seed for the next season. Also either allowing weeds to survive on channels and check banks or repeatedly using the same chemical year after year will not help reduce the seedbank and provide a source of resistance.

We utilise all options – even though we want to maintain our stubbles for many reasons, if the best option is to burn, then the occasional burn will have little long term impact. But once again, it is rare that a burn will eliminate all weed seed in the paddock on its own.

Reliance on one management or technology technique will not be successful in managing weeds in general, let alone in a stubble retention system. When all options are evaluated and applied in a planned and thorough strategy, then weeds and herbicide resistance can be managed effectively under irrigation.

By Damian Jones
Trial Manger
Irrigated Cropping Council
E: damian.jones@irrigatedcroppingcouncil.com.au

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This research is being conducted by ICC as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 ‘Maintaining profitable farming systems with retained stubble in Victoria and Tasmania’).

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