- Ryegrass is the major weed present in the Murray Mallee
- Benefits of stubble come from increasing organic carbon over the long term and reducing slaking and dispersion.
- Ground cover is typically more than 50% meaning that trifluralin is not used as a pre-emergent herbicide option.
Irrigation presents two issues that complicate herbicide use when retaining stubbles. The first is the amount of stubble present and second the amount of moisture that may be present at sowing.
Irrigated cereal crops may yield as high as 10t/ha, with the average over 6t/ha, and are usually sown on narrower (less than 8 inch) row spacing’s. This results in groundcover greater than 50 per cent which, due to stubble interception, eliminates the option of using some of the popular pre-emergent herbicides such as trifluralin unless stubbles are managed post-harvest. Similar issues are also encountered when using triallate.
Group B herbicides such as chlorsulfuron have been more commonly used for pre-emergent weed control due to their low volatility and the fact that they don’t need to be incorporated. Although heavy stubbles do intercept this chemical, subsequent rainfall washes the herbicide into the soil. This reliance on rainfall post sowing, however, can result in poor results if rainfall is insufficient or occurs after weeds have become too large for control.
These challenges have seen a heavier reliance on post-emergent weed control in irrigated cropping systems. This strategy has been successful in the past due to the ‘typical’ irrigation farm being a mix of animal and cropping enterprises. However, as cropping has become more intensive, herbicide resistance has become far more common. Resistance to Group A chemistry is now widespread and some low levels of Group B resistance have also been confirmed.
To get the best results from the herbicides available, rotations that allow the use of alternative herbicide groups are being adopted. A typical rotation of faba beans – canola – wheat – barley offers the opportunity to have two years of grass control in the broadleaf phase and two years of broadleaf control in the cereal phase. Irrigation reduces the risks associated with the inclusion of more drought sensitive broadleaf crops which are assured of a moisture supply. Additionally, there should be few issues with herbicide plantbacks that can result from extended dry conditions.
Canola in the rotation also offers the opportunity to use alternative herbicide groups such as the triazine’s and glyphosate. The use of triazine tolerant (TT) canola would only be a part of a weed management strategy as in 2014 there was a yield penalty compared to the other canola groups. In 2014 canola yields in the Murray Mallee were Conventional 4.4 t/h, Roundup Ready 4.4t/ha, Clearfield 4.0 t/ha and Triazine tolerant 3.4 t/ha.
Similarly, faba beans can offer the option of triazines, but late broadleaf weed control can be an issue.
Irrigation also presents issues for herbicide efficacy. However this can be overcome by using robust water rates when spraying. Water rates of 100L/ha should be considered as a minimum.
Pre-irrigation can result in stubble and/or crop residues forming mats that reduce the effectiveness of pre-emergent herbicides.
If a crop is being watered up (most commonly canola and to a lesser extent cereals and faba beans), the volume of water applied (100-150mm rainfall equivalent) usually results in any pre-emergent herbicides being leached through the soil profile and to a concentration that is ineffective for weed control.
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).