Broadleaf crops such as canola and pulses offer grain growers an opportunity to use different herbicides to control grasses such as the super-adaptive annual ryegrass. Clethodim herbicides such as Select® have provided good control until recently where there have been increasing occurrences of resistance across southern and Western Australia.
Most growers in Western Australia are aware of the rising risks and are keen to know what their options are for regaining the advantage, given there are no highly effective alternatives to clethodim for post-emergent control of annual ryegrass in canola and pulses.
To grow canola and pulses without clethodim growers will need to implement a robust pre-emergent herbicide program, consider tank mixes, and stop seed set through crop topping and harvest weed seed control tactics.
A robust pre-emergent herbicide package combined with a knockdown or double knock is essential to drive down weed numbers early in the crop. Growers can work with their agronomists to develop a plan to rotate and mix pre-emergent herbicides and take advantage of the new herbicides that have been registered for ryegrass control in recent years. Plan to use them carefully to get best results and make them last.
Applying clethodim post-emergent is still a good idea where some level of control is expected. Clethodim is a good low cost option in susceptible populations and is worth protecting through the control of plants that survive treatment. Where resistance to clethodim is present, a mix of clethodim plus butroxydim may give better results than clethodim alone.
Crop topping helps stop seed set. Weedmaster® DST is the only product registered for crop topping in canola. As glyphosate is widely used in crop rotations, using paraquat for crop topping pulse crops is a good resistance management strategy.
Finally, harvest weed seed control should be employed as often as practical to stop ryegrass entering the seed bank and to reduce total weed numbers so that future control methods have a better chance of being effective.
Hybrid cultivars of canola can provide additional competition against weeds like ryegrass, compared with open-pollinated cultivars. This becomes particularly important when post-emergent herbicides like clethodim fail. Combining crop competition with effective pre-emergent herbicides is an effective way of reducing ryegrass seed production. Adding seed set control techniques to this reduces weed seed banks even further.
Planning for weed control across the whole rotation is a complex business that requires growers and their advisors to be well-informed and up-to-date with the latest advances in crop and weed science.