Weed control is an important part of crop management, and the use of herbicides to control problem weeds is a significant part of the management system.
However, the continual use of same group herbicides to control weeds has resulted in increased numbers of resistant plants.
GRDC funded a random roadside weed survey carried out by Southern Farming Systems together with Dr Chris Preston from the University of Adelaide over five years. We found that in the western district of Victoria, resistance levels averaged 80-85 per cent for Group B and Group A (subgroup – FOP) herbicides in ryegrass.
With the increased level of selection pressure (selecting for weeds that can survive a target herbicide application) comes the problem of knowing which products will work effectively in your paddocks.
At this time of year the problem weeds become obvious, as they stand tall above the rest of the crop in paddocks. It is the perfect opportunity to collect seed samples. Results for seed testing can take up to three months, so submit samples early to receive results before pre-season planning.
The process of collecting samples depends on the level of resistance across the paddock. If it’s widespread, collect seeds every 10-20m, following a “W” path across the paddock or problem area. If the problem is more localised, collect seeds from areas or patches where the weed has survived.
To ensure greater accuracy, spread sampling across a large number of plants and collect similar seed numbers from each plant. The more collected, the better.
For smaller seeded grasses like ryegrass, about one cup of clean ryegrass seed is required to complete a resistance test. When sending off samples, double bag them and send in paper bags or envelopes – they can rot in plastic bags. Testing for herbicide resistance can determine which herbicides are still effective for controlling problem weeds on your property. It is then possible to implement a cost-effective weed management plan by using herbicides that are still maintaining some control.