Herbicide Resistance in Australia
Dr Peter Boutsalis from the University of Adelaide presented at the Rural Management Strategies, Managing Herbicide Resistant Weeds seminar in Temora on 22nd May 2014.
The following is a summary of the key messages from his presentation.
Key Messages:
Different herbicides in the same herbicide group can have a different resistance status in the same plant, therefore when using a resistance test, make sure the actual product to be used is tested.
High chemical rates will control more plants in a weed population but will more quickly select out the highly resistant individuals.
It is better to manage a very small population of highly resistant weeds than a large population of weeds with variable resistance levels.
To improve Clethodim (Select/Status) activity on ryegrass consider the following:
Target young weeds because often resistant weeds can be killed when young but not when older - Warmer temperatures improve Clethodim activity - Frost after Clethodim application reduces efficacy
Clethodim can be mixed with Atrazine providing the ryegrass plants are small, actively growing and soil moisture is adequate.
Mixing Clethodim and low rates of Factor can improve Clethodim activity on ryegrass, but also increases the risk of crop damage in canola.
Higher rates of Clethodim can improve control of ryegrass but also increase the risk of crop damage in canola especially after 8 leaf stage.
Wild oat herbicide resistance is only increasing slowly, but is a significant problem where it occurs.
Propyzamide (Kerb) is a Group D herbicide but behaves differently to Trifluralin and requires good soil moisture for activation.
Glyphosate resistant ryegrass is spreading. Fencelines and roadsides are a major source of resistance.
Sowing as close to fencelines as possible (crop competition) and controlling young weeds on fencelines in autumn, preferably with a residual herbicide rather than waiting until spring, will significantly reduce the incidence of Glyphosate resistance.
An adequate Glyphosate rate should be used to avoid accumulated resistance through poor control, however this will quickly select out high Glyphosate resistant individuals.
A double-knock using Glyphosate and Paraquat can assist in management of resistance. The application of Paraquat needs to be 1 to 3 days after the initial application of Glyphosate, both at lethal rates.
There is a fitness penalty associated with Glyphosate resistance. After a 3 to 5 year competitive pasture phase, it is unlikely many Glyphosate resistant ryegrass plants would survive due to a reduced ability of the resistant plants to compete and set seed.
No such fitness penalty exists with resistance to other herbicide groups.
An in-crop herbicide resistance Quick Test which uses fresh plants will provide an answer on resistance status within 4-5 weeks. This can allow for active management in the crop.
A seed test for herbicide resistance can take 8-10 weeks and therefore fits in with a 12 month cycle for decision making from problem identification to action.
Rotating herbicide modes of action plus including non chemical methods of control will decrease the spread at which resistance develops.