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CropLife Australia 2016 Guidelines for Group B Herbicides – Clearfield® Technology High Resistance Risk

Group B resistance exists in Australia in annual ryegrass, barley grass, brome grass, wild oats, paradoxa grass and crabgrass and in at least seventeen broadleaf weeds including wild radish, common sowthistle, black bindweed, charlock, Indian hedge mustard, prickly lettuce, Mediterranean (wild) turnip and  turnip weed. Resistance has developed in broadacre, rice and pasture situations.  In respect to rice three broadleaf weeds, namely dirty dora, arrowhead and starfruit, are known to have Group B resistant populations.

Research has shown that as few as four applications to the same population of annual ryegrass can result in the selection of resistant individuals and as few as six applications for wild radish. A population can go from a small area of resistant individuals to a whole paddock failure in one season.

A significant challenge facing growers managing Group B resistance is the control of brome grass and barley grass in winter cereal crops. Group B herbicides are presently the only in-crop herbicides that provide effective control of these grass weeds and this poses a severe risk of Group B resistance for growers with cereal dominant rotations.

If a pre-emergent application is made with a Group B herbicide for broadleaf or grass weed control, monitor results and, if required, apply a follow up spray; preferably with a non-Group B herbicide for control of escapes and to avoid seed set.  If a follow up group B (post-emergent herbicide) is applied; ensure that complete weed seed set control is achieved.

Whether using group B herbicides as a pre-emergent, or post-emergent application; consider the use of registered tank mixes with herbicides from other modes of action.

When using a group B herbicide for post-emergent broadleaf or grass weed control, this should be preceded by a pre-emergent herbicide treatment with other modes-of-action.

  1. Avoid applying more than two Group B herbicide treatments in any four year period on the same paddock. Where more than two treatments are applied introduce alternative control measures to avoid seed set and seed shed in the paddock.
  2. A Group B herbicide may be used alone on flowering wild radish only if a Group B herbicide has not been previously used on that crop.
  3. In all cases if there are significant escapes following the herbicide application consider using another herbicide with a different mode of action or another control method to stop seed set.
  4. Imidazolinone tolerant crops (Clearfield Systems): Where OnDuty, Midas and Intervix are used refer to the Clearfield Production Systems – best management practice guide ( If Sentry is to be used pre-emergent; consult the Crop Care Best Management Guide (