Before herbicide selection has taken place it is very rare for an individual weed to be resistant to two herbicides. Mixing herbicides at full label rates in a single application takes advantage of this fact.
Within an integrated weed control program, try to make sure there is rotation and mixing going on in each phase – in the fallow, pre-seeding, in-crop and for desiccation. When done in conjunction with a determination to stop seed set and remove survivors, it is possible to keep weed numbers low.
Principle #1 Rotating buys you time, mixing buys you shots
Herbicide products are classified and grouped according to their mode of action (MOA). That is, products that target the same lethal pathway are grouped together.
Rotating between herbicide modes of action has the beneficial effect of ‘buying time’ because if a MOA is used once every two years the lifespan of the herbicide effectively doubles.
Rotation of effective modes of action can significantly delay the onset of herbicide resistance and needs to be built into your crop rotation plan. Herbicides in Group A and Group B are particularly susceptible to multiple exposure resistance with as few as six exposures being enough to select for the resistant mutation.
By mixing MOA groups, either in the same tank mix or applied separately to the same population (like a double knock), those plants that survive one MOA are often killed by the second – this ‘buys you shots’.
Principle #2 Rotate between herbicide groups
Start with a herbicide susceptibility test to find out what herbicides and herbicide mixes are still effective. Rotating between products within the same MOA group is the same as using one product all the time and is a very high risk weed control tactic.
Crop rotation can drive rotation of herbicide MOA. For example, if you use trifluralin in canola, consider another registered MOA option for wheat.
Based on research from the Australian Herbicide Resistance Initiative, the best advice to growers and agronomists is to rotate between these three groups of pre-emergent herbicides – 1. trifluralin, 2. Sakura, Boxer Gold and triallate and 3. propyzamide. Full label rates must be applied.
Principle #3 Use different groups within the same herbicide mix
Mixtures are more effective than just rotating MOA in delaying resistance as mixes generally achieve a greater kill rate. While some mixes are physically compatible, the components might have different requirements for optimal application – such as droplet size and and water rates.
With an increasing number of proprietary herbicide mixes coming onto the market and the broad spectrum of synergistic and antagonistic interactions between potential mixing partners it pays to be well-informed and to seek advice.
Rotating herbicide groups is an effective way of slowing resistance build up and managing resistance while ever there are actives to go to. Mixing herbicides from the same or separate active groups can also give you room to move in holding off resistance or getting more out of some marginally effective products.
In fallows, where there is no additional crop competition, a second (double knock) application to the same weed germination can be very effective. Some common double knock approaches that use the mix and rotate strategy are:
Group M (glyphosate) + Group I (2,4-D or fluroxypyr or picloram) followed by Group L (paraquat)
Group M (glyphosate) followed by Group L (paraquat) + Group G (Sharpen® or flumioxazin)
Group M (glyphosate) followed by Group L (paraquat) + Group K (Dual® Gold)
Group A (Shogun®) followed by Group L (paraquat) + Group K (Dual® Gold)
The only way to stave off herbicide resistance completely is to have low weed numbers and to be vigilant about preventing survivors from setting seed. Have a diverse cropping program, use herbicides to provide early weed control, set your crops up to compete strongly and monitor and remove survivor weeds.
Principle #4 Always use full label rates
Reducing the application rate of herbicides increases a weed’s ability to evolve resistance.
Any saving in chemical costs is significantly outweighed by the risk of the low dose causing faster herbicide resistance evolution.
Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.
Some herbicides require better coverage. In many instances the most important mixing partner is more water.