Published on 01/11/2022
Article supplied to Rural Contractor by BASF
Sustainability is an overused word, but that is what resistance management is all about. We want to be able to continue to grow maize for grain, seed and silage well into the future.
Uncontrolled weeds can reduce maize yields by as much as 70 percent, so we have to make sure the ways we are controlling them now won’t wreck our chances of controlling them in the future.
In the early 1980s fathen was the first weed in New Zealand found to be resistant to an herbicide – in that case to atrazine (Mode of Action Group 5, photosynthesis inhibitors). Switching to dicamba with its different mode of action (MoA Group 4, hormone mimics) worked for some years, but now there are fathen populations with resistance to both herbicide groups.
Now we have atrazine resistance in willow weed and black nightshade, and resistance to glyphosate has been recorded in Italian ryegrass and perennial ryegrass in vineyards and orchards.
The pipeline for chemistry is not free-flowing with an endless supply of new herbicides, and public concern around toxicity has raised the bar for registration. Therefore, we cannot count on new modes of action regularly coming to the rescue, and it is crucial that we protect the ones that we have.
Good agrichemical stewardship is vital, and it starts by applying the right chemical at the right time using the right equipment, correctly calibrated. The next step is to rotate between herbicides with different MoAs.
In maize management, there are three slots for herbicides: burndown, pre-emergence, and post-emergence. Herbicides are grouped into different modes of action and a single group may include chemicals used in more than one of these slots.
The rotation of herbicides starts at burn-down. Glyphosate (MoA Group 9) is by far the most used herbicide at this stage, but herbicides from another MoA group are regularly added to increase its speed and spectrum of control. That ‘spike’ is commonly from MoA Group 14.
Pre-emergence herbicides are applied right after planting. A tank-mix of two or three herbicides from different MoA groups can cover grasses and dicot weeds. Controlling weeds by more than one herbicide also helps prevent resistance build-up.
FAR’s Weed Management in Maize fact sheet says controlling weeds after emergence is challenging because post-emergent herbicides control a narrower weed spectrum and are more sensitive to the size of the weeds and the crop.
All growers must understand that effective weed and herbicide resistance management relies on rotation between MoA groups across the full crop cycle from burndown to post-harvest.
Choose herbicides for their suitability at each stage and for their coverage of the weeds present. Be mindful of which MoA groups you have used before. Some groups have many active ingredients and 20 to 40 products within them.
Follow the resistance management guidelines on the label for each product. Follow the label instructions to give herbicides every chance to succeed and minimise the number of survivors. Treated but uncontrolled weeds are a major driver of resistance.