FOR some years now a double-knock system has been used around Australia and regularly describes two sequential doses of herbicides with different modes of action to control various difficult weeds.
This timing between these applications can be slightly adjusted to current workload and weed reaction times, however, the basic plan is to use the herbicide glyphosate (Group M) in the first pass, which is believed to translocate to the root system and put the weed under some "stress".
This is then followed up preferably seven to 10 days later with the Group L bipyridyls, which is designed to break down and destroy the above-ground part of the plant being controlled.
With glyphosate in the root system and then paraquat or sprayseed as an over-the-top second-pass spray application, plant recovery from this spray regime will be low. This practice has done a good job of slowing down our increasing number of glyphosate-resistant weeds.
A couple of notable points is that mixtures are best done in the first pass, with products like 24-D IPA or Dicamba/Tordon 75-D as admix choices for extra broadleaf control with the glyphosate.
It is not the only time to use mixtures, but the most preferred.
The second point is ever since these bipyridyls such as sprayseed or paraquat have been around, applying to any large weed needs exemplary coverage.
In fact, I will go so far as to say many a time have I been faced with large barnyard and urachloa grass plants, and in those early days of registered aerial applications, I advised against using these types of burn-down products due to a potential poor result.
These last couple of decades have continued with no registered application methods by aircraft or hand-held sprayers, and rightfully so, as these paraquat and diquat herbicides do carry a Dangerous Poison tag.
With nozzle development on our ground rigs getting better every year, I would judge that our twin jets are the best all-rounders at this stage. These appear to give us the best coverage and not so much drift issues.
I really am thankful for closed systems on our ground rigs, which allow us to use and transfer these herbicides in relative safety.
Back in the late 1970s, when these fast-acting photosynthesis inhibiters were released, we were made aware of its very low LD50 dermal and oral status. It was even touted that its original manufacturers, ICI, had to add a pungent odorous chemical agent to dramatically increase the smell of the product to warn users and applicators of airborne particles of the product. These days our use patterns are in the double-knock method, and with our increasing glyphosate resistance problems, the paraquat herbicide group has been well adopted to facilitate this second spray option and really does maintain our efforts to continue to zero till.
The drift problem with errant droplets downwind shows up on any green plant material the droplet lands on, and has been used to practically test shielded sprayers' efficacy for spray leakage from under the hood.
This unique product works fast on green plant material and is generally rain-fast within an hour of spraying. However, like glyphosate products, it dislikes dusty conditions and spray water quality is still important.
So continue to use your paraquats or diquats in your weed control. You just need to be responsible.
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