Some time ago I had occasion to venture down to an Area Wide Management of Weeds field day in southern inland NSW and was dismayed with the weed burden in this photo I took of a recently harvested wheat paddock.
Looking out across that 700 to 800 acre block, it was full of a regrown broadleaf weed named Flaxleaf Fleabane, with a newish botanical name of Erigeron Bonariensis. It was literally infested everywhere inside and outside the paddock fence lines.
Of course there are three main species of Fleabane in Australia with the worst type being what we call Tall Fleabane with some nasty habits or features and very present in our Northern region.
This tough tap-rooted plant can be an all year round growing type and is more of a perennial plant than not. Capable of producing 100,000 seeds from one active growing plant, its positive uses or benefits in my book are nil.
Originally from the Americas, this Fleabane has spread worse than our infamous cane toads I believe.
Fleabane of all three species has spread fairly well across the country into Australian footpaths, seaports, car park areas, grain handling facilities, airports, saleyards, trucking depots, main roads, farmers sheds, peri-urban properties and even on the top of tall city buildings I have found fleabane growing happily.
What is worse is that bio types of this tall fleabane have been for some years (and getting worse ) is the increasing levels of Paraquat and Glyphosate resistance.
However a very old herbicide product is still earning its keep, with reasonable control of this pesky weed and that is the old original 24D and Picloram mix called Tordon 75-D ( was Tordon 50-D ).
Now if this weed is two feet high and been around for several months of tough growing times, then it is only a cold steel or a mechanical operation to kill it.
The Tordon products with Picloram in the mix, can certainly leave a few months soil residual for quite a few emerging broadleaf weeds species, after spray application onto basically bare paddocks.
The broadleaf weed knockdown capability of these Picloram mixes I rate as moderate and yet I rate the residual control capability on broadleaf weeds like stramonium, milk thistle, fleabane and others as fairly good.
Not perfect , however fairly handy on weeds we struggle to control or stop seeding.
A couple of key points with Picloram and to a similar extent with Clopyralid ( originally Lontrel ) is that if soon after your fallow spray application, rainfall occurs, then your residual length be it a good or bad thing for you, can lengthen out.
Good for controlling weeds or bad if you were intending to plant a sensitive crop like Mungbeans or Chickpeas.
Conversely, if no rain occurs for some many weeks or months and sunlight pours down on those surface sitting molecules of Picloram herbicide, then the residual length of this herbicide will be considerably shortened.
By same token, applying these registered Picloram herbicides into a winter cereal crop has the added twist of the crop adsorbing and therefore retaining Picloram in the actual stubble for some time, until the stubble actually breaks down by microbial action releasing the still active picloram into the soil…..more time to pass for sensitive crops planting.
How can you tell if you have a residual herbicide, that may not perform like the label says it should, as far as plant backs are concerned?
Well you have to really know your weed spectrum and you look for normal growth of sensitive plants growing or not growing in that residual herbicide area.
For example, if I was concerned that picloram was still active in the paddock and especially in the thick pin trash trails of the header, I would check for sensitive plants like milk thistle, pigweed, mint weed or even plant some small area of legume crop seed for checking of normal growth.
After all my suggesting the tips and tactical warnings when using these old, but handy herbicide products, I still believe that with knowledge on your side these knockdown and residual herbicides are worth a seat at your farming systems weed control table.
