Herbicide resistant weeds
Closer look at northern plants
.I WILL continue to explain some of the technical terms and curious things weeds can achieve in our northern environment.
We are all aware of the very old selective broad leaf product in 24-D. It has been around since the late 1950s and we still depend on its efficacy for broad leaf weed control in 2018.
Given that we have become more responsible users and applicators of this very old product, then we can enjoy its very cheap and reliable efficacy for many more years... or can we?
Believe it or not there is herbicide resistance to 24-D by some of our broad leaf weed species in Australia.
Weeds like wild radish have become resistant to this very old herbicide.
One method of herbicide resistance development is by using lower or reduced rates of herbicide.
There have been plenty of discussions over the years about how using low rates may or may not lead to faster or confirmed herbicide resistance in a species.
Now this phenomenon may not occur across all weed species, so the arguing can continue, however weeds like annual rye grass from our southern Australia areas, which have cross pollination traits, certainly can accumulate the resistance level gene traits.
By that I mean when they outcross from one plant to another rye grass plant by pollen transfer, that any level of herbicide resistance from either plant can accumulate or add up in resistance levels.
A classic case of two-fold from rye grass plant A and two-fold from rye grass plant B making four-fold. This basic sum could be used to judge folds when we talk about resistance levels and control difficulty. A explanation of what we mean when we talk fold. It means we are judging how resistant a weed is to a particular herbicide.
The effective rate of a herbicide on a weed is the dose required for 50 per cent control of your normal population of a particular weed species.
If the resistant weed population takes twice the normal rate to achieve a 50 per cent result it is said to have a two-fold resistance.
If it takes 20 times the herbicide rate to achieve a 50 per cent result, then it is 20-fold. Therefore the higher the fold number stated, the harder it is to control with that herbicide.
So with in-crop herbicides for a weed like wild radish, you may nearly kill the cereal crop to get any effect on the weed population, if there is a five-fold level of resistance to that mode of action.
Understanding the mechanisms of herbicide resistance is one thing, however you need understand the terms that are used in talking about herbicide resistance to really engage in weed management programs.