I WILL continue on explaining about some of the technical terms and curious things that 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 of broad leaf weed control in the 2018 year.
Given that we 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 too 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 way of herbicide resistance development is by using lower or reduced rates of herbicide.
There have plenty of discussions over the years about 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 ryegrass from our southern Australia areas that have cross pollination traits, certainly can accumulate the resistance level gene traits.
By that I mean when they outcross from one plant to another ryegrass 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 ryegrass plant A and two fold from ryegrass 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 specie.
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 a 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 affect 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.
credit:ruralweekly.com.au