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.


WEED CON­TROL OP­TIONS: Agron­o­mist Paul McIntosh dis­cusses plants that de­velop her­bi­cide re­sis­tance.

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