Why do pre-em herbicides last so long before resistance bites?

Once in a while, someone comes along and asks a question that you have never given much consideration to.
Like, “Why does beer garden beer taste so bad?” or “Why do hotels always give away piles of stuff except for what you actually need, like a toothbrush or a razor?” and “Why do our pre-emergent herbicides last longer than our post-emergent herbicides before resistance bites”?
Sure, we had probably considered this question in the past, but many of us would have just assumed that the gene frequency for resistance to our pre-emergent herbicides was low.
However, Gayle Somerville, who recently completed her PhD at AHRI, has looked at it in a different way and has come up with an alternative answer. She undertook detailed computer modelling to look at the population dynamics of annual ryegrass.
She found that pre-em herbicides are slow to evolve resistance for two main reasons:
1.    It’s all about numbers.  Pre-em herbicides such as trifluralin are short-lived in the soil and may only be exposed to about 20% of the total ryegrass seedbank, whereas post-em herbicides are applied later in the season and may be sprayed over 40% or more of the total seed bank.  The more weeds a herbicide is exposed to the higher the chance of resistance, and;
2.    The post-em herbicide can protect the pre-em herbicides, like a double knock, killing any weeds that have survived the pre-em.
This will have major implications for our cropping future as we are now more reliant on pre-em herbicides and resistance to our post-em herbicides is becoming more common.
This research is hard to put into words, so click through to the website and take a look at the short videos that we have prepared that explain these concepts in simple terms.
Gayle Somerville, pre-em

AHRI PhD researcher Gayle Somerville

Watch the videos!

The easiest way to understand this research is to take a look at the videos. Those of you who prefer reading text or have poor internet speed, a brief description is below.

Short-acting pre-em herbicides

The diagram below shows that a short-lived pre-emergent herbicide such as trifluralin is only exposed to a reasonably small number of ryegrass, whereas the post-emergent herbicide, applied much later in the growing season, is exposed to a greater number of ryegrass.

In simple terms, the pre-em herbicides are exposed to a small number of weeds and may get a double knock when a post-em is applied later, reducing the risk of resistance.

While the post-em is still working, resistance to the pre-em is unlikely. However, this can all change if resistance evolves to the post-em herbicide.

The post-em herbicides are exposed to more weeds and late germinating weeds (cohort 3) only get a single knock because the pre-em herbicide has worn off.

weed emergence, pre-em, post-em

Longer-lived pre-em herbicides

If we now consider pre-em herbicides that persist longer in the soil such as Sakura or Boxer Gold, we see in the diagram below that Cohort 3 is now much smaller. This is closer to being a double knock and the pre-em and post-em herbicides can look after each other a little more. There is still late germinating ryegrass that is only exposed to post-em herbicides but they are now fewer in number.

pre-em, post-em, herbicide

On the flip side, if a long acting pre-em is applied and there is no post-em herbicide option, there’s an increased risk of resistance evolving to the pre-em as it’s exposed to more weeds and there’s no ‘second knock’ (see diagram below).

pre-em, knockdown

What about dry seeding?

When we dry/early sow, we don’t get the opportunity to remove Cohort 1 with knockdown herbicides and we are totally reliant on our pre-em and post-em herbicides. With no knockdown and widespread resistance to post-em herbicides such as Diclofop and Clethodim, we now have a lot of pressure on our pre-em herbicides.


In the past in Australian cropping systems, we have often achieved a knockdown of weeds prior to seeding and we were fortunate enough to have effective pre-emergent and post-emergent herbicides to choose from. Now our modern farming system often relies on early sowing without a knockdown, and resistance to many of post-emergent herbicides is common. We need to understand that while we are fortunate to have several effective pre-emergent grass herbicides, these herbicides are now under greater pressure. Resistance to pre-emergent herbicides has evolved slowly in the past, but this may not be the case in the future.


Contributing authors

Gayle Somerville, Prof. Steve Powles, Dr Michael Walsh and Dr. Michael Renton


Why was resistance to shorter-acting pre-emergence herbicides slower to evolve

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