How do pre em herbicides work?

[Devil's advocate]

I know the basics of pre em herbicides, a fine firm moist seed bed & don't disturb it afterwards.

But how does it actually kill the weed.

Does the weed germinate hit the pre em layer & die?

or Does the weed die as it germinates?

or is the seed discouraged from germinating & may even remain viable for another year.

 

[spin cycle]

i always thought that the weed was killed as the ' growing tip' tried to pass through the herbicide layer

 

[bobk]

It forms a chemical layer in the soil ,

 

[7610 super q]

Yes, as far as i can see, weeds still germinate and grow, but then turn yellow............and die.
Unless it's dry, in which case, bugger all happens........

 

[Devil's advocate]

Yes millions of the little feckers in the sprout field germinated but not through the surface layer of Springbok, I hope they do die. Find another way to kill'em if necessary.

 

[Goodolddays]

Depends on the chemistry, some form a layer near or on the surface that the growing tip of the weed grows through and picks up the chemical. Some move very slightly down in the soil profile where the uptake is via the weed seed or roots. Depending on speed of growth and chemical applied some weed shoots never emerge, some emerge and then die back.

 

[colhonk]

Simple answer....THEY DON`T anymore.

 

[ollie989898]

If I had a tenner for every time a farmer told me residual chemistry doesn't work I'd be a kazillionaire. Of course it works, you see a bit of a miss in the corner of a field or something the stuff is rank with weeds in no time. Even in a dry time the stuff is still there. PDM in particular is still there weeks after application. How each one works differs. PDM for example works by microtubule inhibition, so it stops cell division and assembly directly. Anything that does germinate will be slowed, weakened and smaller than usual until the effect can be 'out grown'.

 

[colhonk]

WHY is my wheat fields full of grass then if it works?

 

[Goodolddays]

WHY is my wheat fields full of grass then if it works?

When did you spray, what did you spray, how did you spray, what did you spray onto?
Soil residuals are impacted by the environmental conditions. Put the wrong thing on at the wrong time in the wrong conditions and they might not work as expected.

 

[ollie989898]

WHY is my wheat fields full of grass then if it works?

If you have weeds germinating from depth, they will be too strong or remain unaffected when they reach the layer of chemical in the soil. PDM is notorious for this. It's not so much dry soil conditions that are to blame, it's the depth from which weeds are emerging.

This is the biggest issue with minimum tillage- things like solos, sumos and topdowns with discs and tines mix huge volumes of soil in a kind of boiling action. This means seeds that have fallen out of the combine or out of the ear beforehand are mixed thoroughly throughout the soil profile. Ploughing, if done correctly, inverts the soil in one action and does not give this same mixing action. You can see this in the differences in levels of volunteers where min-till and ploughing are used.

It is possible that some farms have full blown resistance (probably enhanced metabolism) against some or even all soil acting herbicides however. The fact remains that many of these products cannot kill many established plants at any rate or in any conditions no matter how favourable. The point at which they are most effective is when the tiny white coleoptile (and first seminal root) is emerging from the seed. They absorb a dose of the chemical and cell division is disrupted meaning very poor or zero growth from that point on.

Unfortunately, trifluralin, flufenacet and pendimethalin all share this same kind of mode of action. From memory only IPU was a genuine photosystem inhibitor which made it a bit more effective even on established plants.

 

[Goodolddays]

If you have weeds germinating from depth, they will be too strong or remain unaffected when they reach the layer of chemical in the soil. PDM is notorious for this. It's not so much dry soil conditions that are to blame, it's the depth from which weeds are emerging.

This is the biggest issue with minimum tillage- things like solos, sumos and topdowns with discs and tines mix huge volumes of soil in a kind of boiling action. This means seeds that have fallen out of the combine or out of the ear beforehand are mixed thoroughly throughout the soil profile. Ploughing, if done correctly, inverts the soil in one action and does not give this same mixing action. You can see this in the differences in levels of volunteers where min-till and ploughing are used.

It is possible that some farms have full blown resistance (probably enhanced metabolism) against some or even all soil acting herbicides however. The fact remains that many of these products cannot kill many established plants at any rate or in any conditions no matter how favourable. The point at which they are most effective is when the tiny white coleoptile (and first seminal root) is emerging from the seed. They absorb a dose of the chemical and cell division is disrupted meaning very poor or zero growth from that point on.

Unfortunately, trifluralin, flufenacet and pendimethalin all share this same kind of mode of action. From memory only IPU was a genuine photosystem inhibitor which made it a bit more effective even on established plants.

Alot I would agree with, but parts not. Flufenacet is not the same mode if action to pendimethalin. Check out the HRAC 2021 world of herbicides mode of action poster.
Full blown resistance? Target site resistance is very common amongst UK black-grass and ryegrass populations. This is broadly speaking black and white and once you have it the contact type products like Atlantis will not work anymore. Non-Target Site Resistance which includes enhanced metabolism is much more shades of grey. You can have populations with it but the chemistry will still work. This is why pre-ems are important, as they hit the weed at its youngest growth stage.

 

[wanton dwarf]

It is possible that some farms have full blown resistance (probably enhanced metabolism) against some or even all soil acting herbicides however. The fact remains that many of these products cannot kill many established plants at any rate or in any conditions no matter how favourable

The only thing I would add from my experience is that the total number of weeds germinating seems to be important.

We have good control over most ground but where populations were high last year we have poor control - don't know exactly how we are going to handle that this year as the Sprayer also had a fault during applying pre-ems.

 

[colhonk]

Liquid Avadex Factor and Shooter, (that orange stuff ) pendimethalin / flufenacet full rates.
ploughed land (reluctantly) to get a clean soil to spray onto.

 

[Goodolddays]

Liquid Avadex Factor and Shooter, (that orange stuff ) pendimethalin / flufenacet full rates.
ploughed land (reluctantly) to get a clean soil to spray onto.

Drilled when roughly, September, early or late October or later?
Cloddy seedbed or fine and consolidated?
Any autumn post-em follow up?
Do you know if black-grass or ryegrass?

 

[alomy75]

Liquid Avadex Factor and Shooter, (that orange stuff ) pendimethalin / flufenacet full rates.
ploughed land (reluctantly) to get a clean soil to spray onto.

Unfortunately in my experience that mix will give you 70% control at best. 30% of a lot is still a lot. Swap the liquid for granular Avadex and add hurricane to your shooter; this is probably the best bang for your buck. Or wait for BASF’s new offering this autumn…

 

[colhonk]

Early October sown. in between seedbed, double rolled. ryegrass.
SSSHHH, don`t tell anyone but accidently put liqid on to the same rate of product as granular. Palio in April.

BASF’s new offering this autumn…

Tell me more.

 

[Goodolddays]

Early October sown. in between seedbed, double rolled. ryegrass.
SSSHHH, don`t tell anyone but accidently put liqid on to the same rate of product as granular. Palio in April.



Tell me more.

So might have needed more, an autumn post-em, more flufenacet maybe some prosulfocarb as the ryegrass might have continued to germinate throughout the autumn!?

Hearing good things about it, particularly on ryegrass.

 

[alomy75]

Early October sown. in between seedbed, double rolled. ryegrass.
SSSHHH, don`t tell anyone but accidently put liqid on to the same rate of product as granular. Palio in April.



Tell me more.

It’s pretty epic. Cost yet to be released as far as I know. Won’t be cheap.

 

[robs1]

WHY is my wheat fields full of grass then if it works?

Because a lot of sprays run out of steam before spring and it seems bg in particular has adapted to germinate in late march/april, this is becoming a major issue