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Why spray every weed?

Last post Thu, Sep 14 2006 14:29 by anonymous. 9 replies.
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  • Thu, Sep 14 2006 14:29

    Why spray every weed?

    Direct Talk on...Weeds expert John Cussans says ...

    It’s easy to make bold statements about the level of herbicide use by the industry as a whole - and they are the messages taken up by many political lobbyists.

    The conundrum is that it’s very hard to say exactly how an individual agronomist or farmer could sensibly reduce their use.

    We know much about the impact on crops of individual weed species, and overall it seems the level of weed control (and hence herbicide use) is too high to be justified purely on economics.

    Recent trials in winter wheat by ADAS and ourselves found that in over a third of the nearly 200 comparisons carried out weed control could not be currently justified.

    So why are almost 100% of arable fields sprayed with herbicide – often more than once?

    Not all weeds are the same, but that’s a secret we seem to be keeping from the lobbyists.

    Some species can easily be tolerated. Others, like blackgrass, wild oats, ryegrass and cleavers cannot.

    We shouldn’t criticise others while not remembering this basic principle ourselves, namely not all weeds are the same and we must focus on the most damaging species.

    What justification is there in chasing that last little bit of poppy, mayweed or groundsel?
  • Thu, Sep 14 2006 15:24 In reply to

    • 2690695
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    • Joined on Sun, May 22 2005

    Re: Why spray every weed?

    John, I would dearly love to only spray once, but I get the little problem of subsequent flushes of weed growth.

     However that said, I do suffer from weed flushes after "late season"rains especially in beans and spring barley. These flushes normal coincide just outside lable last timings!!

  • Thu, Sep 14 2006 20:42 In reply to

    Re: Why spray every weed?

    Fair comment Stephen...

    A one size fits all recommendation (in this example about spraying only once) is not sensible - if it's a problem then it needs to be dealt with.

    The main aim of the article was to get people to think again about whether sprays are really justified - to focus in on what really is a problem (and not respond to all weeds in the same way I guess).

    What sort of (weed) species do you get emerging late and how late compared to the crop sowing date - if they're emerging much later than the crop in your spring barley do they really represent a threat to your yields?

  • Fri, Sep 15 2006 19:44 In reply to

    • Landy
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    • wilts

    Re: Why spray every weed?

    I agree there is no point in chasing every weed, however if you let things slip you soon become over run with such weeds.  Many weeds eg. groundsel can be cleared up cheaply with a preharvest glyphosate which also helps harvesting, so again it is controlled. 

    Still my thoughts are you need to target a very broadspectrum to control as much as possible.  If you have the majority of weeds under control rates of certain products can also be cut back slightly.

  • Mon, Sep 18 2006 10:04 In reply to

    Re: Why spray every weed?

    Valid points - particularly w.r.t the future impact of reduced weed control (which I'm dodging!).

    So OK broadspectrum weed control is your strategy - are you paying a premium for that broad specturm control- over and above what it would cost just to target the priorities?

    Are you using pre-harvest glyphosate to dessicate for harvesting reasons or as a herbicide - if for the later what is it costing you to control (what I assume to be a really quite low density of) groundsel?

    I guess my question (for most growers) is has the change in the economics of crop production (post SFP) changed you atitude to weed control?

  • Mon, Sep 18 2006 11:26 In reply to

    Re: Why spray every weed?

    John, all


    From an agronomists point of view it is very difficult to be very selective in weed control. I am often advising growers that the weed/s left are not of economic importance, particularly late season and that a pre-harvest glyphosate would be a more effective route with all the other assosiated benefits, however, in targetting the key weeds in autumn or early spring the products we now use as a default i.e Atlantis, Lexus, Flufenacet, pendimethalin, Picolinafen and DFF in autumn, not to mention the rangeof spring applied SU's, all pick up a very broad-spectrum of weeds and of course we always have a watchful eye on the following crop as a simple weed in cereals can be an expensive target in other broadleaved break crops. One further consideration is the harvesting machinery and seed size with regard to crop contamination. Rotary style seperation units on combines are now far more common and these machines will not tolerate green material and harvesting efficiency is rapidly lost. 


  • Mon, Sep 18 2006 14:52 In reply to

    Re: Why spray every weed?

    I agree that it isn't necassary to clean out 100% of weeds present in a crop but it takes a brave man to make the decision which could mean you're either agronomically and financially astute or the local laughing stock of all your neighbours! 

    Does he worry about carry over of pests and diseases in weeds especially with the advent of the Entry Level Scheme and over-wintered stubbles?

    Mark Ireland

  • Mon, Sep 18 2006 20:34 In reply to

    Re: Why spray every weed?

    Mark -
    That's an observation that I 100% agree with - I do worry that the fear of what the neighbours will say is driving decision too much. Is perhaps another factor where farmers and growers are getting their advice from 'on-farm'. Does the rise of the self employed advisor - maybe introduced a built-in conserservatism to the decision making process - there's a fear of failure (or perceived failure) leading to loss of the contact?

    As for the comment about stubbles - as weed scientists we need to know much more about what's happening to weeds in stubbles. In theory over-winter stubbles as well as having the obvious and proven advantages for over-wintering bird populations they could be a great opportunity to reduce population levels of some key grass-weed species but it's not an area we've done enough work on. Perhaps I digress but it's worth noting that some nice work (done I think by the BTO and Long-Ashton research station (RIP)) showed that effective weed control in the crop before the over-winter stubble almost negated the positive biodiversity effects
     of the stubble period itself!

  • Wed, Sep 27 2006 9:53 In reply to

    Re: Why spray every weed?

    There’s a lot of peer pressure from farming neighbours which gets passed back to the advisers! Farmers still very proud of how crops look. They fear weed seed build-up.

    If the farmer wants total weed control then I have to follow but I’m hoping the 21st Century will see a change in mindset to allowing a few weeds as long as they’re not the most damaging ones and can be controlled elsewhere in the rotation.

  • Tue, Nov 14 2006 11:10 In reply to

    Re: Why spray every weed?

    Not wanting to get a backlash from child welfare - get the sons to do it by hand. There is no-way to get rid of all weeds, but as a mechanised arable farmer on heavy land I find the easiest way to go for that last head of blackgrass is through a reasonably robust herbicide program. Come some pleasant sunny spring days, get roguing.

    I have no sons of my own to do it yet, but being marched through sugarbeet fields by my father pulling weed beet has made me a tight-fisted perfectionist. Black grass, wild oats, bromes, cleavers are suprisingly easy to get through providing the stuff my agronomist has sold me earlier has not left vast swathes.

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