Occurrence

In the UK, there are five main species of perennial grass weeds of arable crops that are commonly known as couch or twitch.  These are:

Elymus repens (= Elytrigia or Agropyron repens) – common couch.  Very common throughout England and Wales, more scattered in Scotland.  Occurs in a wide range of habitats, including field margins, hedgerows and rough grassland as well as arable fields.  Up until the mid-1970s this was the most important weed of arable crops in the UK.

Agrostis gigantea black bent.  Widespread in England, but mainly on lighter soils than common couch.  Probably under-recorded.

Agrostis stoloniferacreeping bent.  Very common throughout the UK in a very wide range of habitats but less of a problem as an arable weed than common couch.

Arrhenatherum elatius var. bulbosum onion couch.  A variant of false oat-grass which is abundant throughout the UK in field margins, hedgerows, rough grassland and waste ground.  The onion couch form has a much more restricted distribution as an arable weed, being largely confined to southern and central England.

Holcus mollis creeping soft-grass.  Widely distributed throughout the UK and occasionally a weed of arable land, usually on lighter, acidic soils.

Identification

Common couch (Shown in picture, via Wikipedia) is characterised by having creeping, white underground stems (rhizomes), shoots with small auricles and a short ligule at the base of the leaf blade.  The heads (spikes) superficial resemble perennial rye-grass, but the spikelets sit flat against the stem rather than with the narrow, rounded side adjacent to the stem as in rye-grass.

Black bent also has rhizomes but has a long ligule, no auricles and the heads are large, loose panicles.  Creeping bent is somewhat similar to black bent, although mature plants are shorter and produces creeping stems (stolons) on the soil surface, not underground rhizomes.  Creeping soft-grass produces rhizomes, has conspicuously hairy nodes on the stem and a compact panicle.

Onion couch looks like false-oat grass with large loose panicles producing relatively large seeds with awns.  However, the stem bases are swollen to form several round, onion-like corms, up to 1 cm in diameter, hence the common name. 

Damage

Dense infestations of couch may produce several hundred heads/m2 reducing cereal yields by 25% or more.  High populations of plants can slow harvesting and the rhizomes interfere with cultivations and the harvesting of root crops by getting wrapped around equipment.  Couch acts as alternative host to the cereal take-all fungus which can have implications for the value of break crops in cereal rotations and take-all decline.

Why was couch a major problem in the UK?

In a survey published in 1910, couch was reported to be the most problematic weed of arable crops in the UK, just ahead of charlock.  This situation continued right up until the mid-1970s due to the lack of effective herbicides and the limitations of cultivations as a means of control.  This all changed with the introduction of glyphosate – a rare example of how a single highly effective herbicide can almost totally eliminate a major weed problem.   Couch continues to be a problem on organic farms and in some perennial crops where it can be difficult to apply glyphosate without damaging the crop. 

Agroecology – implications for control

  • Couch grasses owe their success to their ability to spread by means of horizontal stems growing through the soil (rhizomes) or across the surface (stolons).
  • These stem structures, which may extend to several metres, act as storage organs for food reserves.  Buds and adventitious roots are located along the length of these at frequent intervals.
  • The majority of the buds remain dormant unless plants are disturbed (e.g. by cultivation) when new aerial shoots or rhizomes will arise from many of them.  In uncropped land, rhizomes growth is greatest from May to November, with peak growth in June and July, although this may be disrupted by cultivations.
  • More couch plants arise from seeds than is generally supposed, especially with black bent, although the chief means of propagation in arable land is by means of the creeping rhizomes or stolons, fragments of which can produce new plants.
  • Non-inversion tillage, and especially direct drilling, greatly encourages couch as there is less disturbance to rhizomes and stolons than occurs with deeper cultivations.

Management recommendations – herbicides

  • In the UK, the introduction of glyphosate in autumn 1974 revolutionised the control of couch.  Prior to that, the herbicides available were aminotriazole, atrazine, bromacil, chlorthiamid, dalapon, dichlobenil, EPTC, propyzamide, TCA and terbacil.  None of these were as effective as glyphosate and most are no longer commercially available.
  • Pre-harvest applications of glyphosate are approved for use in wheat, barley and oats, combining peas, field beans, oilseed rape and linseed.  Applications must be made no less than 7 – 14 days pre-harvest, depending on crop.   Control of couch pre-harvest is dependent on adequate green leaf and favourable conditions.  If couch is senescing due to hot/dry conditions, control is likely to be reduced due to inadequate translocation to the rhizomes. 
  • The best control of onion couch is likely to be achieved by pre-harvest applications of glyphosate to earlier harvested crops, such as winter barley or oilseed rape, because it tends to senesce earlier than common couch.  
  • The most effective post-harvest technique is to leave stubble uncultivated and allow couch to grow to 4-5 leaves, typically 10 – 15 cm high, before spraying with 1.44 kg a.i./ha of glyphosate.  Cultivating prior to spraying reduces overall control as new plants are likely to emerge post-spraying from damaged fragments of rhizome.
  • Glyphosate is not very rainfast, and it important that no rain falls for at least 6 hours, and preferably 24 hours, after spraying.
  • Efficacy can be reduced by hard water so using a water conditioner can be helpful.
  • Glyphosate is absorbed through the leaves and translocated to the rhizomes.  To allow this to happen effectively, do not cultivate or drill for at least 5 days after spraying.
  • Activity against couch is best when growing conditions are favourable – control is likely to be reduced if plants are wilting or cold or heat-stressed.
  • Although glyphosate is highly effective against couch, it is slow acting and complete kill may take several weeks, especially in cooler conditions.
  • Other herbicides that give selective control of couch within crops include propoxycarbazone-sodium (wheat) and propyzamide (various crops).

Herbicide resistance

Only one case of evolved resistance to herbicides has been recorded in any species of couch worldwide - resistance to amitrole in creeping bent in pears in Belgium in 1986.  Couch appears to have low risk of evolving herbicide-resistance – almost certainly because it is largely propagated vegetatively, rather than by seed.

Management recommendations – Non-chemical control

  • Use of glyphosate is much the best method of couch control.  It is more effective, quicker and cheaper than non-chemical methods which may involve repeated cultivation of the soil, which is expensive, has a high energy requirement and is potentially damaging to soil structure.  Non-chemical methods should only be adopted when glyphosate cannot be used, such as in organic systems.
  • Map couch patches in June/July and focus management on these areas.
  • The primary aim of cultural control is to damage the rhizomes (or stolons) as much as possible in order to exhaust their food reserves.  Food reserves in fragmented rhizomes will decline until new growth has reached the 1 to 2 leaf stage, typically after 2 – 3 weeks, after which food reserves will increase again.  Consequently, repeated cultivation, whenever new growth reaches 2 leaves, will progressively exhaust the food reserves and reduce couch infestations.
  • Repeated cultivations are necessary as a single cultivation can be worse than none, as it fragments the rhizomes, breaks the dormancy of buds, stimulates new growth and results in an increase in the number of individual plants.  Ideally cultivations need repeating at least 3 times to achieve a reasonable degree of control.
  • Correct choice of cultivator is important.  Rotary cultivators are much more effective at fragmenting couch rhizomes than tine cultivators.  In an experiment set up in autumn 1968, but assessed a year later after a spring barley crop, a sequence of 3 rotary cultivations achieved 71% reduction whereas a similar sequence using rigid tine cultivators achieved only 44 – 56% reductions in couch rhizome dry weight.
  • Ploughing can reduce couch infestations but only if all parts of the weed are covered by at least 15 cm, and preferably 20 cm, of soil.  This requires ploughing to a depth of at least 30 cm and the use of efficient skimmers.  Subsequent cultivations should be relatively shallow to avoid brining rhizomes back to the surface.  Poor or shallow ploughing will achieve little and may distribute rhizomes throughout the plough depth making future control more difficult.
  • Desiccation of rhizomes has been used as a means of control.  This may be achieved either by using tine cultivators to drag out rhizomes which are then left on the surface or, on heavy land, producing large dry clods in which rhizomes desiccate.  Neither method is very effective unless hot, dry conditions persist for a long period.
  • Prevent importation of couch rhizomes or seed – many infestations originate from field margins where couch commonly occurs, so active headland management can prevent further ingress.
  • Avoid moving rhizomes on cultivation or harvesting equipment and try to prevent seed return, although this is a less important method of propagation than rhizomes.
  • Crop competition can enhance the control of couch weakened by burial or fragmentation.  Seedlings of couch are more sensitive to crop competition than regenerating rhizome fragments.  Couch is sensitive to shading and when continually shaded, tends to die out.
  • Perhaps surprisingly, given its biology and the fact that it is encouraged by non-inversion tillage, couch does not tend to persist in intensively managed grassland.  A three-year grass ley can greatly reduce couch infestations, provided the grass is cut or grazed frequently.

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