30 August 2002

Knowing your couch type

Couch grasses are the

bane of many cereal

growers lives. But fine-tune

control to the species

present and numbers can

be reduced, says an

industry expert

MANY UK farmers are failing to get on top of their couch grass problems, because the weed in the field is not the weed they think it is, warns a leading technical expert.

Nearly 30 years on from the advent of glyphosate and with no known instances of resistance to the herbicide anywhere in Europe, couch really should not be a problem, maintains Colin Stride of herbicide supplier Monsanto. That is particularly so given the low treatment cost, which is now little more than £10/ha (£4/acre).

So why does the weed continue to present problems on arable units up and down the country? "It is really very simple," he says. "Time and again when I visit people having couch control problems I find couch is not, in fact, the problem they have. Or more specifically, it is not common couch.

"They mostly have onion couch or Yorkshire fog or creeping soft-grass or great, black or creeping bent. All look remarkably alike when they are small. And they are not that easy to tell apart at first sight even when they are larger. So they tend to be lumped into the single category, couch."

He urges growers to look at the weeds in more detail and tailor control accordingly (see table).

The most notable difference is in the way the different species grow and survive from season to season.

Despite a variety of tough and extensive roots, rhizomes, bulbils and stolons, all can be successfully controlled with glyphosate, he says. But the treatment programme must be carefully matched to their biology.

All the perennial grasses survive through a tough "perinating organ". This is packed with carbohydrates, which are mobilised upwards to support new foliar growth each season. As the new growth matures the nutrients it produces are progressively redirected back down the plant to swell its over-wintering reserve.

Different patterns of growth and maturity between the species, combined with a wide range of different perinating organs, mean a single treatment programme will not be equally effective against them all.

Most fields have a preponderance of one species of "couch", Mr Stride notes. That and the fact that modern arable rotations provide several annual opportunities for perennial weed spraying makes excellent control in a single season achievable with almost all populations, he says.

Set-aside is one of the best opportunities for getting on top of all perennial grass weeds, he says.

The key is to spray once all the ears are fully emerged, but while the seed heads and leaves are still green.

"This will vary with species and season from early June to early July. In particular, be aware that common couch, with its deeper roots and greater drought tolerance, will mature rather later. It is important you hit the weeds at the right time so translocation will maximise downward glyphosate flow to the roots for the best long-term kill. Go in too early for your main target and less complete downward translocation will limit your control.

"Pre-harvest treatment of cereals or oilseed rape as part of a harvest management programme can give very valuable perennial weed control," he adds. "But, it is really only suitable if you are tackling the later maturing common couch. Onion couch, Yorkshire fog, creeping soft-grass and the bents will generally have senesced in July so you are unlikely to get consistent control in wheat. Oilseed rape and winter barley can present pre-harvest opportunities for these weeds, though."

Stubble control

The fact that all the couch-like perennials behave in a similar way in the autumn makes stubble control another attractive option.

Here, too, Mr Stride insists success depends on a clear understanding of the biology of the target weed.

"In all cases, cultivations are the best way of making your problem worse," he says. "They chop and spread the roots, rhizomes, bulbils and stolons to multiply the number of plants many-fold. A few tufts one year can easily cover a field the next.

"So if you want to control perennials between harvest and drilling, do not cultivate the stubble until you have been on with the Roundup. And make sure you leave it for long enough to allow the greatest possible shoot emergence from new rhizomes or bulbils or stolons before you spray – late September or early October is ideal. This does not fit comfortably with moves to reduced tillage or early drilling, so careful planning is vital.

"Bear in mind with common couch that regular ploughing can divide the rhizomes into two distinct populations. Those within the plough depth will emerge rapidly and readily in the stubble. But those below it will grow up into the ploughed soil to emerge later, if at all. Many, indeed, have sufficient reserves to survive underground to be chopped off by the next ploughing. In this way they are able to continually top-up the shallower population.

"Onion couch presents an even greater autumn challenge," insists Mr Stride. "Its roots take the form of bulbils – strings of up to six small bulbs joined by what is, by the end of the season, dead tissue. Shoots from the upper two bulbils reach the surface quickly. But if you spray too early then all you kill is the upper bulbils, leaving shoots from the lower ones to emerge unscathed later in the season.

"Even if you kill all the bulbils you also need to watch out for seedlings with onion couch," he says. "Being just about the only perennial that seeds readily under normal conditions, it can catch you out.

"You do not expect it to be there, so it is easy to mistake for annual meadow grass. In the light and chalky ground in which it grows so well you are unlikely to be needing a robust autumn annual grass weed programme. So you leave the seedlings until the spring then go in with IPU.

"While this would have been effective early enough in the autumn, it cant touch the first bulbils that will already have been formed by February. True, the onion couch is still susceptible to Monitor (sulfosulfuron) or Topik (clodinafop propargyl). But you would not think to use it unless you knew what your couch was, or had a brome or wild oat problem."

As well as planning the timing of Roundup spraying to account for the differences between the couch-like perennials, Mr Stride advises growers to match dose rates, formulations and spray volumes carefully to the species they need to control.

"Common couch is relatively easy to control. Recommended rates for Roundup Biactive increase from 2 to 4 litres/ha, depending on timing and weed population. With the others you must use the full 4 litres/ha rate regardless of numbers.

"Particularly good wetting is essential too with the hairy leaved species like Yorkshire Fog and creeping soft-grass. You are always better off using 200 litres/ha rather than just 100 litres on these. And you should be including a higher concentration of wetter.

"The faster uptake and improved performance under tough conditions offered by new 450g/litre Roundup Gold, with our patented Transorb technology, is particularly valuable when tackling the tougher couches.

"Its lower sensitivity to environmental conditions and wider application window is especially welcome when it comes to getting on top of these perennials," he concludes.

KNOW YOUR COUCH TO BOOST CONTROL

Key characteristics Species Best control advice

Large underground rhizomes. Leaves smooth, ligule & auricles v short. Common couch Treat in set-aside or pre-harvest or autumn stubble. Use rate appropriate to plant population.

Short underground rhizomes. Leaves striate, long toothed ligule. Great/black bent Treat in set-aside or early pre-harvest or autumn stubble. Beware light soils pre-harvest. Watch out in wheat. Use highest label rate. Beware lighter drought prone soils.

Stolons along soil surface. Leaves striate, with long narrow ligule Creeping bent grass Treat in set-aside or early pre-harvest or autumn stubble. Watch out in wheat. Use highest label rate. Beware lighter drought prone soils.

Bulbils in chain 2-6 units long. Leaves keeled, long torn ligule. Tufted. Onion couch Treat in set-aside or early pre-harvest or autumn stubble. Watch out in wheat. Use highest label rate, or a split sequence though the season. Beware lighter soils. Treat seedlings in stale-seed-bed or in-crop with IPU or clodinafop.

Leaves softly hairy, striate, no auricles. Leaf sheaths hairy, often violet striped. Tufted with simple roots. Yorkshire fog Treat in set-aside or early pre-harvest or autumn stubble. Watch out in wheat. Avoid low water volumes and ensure sufficient surfactant used. Use highest label rate. Beware lighter drought prone soils.

Leaves softly hairy, striate, no auricles. Leaf sheaths hairy, white or brown. Tufted with short underground rhizomes. Creeping soft grass Treat in set-aside or early pre-harvest or autumn stubble. Watch out in wheat. Avoid low water volumes and ensure sufficient surfactant used. Use highest label rate. Beware lighter drought prone soils.

Couch species and control methods

Key characteristics Species Best control advice

Large underground rhizomes. Common couch Treat in set-aside or pre-harvest or autumn

Leaves smooth, ligule & stubble. Use rate appropriate to plant

auricles v short. population.

Short underground rhizomes. Great/black bent Treat in set-aside or early pre-harvest or autumn.

Leaves striate, long toothed ligule. stubble. Beware light soils pre-harvest. Watch out

in wheat. Use highest label rate. Beware lighter

drought prone soils.

Stolons along soil surface. Creeping bent Treat in set-aside or early pre-harvest or autumn

Leaves striate, with long narrow ligule. grass stubble. Watch out in wheat. Use highest label

rate. Beware lighter drought prone soils.

Bulbils in chain 2-6 units long Onion couch Treat in set-aside or early pre-harvest or autumn

Leaves keeled, long torn ligule. stubble. Watch out in wheat. Use highest label

rate, or a split sequence though the season.

Beware lighter soils. Treat seedlings in stale-

seed-bed or in-crop with IPU or clodinafop.

Leaves softly hairy, striate, no auricles. Yorkshire fog Treat in set-aside or early pre-harvest or autumn

Leaf sheaths hairy, often violet striped. stubble. Watch out in wheat. Avoid low water

Tufted with simple roots. volumes and ensure sufficient surfactant used.

Use highest label rate. Beware lighter drought

prone soils.

Leaves softly hairy, striate, no auricles. Creeping soft Treat in set-aside or early pre-harvest or autumn

Leaf sheaths hairy, white or brown. grass stubble. Watch out in wheat.

Tufted with short underground rhizomes. Avoid low water volumes and ensure sufficient

surfactant used. Use highest label rate.

Beware lighter drought prone soils.