Know Your Enemy In 'Couch' Control

July 2006 - Many farmers across the country are failing to get on top of their couch because it's not the weed they think it is, warns Bob Mills of Frontier Agriculture.

More than 30 years on from the advent of Roundup (glyphosate) and with no known instances of resistance to the herbicide anywhere in the UK couch really shouldn't be a problem. All the more so with treatment costs as low as they have are now. But the weed continues to present problems on arable units up and down the country. Why ?

"It's really quite simple," explains Mr Mills. "All too often when I visit people having couch control problems I find couch is not, in fact, the problem they have. Or more specifically, they don't have common couch.

"Instead, they have a range of problems, including onion couch, Yorkshire Fog, creeping soft-grass and great, black or creeping bent. All look remarkably alike when they're small. And they're not that easy to tell apart at first sight even when they're larger. So they tend to be lumped together under single heading, couch.

"Look at the weeds in more detail, though, and you'll see clear differences in the way they 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 the powerful new, Roundup Ace herbicide. But the secret of success is undoubtedly to match the treatment programme carefully to their biology (Table )."

All the perennial grasses have one thing in common. They survive through a 'perennating 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 re-directed 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 perennating organs, mean a single treatment programme can never be equally effective against them all.

Thankfully, in Bob Mills' experience, most fields have a preponderance of one species of 'couch'. This, and the fact that modern arable rotations provide several annual opportunities for tackling perennial weed makes excellent control in a single season perfectly achievable with almost all populations.

"Set-aside is one your best opportunities for getting on top of all perennial grass weeds," he points out. "The key here is to spray Roundup Ace 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 than its partners in crime. It's important you spray 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 too," he adds. "But, it's only suitable if you're tackling the later maturing common couch. Onion couch, Yorkshire fog, creeping soft-grass and the bents will generally have senesced in July so you're unlikely to get consistent control in wheat. Oilseed rape and winter barley can present pre-harvest opportunities for these weeds, though."

The fact that all the couch-like perennials behave in a similar way in the autumn makes stubble control another attractive option. Here too Bob Mills stresses success depends upon 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, don't cultivate the stubble until you've 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 - mid to late September is ideal. This doesn't fit comfortably with moves to reduced tillage or early drilling, so careful rotational 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 Mills. "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 ones, 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 continues. "Being just about the only perennial that seeds readily under normal conditions, it can catch you out."

As well as planning the timing of Roundup spraying to account for the differences between the couch-like perennials, Bob Mills 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," he says. "Recommended rates for Roundup Ace increase from 2.4 to 3.2 litres/ha, depending on timing and weed population. "With the others you must use the full 3.2 litre/ha rate regardless of numbers.

"A 2.4 litre/ha application may do a good job on a relatively low infestation if it's common couch. But if your problem is, in fact, one of the other species it could be a waste of time and money.

"Particularly good wetting is essential too with the hairy leaved species like Yorkshire Fog and creeping soft-grass. So you're always better off using 200 litres/ha rather than just 100 litres on these. And where you have hard water I'd recommend including the specialist additive X-Change at 0.10% to 0.25% of spray volume to optimise activity."