23 February 1996

Annuals love sterile strips

Herbicides and hedges are poor companions. But research shows weed-killers may be used to good effect – if they are selective. Andrew Blake reports

GROWERS creating sterile strips along hedge bottoms using total weed-killers are wasting time and money, claims agrochemical specialist Willmot Pertwees Marek Nowakowski.

Field margins, where the habitats of crop and hedge meet, provide some of the most environmentally valuable and interesting parts of arable farms. "Yet these so-called ecotones are often the most cultivated and abused areas," says Mr Nowakowski, who describes himself as "an agronomist with environmental leanings".

"Hedges harbour a range of desirable and undesirable flora. But there are two main weeds which spread into arable fields – cleavers and sterile brome." With appropriate margin management they can be contained, he believes. A "barrier" of bare ground between hedge and crop by spraying off all vegetation seems simple. But over time its disadvantages far outweigh the benefits, he believes.

The main problem is that weeds which tend to recolonise the cleared area are the most aggressive annual species – like cleavers and brome. Growers using sterile strips struggle to prevent them re-invading, he says. "Annuals love bare ground. If sterile strips are so good, why do growers have to keep making them each year?"

By comparison most perennial weeds are less invasive. "Research proves if you create a desirable perennial flora where annuals cannot get in and spread you will nail things like cleavers and brome."

Willmot Pertwee costings over six years suggest the "sterile" approach costs "only fractionally less" than sowing and managing a 2m (6ft) wide, well chosen sown grass/wildflower mix. Even the supposed benefits of sterile strips are debatable, argues Mr Nowa-kowski. "They do give a clean edge for the combine driver, but no more so than short-clipped grass."

Their merits as dusting patches for game birds have been greatly overplayed, he adds. "There are plenty of other areas, such as farm tracks, where partridges can go." But one potential benefit, he admits, is that some rare arable weeds such as pheasants eye thrive on annually cultivated ground. "If you had all your field margins down to a perennial mix you would be putting those weeds under more pressure. What is needed is a tapestry of habitats."

Growers adopting managed field margins face three possible scenarios, the first two achieved "free". "If they are lucky they will already have a hedge bottom with naturally occurring non-invasive species." Suitable management should allow the strip nearest the hedge to become rough and tussocky, says Mr Nowakowski. "If you mow so that annuals dont produce viable seed you gently swing it in favour of perennials."

A more likely picture is that natural vegetation will contain "nasties" like brome. "But by mowing and/or using selective herbicides you can tip the balance towards more desirable species."

Fusilade (fluazifop-P-butyl) and Laser (cycloxidim) may both be used in field margins. "But there are subtle differences between the two on some desirable species. For example, Fusilade is safer on crested dogs tail if the plants are big enough and the dose rate is correct." A range of effects can be achieved according to growth stage and dose, he explains.

For tackling cleavers the experimental herbicide quinmerac has proved "extremely selective and very useful". Unfortunately the BASF chemicals progress towards full UK approval has been delayed.

Eagle (amidosulfuron) has also shown promise. It could be especially useful in early cold conditions when weeds threaten an autumn-sown mixture.

Growers with sterile strips could leave them to revert to natural vegetation. "But that can be extremely slow," says Mr Nowakowski. "Most of the species you want dont have very long-lived seed."

Establishing a new hedge margin by sowing a specific mixture may also be tricky, especially if soil fertility is high from careless fertiliser broadcasting. "The establishment phase from sowing to the first spring is critical to avoid the sown species being smothered by blackgrass, cleavers and brome.

"The rules of good agriculture do not change. You must produce a good clean seed-bed. Choose species to match the site, and be prepared to treat it like a crop." &#42


&#8226 Field/hedge margins valuable wildlife habitats.

&#8226 Scope for selective herbicides.

&#8226 Sown mix and sterile strip costs similar over six years.

&#8226 Cleavers and brome can be contained.


HEDGE BOTTOMS

Sterile strips encourage aggressive annual weeds, which invade crops, says Marek Nowakowski (inset). Sown, managed margins are better.