12 April 2002

How to control weeds the organic way

Weeds are arguably the

biggest worry when going

organic. Andrew Blake

asked for Elm Farm

Research Centres advice and

visited one organic grower

who is clearly in control of

the weeds on his farm

GROWERS previously dependent on herbicides for controlling weeds have plenty of alternatives when going organic, says Elm Farm Research Centre.

But without access to livestock they can expect a gradual build-up of problem weeds under organic cropping. Studies suggest annuals can be contained, but perennials remain a major problem.

"The challenge for organic farmers is to manage weeds in a way that accommodates their beneficial effects and still produces acceptable crops," says researcher James Welsh.

The aim is to maintain low populations to provide food and habitat for beneficial organisms, such as ground nesting birds and invertebrates, while avoiding excessively weedy crops which will suffer on both yield and quality.

"Crop rotation is the key to long-term weed management and grazed grass breaks give the best results." Stockless systems using red clover/ryegrass as a break can be viable, especially if potatoes are part of the rotation, EFRC work suggests. But they are more challenging to manage, warns Dr Welsh.

After foot-and-mouth, the use of flying flocks and herds to suppress weeds within grass breaks is likely to decline. But neighbours could get together more to offset that, he suggests.

When it comes to mechanical weed control there are two main options. The most widely used is spring-tine harrowing, but inter-row hoeing has much to offer and is proving successful on a few pioneering farms.

One such unit is Richard Steeles Chapel Farm, at Netherton, Worcs. Sound rotation, with grass/white clover leys grazed by sheep, careful combining and a combination of stale seed-beds, inter-row hoeing and weed topping are keeping the organic milling wheats and oats economically clean.

In reverting to the technique introduced by seed drill inventor Jethro Tull, Mr Steele says he finds sowing cereals in slightly wider rows and hoeing between them gives good results. In due course that should allow him to grow more crops for seed.

"The main reason we went organic was because we had resistant blackgrass," says Mr Steele, whose 338ha (835-acre) farm was fully converted by 1997.

"With wheat we aim for 7t/ha rather than the conventional 10t. We probably average five, but with the premium it still pays."

At least half an organic rotation must have legumes for fertility building, he believes. At Chapel Farm these come in the form of clover/grass leys grazed by sheep which control broad-leaved problems like charlock very effectively. Couch and brome are mostly confined to the headlands.

"Red clover in silage leys is particularly good for smothering creeping thistle." Oats, grown as a second cereal, need less nutrient and compete well with weeds, he adds.

Son Adrian, who now runs the farm, grows potatoes where soil types permit. "They are useful because we can justify hand labour to clean up wild oats and docks."

An Einbock spring-tine weeder is used when establishing leys, but on the heavy land over-wintering weeds in cereals can become too well-rooted for control by this method. Inter-row hoeing between November and March solves the problem.

"We use a 4m Stanhay Webb off-the-shelf machine. Every field gets at least one pass and we may go through up to three times," says Richard.

The rear-mounted equipment is carefully matched to the tractor wheel spacings and the Kongskilde Demeter drill, which sows in 28cm (11in) rows. Disc guides help the hoe run true and mounting it on an articulated Holder tractor allows bends in headland rows to be followed without damaging the crop.

For the first pass 10cm or 12cm A-blades with side baffles are fitted. Later passes use 20cm blades without baffles. Being deep-rooted, cereals suffer little damage. But peas have proved impossible to hoe late in the season.

Despite the wide rows, conventional seed rates are used so dense mini-hedges of crop are formed in which weeds find it hard to grow. The extra ventilation provided by the wider rows means diseases never exert significant pressure. "And we havent had any lodging because we do not top dress and crops stay healthy," says Richard.

The hoe works best in trash-free conditions, so ploughing has been the norm. But that makes it hard to achieve the ideal level finishes and the Steeles are turning to a min-till stale seed-bed approach using a Knight Triple Press cultivator.

In summer, a hydraulically-driven topper based on a US maize tassle remover fits on the tractors front-end loader and shears off wild oats and other weeds as they appear above the crop. The falling seeds are usually immature and die. Using relatively short varieties instead of the taller ones favoured by some organic growers helps the operation.

"We can do four acres/hour with the hoe, but we are always looking for economies of scale." Incorporating auto-steering, such as the Robydome system, into a wider-bout operation could be the next move, he believes. &#42

Make way for the hoe… Richard Steele (below) sows wheat in rows 28cm (11in) apart so the Stanhay hoe can slice through weeds. While spring-tine weeders are more widely used, hoes have much to offer, says Elm Elm Farm Research Centres James Welsh.

&#8226 Many strategic options.

&#8226 Grazed grass very useful.

&#8226 Inter-row hoeing promising.

&#8226 Auto-steering next step.

Option EFRC comment

Crop rotation Vital. Weed management harder if all-arable.

Crop choice Oats and winter rye best, then triticale, barley and wheat – in that order. Spring beans OK, but peas

and winter beans uncompetitive.

Variety choice Pick vigorous types with ability to shade weeds. Potential from mixtures.

Stale seed-beds Help ease weed burden in next crop.

Time of sowing Avoid early autumn. Allow time for stale seed-beds.

Seed quality Home-saving can lead to problems.

Seed rate High level favours crop over weeds, but at extra cost.

Night cultivations Working soil in darkness produces inconsistent results.

Crop architecture Prostrate varieties suppress weeds. Tall ones can be useful, shading is key factor.

Sowing direction East/west may be better further north, but row width interaction.

Crop vigour Crops nearest fertility-building phases most competitive.

Undersowing Useful against annuals, as are cereal/pulse mixtures.

Allelopathy Sunflower & rye residues may help suppress weeds.

Cleanliness Wash equipment after working in weedy fields.

Source: Weed Control In Organic Cereals And Pulses by Ken Davies (SAC) and James Welsh (EFRC).