Field Margins

Field margins can provide and enhance wildlife habitats across arable farms without changing cropping patterns.

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Field margins can provide and enhance wildlife habitats across arable farms without changing cropping patterns.

Source: UK Agriculture Departments, Cereal Field Margin Habitat Action Plan (1998)

The UK biodiversity action plan has set targets for the maintenance, restoration and improvement of 15 000 ha of cereal field margins by the year 2010. This is broken down thus:

	12 725 ha in England
	2 025 ha in Scotland
	250 ha in Wales

It has been estimated that the area of cereal field margins now exceed 21,000 ha (Figure from UKBAP in 2002).  This is thought to be mainly as a result of uptake through agri-environment schemes.

The Countryside Survey 1990 (Barr et al., 1993) demonstrated the importance of field margins as refuge for botanical diversity in lowland agricultural landscapes. A series of  studies have also demonstrated the importance of field margins as over-wintering sites for a wide range of invertebrates. Whilst the idea of field margins as corridors for animal and plant movement between habitats has only been clearly demonstrated for forest beetles, these landscape elements are used by a wide range of birds, mammals and insects and are important for plants. Pipistrelle bats are almost always found foraging close to hedges and treelines in the Netherlands (Verboom & Huitema, 1997). Studies on weed seed banks in arable fields have demonstrated that the field edge has the most diverse and abundant seed bank. Thus, the conservation of rare cornfield flowers is likely to be most successful at the field edge.

 The role of field margins

Field margins exist in the landscape as they have, or had in the past, true agricultural functions. In stock farming areas, hedges and walls were maintained to keep stock in or out. In arable land, field margins delineate the field edge and land ownership. In more recent time, a series of subsidiary roles have been identified, reflecting agricultural, environmental, conservation and cultural or historical interests.

Traditional agricultural landscapes showing field boundaries

Original roles and requirements:

	To define the field edge
	To be stock- or trespasser-proof, to keep animals in or out
	To provide shelter for stock
	To provide shelter for crops, particularly as windbreaks
	To reduce soil erosion by wind or water
	Not to compete with the crop for light, moisture or nutrients
	Not to harbour weeds, pests and diseases
	To harbour beneficial plants and animals
	To act as a refuge or corridor for wildlife
	To provide a source of fruits and wood

Current and potential functions of field margins:

	Promotion of ecological stability in crops
	Reducing pesticide use:
	Exploiting pest predators and parasitoids
	Enhancing crop pollinator populations
	Reducing weed ingress and herbicide use
	Buffering pesticide drift
	Reducing fertiliser and other pollutant movement, especially in run-off
	Reducing soil erosion
	Promotion of biodiversity and farm wildlife conservation
	Maintaining landscape diversity
	Promotion of game species
	Encouragement of “countryside” enterprises
	Maintenance of historical features, heritage and ‘sense of place’

As linear features, field margins are also thought to act as corridors for the movement of fauna and possibly flora. Evidence for this has been shown for carabid beetles of forest and woodland in Brittany (Burel, 1989). Further, it is known that bats utilise margins to fly along as part of their feeding behaviour (Verboom & Huitema, 1997). Field margins are also known to be important over-wintering habitat for many insects that move into adjacent arable crops (Sotherton, 1984; Thomas et al., 1994; Wratten, 1988). However, it has also been shown that field margins can be barriers to the movement of such species between fields. Initiatives over recent years have been taken to modify the management of arable field margins for a series of different objectives, often with the aim of enhancing wildlife while providing agronomic benefits, in terms of reduced weed ingress or enhanced populations of beneficial invertebrates. These have been widely investigated, with modifications, across Europe. 

The terminology used here follows that of (Greaves & Marshall, 1987), in which the term field margin includes any pre-existing boundary structure, such as a hedge, a boundary strip and the crop edge, where conservation headlands are located. The diversity of conservation management approaches for field margins can be best summarised as follows:

	Boundary Strips: Grass strip
	Grass and wild flower strip
	Sterile strip: just cultivated
	Uncropped wildlife strip
	Set-aside margin
	Sown wildlife mixtures (strips or blocks)
	Crop edge: Conservation headland
	Across fields: Beetle bank

Within these main approaches, variations are available. For example, within a grass margin, the area nearest the hedge may be managed for tussocky grasses to provide nesting cover for gamebirds and over-wintering habitat for beetles and other invertebrates.

 Specific options related to field margins aim to increase:

	Conservation headlands
	Field margin strips
	Uncropped wildlife strips
	The flora and fauna associated with these areas.

Uncultivated headlands

Wildlife

Existing field boundary structures, including hedges, walls, grass banks and ditches, comprise a major part of the semi-natural habitat mosaic of farmland. Many of these have been degraded or lost. A variety of methods of extending and recreating margin habitats are available, many of which buffer adjacent habitat from disturbance from farming operations. Grass margins and beetle banks provide nesting areas for grey partridge in tussocky grass and for skylark in shorter grass. They also provide habitat for insects and small mammals, feeding areas for owls and other birds of prey and over-wintering habitat for many invertebrates. Common flowers can be important sources of pollen and nectar for bees and other insects. Uncropped wildlife strips at arable field edges provide conditions for rare arable weeds to germinate and set seed. The seeds produced will in turn provide forage for a range of bird species. Fields that been under intensive arable cultivation for many years are likely to have impoverished seed banks. However, the seed banks are larger and more diverse at the field edge. Thus, wildlife strips at the field edge are more likely to promote rare annuals than in the field centre. Nevertheless, targeting fields known to support rare annual flowers is to be recommended. Fields that have not been in arable production for long are unlikely to support a seedbank of rare annual plants.

Uncultivated areas adjacent to water courses with a wide range of shrubs and trees not only create valuable habitats but act as a sink for leached nutrients and pesticides

Pollution control

Extending grass margins at arable field edges results in farming operations, particularly pesticide and fertiliser applications, taking place further from pre-existing habitat. This in itself provides some protection from drift. Grass margin strips can act as buffer strips when sited next to watercourses. Margin vegetation may act as a physical buffer to drift and to surface movement of water from fields. This may reduce the movement of nitrogen, phosphorous, pesticide and silt into surface waters, fulfilling requirements under the Codes of Good Agricultural Practice.  Placement of beetle banks strategically across large fields with areas prone to soil erosion can reduce soil losses by reducing overland flows.

Agricultural practice

Wide margin strips may provide easy access for hedge trimming in late winter, after berries have been eaten, without damage to adjacent arable crops. Strips are also one way of satisfying the requirement not to apply an increasing range of pesticides within 6 m of watercourses. Nevertheless, wide strips in small fields may have significant impacts on the working area within fields. Where annual weeds dominate the field boundary, notably barren brome and cleavers, creation of a perennial grassy margin can form a barrier to weed spread into the adjacent arable crop. Over time, reduced disturbance will also enhance perennials in the boundary, reducing annual weed populations. Provision of semi-natural habitat for beetles, spiders, bees and hoverflies will enhance their populations. Many of these species are beneficial to adjacent arable crops, either as pollinators or as predators of crop pests. Some hoverfly species, for example, require pollen and nectar to feed on as adults, before seeking out colonies of aphids in which to lay their eggs. The emerging hoverfly larvae are voracious aphid predators. Set-aside regulations allow margin strips to be included. Such strips may be for rotational set-aside and moved from field to field, or for non-rotational set-aside. In all cases, the width of set-aside margins has to be 20 m wide, under current EU regulations.

Management

Agricultural soils are often highly fertile. High fertility promotes tall, fast-growing plant species at the expense of shorter species, resulting in low species diversity (Marrs, 1993)..Where tall, tussocky vegetation is necessary for the nesting of grey partridge, little management may be required. Where a shorter, more diverse perennial plant community is required, particularly with grass margins adjacent to arable crops, management should aim to reduce fertility or the height of perennial vegetation. However, care needs to be taken to avoid disturbance during the breeding season, particularly of ground-nesting birds.

Management of features in the above diagram (as suggested by the Game Conservancy Council:

Hedges:

	Trim every other year
	Keep to maximum height of 2m
	Do not allow to overgrow adjacent grassy strip

Grassy bank/nesting strip

The area used for nest sites by game birds and for over wintering of beneficial insects:

	At least 1m wide and preferably sited on a bank
	Composed of perennial grasses and other non weedy herbaceous species
	Avoid spray and fertilizer drift into this area
	Allow build up of dead grass material (essential for successful nesting)
	Top the vegetation every 2 – 3 years to avoid scrub encroachment

Sterile strip

The purpose is to prevent invasion of crop by barren brome and cleavers where they have become abundant. This area is not essential for conservation purposes but is for crop protection and weed management:

	Drill crop further out into field to leave area of bare uncultivated land
	At least 1m wide
	Maintain by rotavation and herbicides (i.e. atrazine) in February/early March
	Do not spray out grassy bank
	Avoid spray drift by using nozzle shielded down to ground level

Conservation headlands

The area between the crop edge and first tramline:

	Usually 6m wide, depending on sprayer boom width
	Treated with selective pesticides only to control grass weeds, cleavers and diseases whilst allowing broad leaved weeds and beneficial insects to survive
	Ploughing of headlands is recommended especially on heavy soils or where grass weeds are a problem
	Avoid turning furrow on to grassy strip as this area can create ideal conditions for annual weeds
	Choose headlands next to good nesting cover
	Avoid headlands infested with difficult weeds (particularly barren brome and cleavers)

Sprayed crop:

	Treat as normal
	Avoid drift onto conservation headland
	Use only ‘safer aphicides’

Pause for thought……..Would conservation headlands be viable on small farms with small field sizes, or would the economic losses be too great?

Left: Arable monoculture, Right sheep grazing amongst relatively new trees

Beetle banks

A grassy strip across a field (beetle bank) can provide new habitat for birds, small mammals and invertebrates. The width of the beetle bank should be 2 to 3 m wide and about 0.4 m high ideally. A 3 m strip may be created in the first year and reduced to 2 m subsequently. The technique is used to create semi-natural habitat within large fields, dividing large blocks into smaller areas with a new field margin. This can have agricultural benefits in enhancing beneficial insect populations, as well as wildlife benefits in providing new feeding and breeding habitat.

By careful location across slopes, beetle banks can reduce within-field erosion. Although originally designed as temporary features within fields, beetle banks can provide better habitat structure, if connected to and connecting other semi-natural habitats. Connections between woods or hedges should be encouraged.

The beetle bank may connect to existing field edges or have gaps up to 25 m wide at either end to allow machinery access. Combination with conservation headlands either side of a beetle bank is likely to enhance wildlife benefits:

	Provide a method of recreating a linear margin habitat across fields
	Provide nesting areas for grey partridge in tussocky grass and possibly for skylark in shorter grass
	Provide habitat for insects and small mammals, feeding areas for owls and other birds of prey and over-wintering habitat for many invertebrates. Many of these species are beneficial to adjacent arable crops, either as pollinators or as predators of crop pests. Some hoverfly species, for example, require pollen and nectar to feed on as adults, before seeking out colonies of aphids in which to lay their eggs. The emerging hoverfly larvae are voracious aphid predators
	Common flowers can be important sources of pollen and nectar for bees and other insects
	In combination with conservation headlands either side of a beetle bank, wildlife benefits are likely to be enhance
	Placement of beetle banks strategically across large fields with areas prone to soil erosion can reduce soil losses by reducing overland flows
	This may reduce the movement of nitrogen, phosphorous, pesticide and silt into surface waters, fulfilling requirements under the Codes of Good Agricultural Practice

Conflicts

The effect of introducing a beetle banks across a field, may be to change the machinery working patterns within the field. This may have practical consequences for the farmer, in terms of direction of work, timing and efficiency.