Bracken(Pteridium aquilinum), is one of the most successful plants in the world. It has the ability, by virtue of its underground rhizomes, to spread considerably. One plant alone can occupy a large area and fragment into separate plants which themselves have similar capacity. During the last fanning depression, in the late 1920s to 1930s, when marginal land was abandoned from cultivation and adequate stocking, bracken crept down the hill to invade what had previously been reasonable upland pasture. At that time, the late Professor Stapledon estimated that well in excess of a fifth of the upland of Wales and Scotland was rendered agriculturally of poor value because of bracken encroachment. No-one knows the total area in the world which this plant occupies; it must be many many millions of hectares, much of which could provide a more useful grazing. It is certainly a successful plant.

Bracken, Pleridium aquilinum, has long been present in Britain as part of the native flora. Before the arrival of man, Britain was a wooded and forested country, and in this vegetation bracken was an open woodland and forest margin species, which played only a relatively minor vegetational role. Its biology is such, however, that this plant is able to spread rapidly in response to removal of the forest canopy, developing into colonies which are much more ecologically aggressive and dominating than was the original habit of the plant when undisturbed.

In open conditions, bracken also has a far greater spore output than in more shaded ones. It is able to establish anew from spores only in relatively virgin soil conditions, particularly those produced by burning of other vegetation. Burning also benefits established bracken by removal of its competitors, whilst heavy and selective grazing by sheep also slowly reduces the natural vegetation competition in favour of bracken's further spread.

The history of the spread of bracken in Britain from palynological and historical evidence is reviewed in the light of this biology and, in particular, its spread as man has progressively removed the forest canopy,introduced steady heavy grazing pressure, and has burned, and continues to burn, vegetation.

The only serious attempt to quantify the area of bracken in Scotland took place in 1957. At that time the Department of Agriculture for Scotland, through the medium of the June Agricultural Census, asked farmers to assess the area of bracken on each holding.

The survey produced a figure of 187,500 ha of bracken-infested land throughout Scotland, of which some 50% was found in the Central, North and West Highland counties of Perth, Inverness and Argyll. One quarter (46,875 ha) of the Scottish total was located on the mainland and islands of Argyll.

Distribution is governed to a great extent by the soil type, exposure and rainfall, with the free-draining slopes of brown soil on the lower hill faces and glen sides providing the ideal habitat for the plant. The poorly drained acid peat as found over much of the Highland areas is an unsuitable medium for bracken growth but thin peat overlying mineral soil may be subject to bracken encroachment.

The crux of the problem lies in the imbalance of brown soils to peat land.

The limited areas of the mineral soils, which are located on the accessible lower slopes, have a marked potential for improved grazing and, by virtue of better drainage, are capable of more complete utilisation in an intensive stocking system.

Bracken control and subsequent grass sward establishment can form the key to greatly increased levels of productivity. This is illustrated by detailed reference to one farm in Argyll where over the past five years an attempt has been made to operate the Hill Farming Research Organization (H.F.R.O.) concept of a Two Pasture System.

Bracken grows on sites very suitable for forest tree growth allowing a wide choice of tree species. However, the trend in afforestation is towards poorer soils and the area of bracken land being planted is declining. Bracken will damage young trees and methods of its control are reviewed. Hand weeding still has a part to play, but most weeding of heavy bracken growth is by herbicides. Experiments using dicamba, chlorthiamid, picloram and more recently asulam and glyphosate demonstrate a gradual improvement in the selectivity and cost effectiveness of herbicides. Dicamba can only be used before planting and its use is limited to sites where control over three years is assured. Picloram and chlorthiamid at the rates necessary to control bracken are damaging to conifers. Asulam is selective and can be applied after planting giving good control of the bracken. Glyphosate is more damaging to trees than asulam but its use may be justified where a wider spectrum of weed control is required.

The toxicity of bracken fern (Pteridium aquilinum (L) Kuhn) is complicated because this plant elaborates more than one type of agent potentially harmful to livestock.

When simple-stomached animals are fed a diet containing some green fronds or rhizomes for a few weeks, they develop anorexia and inco-ordination symptoms; this polyneuritic syndrome shows all the characteristics of thiamine deficiency. Thiamine therapy is effective in curing the animal provided it is administered at the onset of clinical symptoms.

Bracken staggers in the horse and rhizome poisoning of pigs are diseases of this nature,i.e. an avitaminosis B1 has been induced. The causative agent in the fern is a steam-labile enzyme, thiaminase I, which destroys thiamine in such a way that it is no longer active in animal nutrition.

Acute cattle bracken poisoning occurs after frequent eating of the fern over a period of weeks.Haemorrhages and fever are the characteristic clinical symptoms, and the animal usually dies within a few days. The fundamental lesion is bone marrow aplasia resulting in a severe leucopenia and thrombocytopenia.Thiaminase I is not involved in this disease. Bracken contains an organic chemical(s) which poisons rapidly dividing cells in the bovine animal. Sheep are less sensitive to this toxic agent, although the condition has been described along with 'bright-blindness* in some parts of Britain (e.g. North York Moors).

Purification of the active principle causing cattle bracken poisoning from green fronds and rhizomes is far advanced; these methods are described, along with calf toxicity tests on each fraction. Chemical investigations are also under way to determine the nature of the components.

Whether the cattle factor, in small doses over an extended period of time (years), is also the agent causing tumours in livestock, is probable but not certain. Some bracken constituents shown to be carcinogenic in small animals, e.g. shikimic acid, quercetin or the tannin fraction, are not capable of producing acute cattle bracken poisoning; neither does safrole or 1-indanone.

Some of the sesquiterpene glycosides—the 'pterosides' and/or their aglycones, the 'pterosins', of which over 30 have been identified in bracken—are likely candidates. On the other hand, the active principle(s) may be something quite different; the search continues.

Information concerning the carcinogenicity of bracken fern (Pteridium aquilinum) in different species and organs is reviewed. Various methods used in attempts to identify the plant components responsible are described and the results evaluated. Reference is made to recent and current research in this field (University College of North Wales, Bangor) and includes mutagenic and sterility studies, and the introduction of teratogenic screening by injection of Japanese quail eggs. The many problems still challenging bracken workers are indicated, and a final section deals with the potential hazard to human health in an attempt to link experimental work with epidemiological surveys of human cancer incidence. One important result of the study of bracken carcinogenicity is that active components, previously thought to be harmless, can be found in a wide variety of regular dietary items.

There is a high incidence of cancer in beef cows in certain sharply localised geographic calf-rearing areas in Britain and also in cattle in several other parts of the world. The tumours occur on farms which are highly infested with bracken fern and which have a history of bracken poisoning. Bracken is known to cause intestinal and bladder cancer in rats and recently the flavonol, quercetin, has been shown to be a mitogen and to be carcinogenic in rats. Bracken contains large amounts of quercetin. In addition the animals have a high incidence of papillomas of the alimentary tract and these transform to malignant tumours. A new and unique papilloma virus has been isolated from these tumours. Studies using genetic engineering and DNA hybridisation techniques have shown that both alimentary and urinary bladder tumours contain viral DNA sequences. The relationship between bracken, quercetin and viral DNA sequences is under investigation both in cattle and at the molecular biology level as two possible stages in a multi-stage process in the aetiology of a high incidence naturally occurring cancer.

The efficiency of bracken control using foliage-applied translocated herbicides depends upon the accumulation of a potentially lethal concentration of the active ingredient at the sites of action, the rhizome buds. There is evidence that the level of herbicide accumulation at these target sites is determined by a number of factors including the efficiency of cuticle penetration, absorption, translocation, metabolism and activity at the enzyme target sites. The level of metabolic activity of the rhizome/frond buds may be critical, dormant buds possibly receiving a sublethal dose. These buds may later become active and act as foci for reinfestation.

The nature of these problems is discussed with particular reference to studies carried out on the mode of action of [14C]asulam.

Work done by the authors in relation to bracken control is reviewed. These studies are largely concerned with the use of additives to enhance both the uptake and translocation of aminotriazole and asulam using a number of approaches: (a) estimating uptake by applying known herbicide concentrations in the presence of additives followed by washing off and residue determination after set times; (b) bioassay studies for selecting additives which can reduce herbicide scorching and therefore possibly enhance translocation; (c) field trials with selected formulations.

The chemical, physical and biological properties of asulam make this herbicide well suited for the selective control of bracken in various situations. These properties are reviewed and conditions considered to optimise the control of bracken and minimise the effect on the environment on both a short- and long-term basis.

The role of surfactants and mineral oils is considered.

South-west and west Scotland contains a large proportion of the bracken-infested hill pasture in that country and considerable effort has been expended over the past five decades on a scientific approach to bracken control. The trials of various, more or less successful, herbicides culminated in the current work based on the use of asulam or glyphosate which will control, but only in rare cases eradicate, bracken. Current work is aimed at improving the control effect by investigating and refining timing, rate and method of application, additives and mixtures. Particular emphasis is placed on techniques of improving the underlying sward, where present, and of investigating the factors influencing its growth. Experimental work has not shown any degree of allelopathy towards sown-in grass and clover seed growth, but depth of bracken litter is shown to be important. Doubt is expressed regarding the requirement for a total programme of bracken control and follow-up fertiliser treatment in all cases before grant aid is approved

Bracken is not a major weed problem on hill land in the north of Scotland, being restricted by environmental factors to the lower slopes of sheltered glens. There is no evidence of widespread stock poisoning by bracken in the area.

Ploughing and reclamation, forestry planting and spraying with the selective herbicide asulam have considerably reduced infestations of bracken on land worthy of improvement. At the present time there is little scope for further economic control of bracken.

Field experiments have defined full frond emergence from mid July to mid August as the most effective time for application of asulam. Reinfestation in about five years has been shown to follow incomplete control on small plots but an area sprayed by helicopter in 1972, where a very high degree of initial control was achieved, remained almost free of bracken in 1980.

Easier handling of stock and increased availability of summer grazing are reported following bracken control. An increase in herbage production has been measured experimentally in the second year after removal of the bracken by spraying.

‘Life can only be understood backwards but it must be lived forwards’.Thirty years of professional but spasmodic interest in the problem of bracken on the Scottish hills brings home the aptness of Kierkegaard's aphorism. Despite rapid technological change —from a man with a scythe to the pilot with a helicopter—the fact remains that control of this serious weed problem is currently running at only a fraction of the rate in the early fifties. Clearly, financial factors have played a major part in this situation but it is facile to lay the blame wholly at the door of economics. The objective of the paper is to summarise the dominant technical and economic aspects which have impinged upon the problem over three decades and to attempt to draw broad conclusions as to why we are in this situation and to outline more fruitful possibilities for the future.