Giuseppe Parente and John Frame
Centro Regionale per la Sperimentazione Agraria, Via Sabbatini, 5 33050 Pozzuolo del Friuli (UD), Italy
Ard Choille, 13 St Vincent Crescent, Alloway, AYR KA7 4QW Scotland, United Kingdom
Support to arable cropping
Horticulture and viniculture
Extensification and diversification
Food and wine
Amenity and reclamation
White clover is increasingly used in a number of non-traditional ways. Living or dead mulches, green manure or a living understorey for arable cropping or horticulture are notable examples in which clover's N contribution is a significant factor. It is also a valuable component of seed mixtures for set-aside arable land, horse grazing or deer farming and plays an important role in honey production. It has been used in seed mixtures to attract wild geese onto sanctuaries, thus reducing damage to farmland. With other forage legumes, white clover is an integral part of various organic farming systems, whether stockless or livestock orientated, and it is utilizable as a "vegetable" in human nutrition or for a "wine". Clover can be used in many ways in extensive grassland farming or in amenity grassland but its role as a constituent of wildflower meadows is noteworthy for both its beauty and agronomic qualities.
The agriculture significance of white clover is well known and documented - rhizobial N-fixation capacity, soil improvement properties, excellent feeding value for animal production. These advantages have been neglected in Europe in the recent era of heavy fertilizer N application to grassland (Novoselova and Frame, 1992). However, the new European Community (EC) agricultural policies, e.g. EC Council Regulation No. 2078/92, aimed at environmentally friendly production methods and sustainable low input systems, will undoubtedly increase the exploitation of white clover (and other forage legumes); there is also similar burgeoning interest in non-EC countries.
It is perhaps not widely realized that white clover offers the opportunity of a number of alternative uses in the rural land economy, some closely related to agricultural production systems but others of a more unconventional nature. The purpose of this paper is to review and highlight these alternatives, using selected examples for illustration.
An important alternative use for white clover is as a support to arable cropping systems, mainly through its contribution of N. Various methods have been documented. While ploughing-in is a traditional way, more novel methods have been investigated, e.g. the direct drilling of maize into a perennial ryegrass/white clover mixture previously harvested for silage, with the regrowth between cultivated strips being cut and left as a mulch (Ammon et al., 1990). This process was repeated for three years and while maize yields were generally lower than those from traditional maize cropping, nitrate leaching losses were lower and there was less soil erosion or compaction; also, herbicide use was reduced. Destruction of a white clover cover crop, e.g. by flaming, increased maize yields but at the expense of continuity of cover crop (Werner, 1986; Lake, 1991). Rototilling a clover mulch resulted in sweetcorn yields comparable to those from conventional cultivations but with less disease, while yields were lower with either unsuppressed or periodically mown clover (Grubinger and Minotti, 1990).
Preliminary work on the concept of growing cereals in a white clover understorey has shown promise (Williams and Hayes, 1991; Jones, 1992). The former described trials in which spring barley and spring oats were sown into a clover sward with and without band spraying of paraquat or glyphosphate. Grain yields from the band-sprayed treatments were 60-78 per cent of those from drilling into bare land, but were substantially lower when drilled into the untreated clover understorey. When spring barley, spring oats and winter barley were grown in a white clover understorey, the yields of whole crop cereal for silage or of cereal grain were similar to those from traditional methods (Jones, 1992). In one experiment clover survived to be used again but in a second it was killed by a high slug population. Where clover survived, no weed, insect or disease problems were encountered. Clements and Asteraki (1993) suggested that the low levels of pest attack could be the result of a build-up in clover of predatory insects and spiders which attack aphids and other pests; also, the emerging cereal crop may be camouflaged by clover against aphid attack. On account of the trampling effect, heavy sheep grazing after cereal sowing and again after cereal harvest was proposed as a means of slug control.
So-called set-aside land, i.e. land taken out of arable cropping because of overproduction, is assuming greater importance in EC agricultural policies e.g. EC Council Regulation 1795/92. A dense soil cover is advantageous in preventing soil erosion, leaching of N and weed invasion. In contrast to natural revegetation, a grass/white clover sward fulfils these objectives and soil N status is improved for subsequent cropping. White clover is particularly suited for long term set-aside.
Green manures in general have an obvious role in rotations -in organic systems in particular - and are of benefit to soil physical and chemical conditions. Advantages include augmentation of organic matter, accumulation of nitrogen, reduced soil erosion and nutrient leaching and a degree of weed control (Kahnt, 1983). White clover is generally considered to be shallow rooted in relation to red clover or to several grass species (Caradus, 1990) but nevertheless, some degree of recycling of mineral nutrients from the soil profile can be postulated.
In organic stockless system of arable cropping, leguminous mixtures including white clover as a component are typically sown as a green manure (Lampkin, 1990); the crop may be cut periodically during the growing season and the material left as a mulch. Specific uses of white clover as a green manure have been discussed by Barney (1987) and Ten Holte and Van Keulen (1989); for example, in trials in The Netherlands, the latter obtained increased sugar yield and N uptake in sugar beet and increased tuber yield and N uptake in potatoes following the ploughing-in of clover sown under a winter wheat crop.
White clover, pure-sown or in association with grass, fulfils several functions as a soil cover crop or living mulch in various types of orchards, market gardens and vineyards although irrigation may be required during drought periods to aid its persistence. Engel (1988) listed several advantages of sowing legumes, including white clover, as: rapid soil cover, weed growth inhibition, improved soil structure, increased soil N and organic matter content, increased availability of nutrients; reduced soil surface temperatures in hot summers could also be added. However, potential disadvantages were also noted: high seed costs, a haven for certain pests, e.g. rodents, increased risk of radiation frosts, excessive soil N enrichment leading to adverse effects on the chemical composition of the fruit. Tillage to establish cover crops may damage the roots of trees or vines but direct drilling (= slot seeding) offers an alternative method of establishment and has been successfully used to establish white clover in vineyards (Parente and Beerbohm, 1990; Bozzo and Parente, 1992).
The wave of European extensification policies is resulting in positive measures to conserve plant and animal wildlife, and enhance landscape beauty, by more emphasis on floral diversity in swards. This diversity flourishes in low fertility situations but the cost of wildflower seed mixtures and the low output from such situations militate against the widespread creation of wildflower meadows. In Scotland, a number of commercial wildflower mixtures formulated by The Nature Conservancy Council (Wells et al., 1981) were evaluated by Frame and Tiley (1993). Mixtures containing white clover (and other forage legumes) were more productive by up to 2.5 t/ha DM annually and richer in mineral concentrations than mixtures without forage legume components, yet a satisfactory floral diversity was maintained. They concluded that clover-containing mixtures offered a "middle way" between low fertility and herbage production from classical legume-sparse wildflower meadows and high production from intensively N-fertilized grass swards. The efficiency of this approach in extensive stock rearing enterprises was demonstrated by Fisher and Roberts (1992) for dairy youngstock.
In New Zealand a parallel approch by Ruz-Jerez et al., (1991) compared a forage herb-rich mixture (of particular but not exclusive interest to organic farmers) with a conventional grass/white clover sward and an intensively N-fertilized grass sward (400 kg/ha/year) under sheep grazing. White clover made a substantial contribution to herbage production from both mixed-species swards and relative to production at 100 from the N-fertilized grass sward, the herbal ley and grass/clover sward gave values of 90 and 69 respectively.
Organic animal husbandry
Clearly white clover as a component of productive mixed-species swards plays an important part in organic animal husbandry and its products such as organic milk or meat, and several success stories have been documented (Lambkin, 1990). In his work on organic beef production, Younie (1989) noted the difficulty of maintaining the soil potassium (K) status under grass/white clover swards due to the removal of heavy silage crops, even altough animal manures were returned to the silage areas. Clover depletion in the swards could eventually result since clover's sensitivity to K deficiency is well documented.
White clover is a valuable constituent of seed mixtures for deer farming. In a comparison of tall fescue versus perennial ryegrass swards in New Zealand, the tall fescue-based pastures had significantly higher carrying capacities because of a better clover balance (Stevens et al., 1992).
Horses and ponies
The number of horses and ponies kept for recreational riding has increased in recent years particularly in semi-rural areas. White clover is incorporated in special seed mixtures both to improve the nutritive value of the herbage and to enhance soil fertility.
There is some interest in the use of white clover as a feed for pigs, mainly because of its richness in protein. For example, in Switzerland Jost (1984) successfully fed grass/clover silage to pregnant sows.
The wintering of migratory wild geese has led to increasing conflict with agriculture in various countries since sown grassland in certain localities is subjected to high grazing intensity during winter. The two main wild geese species in Scotland are the barnacle (Branta leucopsis) and the greylag (Anser anser). Experiments showed that their winter grazing substantially reduced the herbage available for subsequent spring grazing by cattle or sheep or for silage cropping (Patton and Frame, 1981). White clover is a "preferred" species by geese for grazing and use of this fact is made in refuges where grassland management is geared to maintaining clover-rich swards in order to attract geese from farmland. Many species of wild birds utilize clover as part of their diet, especially in winter but wildfowl are possibly the most significant with respect to sward damage; nevertheless, in autumn-established grass/clover mixtures bird damage to clover, e.g. by wild pigeons, can be severe.
The importance of flowering white clover for honey bees and hence honey production is well known and documented. The honey bees are also responsible for pollination in clover seed crops. A clover-rich sward is encouraged and the clover allowed to flower in the avenues of a recently-established arboretum at The Scottish Agricultural College, Ayr, partly for the benefit of honey bees, but also from a landscape beauty point of view.
White clover is regarded as a plant edible by humans (Launert, 1981): "An infusion of the dried flowers makes a fine tea substitute. The young leaves, gathered before flowering, can be added to salads, sauces and soups? on their own they can be used as a vegetable and prepared like spinach." Moderation in use is recommended possibly because of the cyanogenic potential of some clovers varieties; Wheeler and Vickery (1989) found that north American varieties had notably lower cyanogenic potential than most European varieties. Again, it is perhaps not appreciated that a type of dry "wine" can be made by fermenting flowerheads, sugar and wine yeast.
The stoloniferous growth habit of white clover makes it suitable for inclusion in seed mixtures for the stabilization of slopes using various establishment techniques including conventional sowing methods, slot seeding or hydraulic seeding (Roberts and Bradshaw, 1985; Bozzo and Parente, 1993). Its N input is also valued for stimulating the growth of associated grass species and this is a major reason for including it in mixtures when reclaiming degraded land after opencast mining or mine spoils and tailings (Muncy, 1985; Poser and Jochimsen, 1989).
White clover is of value in various kinds of amenity or recrational grassland, partly for its agronomic properties, but also for its attractiveness when flowering. It could also be used more widely in gardens than at present, although its stoloniferous habit could be regarded both as an advantage, e.g. ground cover, and a disadvantage, e.g. its unwanted spread into neighbouring areas. In some countries, fourleaved clover is regarded as a good luck symbol and living plants may be sold or the leaves coated with a precious metal such as silver or gold and sold as jewellery.
Thanks are given to Rossella Ardiani, Mauro Scimone, Ileana Napoleone, Hans Lex and Päivi Nykänen-Kurki; for assistance in searching the literature for references.
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