N.W. Pirie
N.W. PIRIE is with the Rothamsted Experimental Station, Harpenden, Herts., UK. This article is a shortened version of an article which appeared in Appropriate Technology, London, Nov. 1978.
Grasses and other low-lying plants are the main food of domestic grazing animals in Europe and North America. But even in the United Kingdom, deer in the highlands can yield more meat than sheep in a similar area, as heather and ling are important foods for them (1). Deer in Hertfordshire parkland browse more than they graze, they only get about 10% of their food from grasses (2). Domestic animals in tropical regions depend on browsing perhaps more, than they do on grazing.
Agricultural advisers trained in regions where grazing predominates were slow to recognize the importance of browsing but now that it has been accepted, trees and bushes are at last getting more attention. An important factor in favour of trees and bushes is that they have permanent and often deep roots which help to stabilize the soil where it is liable to erosion in heavy rains, and can tap deep sources of water and nutrients that are not available to most grasses. Consequently, they often remain green while grasses wither. Hoppe (3) suggests that browsers do better than grazers on semiarid rangeland because they select the juicier types of leaf.
Recognition of the importance of browsing has led to a few studies on the nutritive value of tree leaves. No one argues that they are better than conventional forage, but they are well worth using and are often found in marginal lands that would not otherwise be used. Increased non-arable food production is one way to conserve forests. Many species were studied by McLeod (4), Rees (5) Finzi and Giulini (6), and McKell (7). Leaves and other byproducts from the olive oil industry make good cattle fodder (8). Oak leaves are though!: to have some value in poultry feeding (9), although those that fall spontaneously contain only 1% nitrogen.
Animals foraging for themselves avoid the leaves that are poisonous to them, but if these were processed into feeding meals, they probably would not be able to distinguish them. Fortunately, there are few poisonous species that must be rigidly excluded; an example is the needle from the ponderosa pine (10). The leaves of Leucaena are sometimes poisonous, but nontoxic varieties are being produced (1.1).
Tree leaves harvested before senescence contain 2-4% nitrogen (1.2). In this respect they resemble the usual forage plants. Judging by analogy with other leaves, 60-80% of this nitrogen is probably present as protein, but measurements have not been made systematically. Similarly, the nutritive value of the protein, after freeing it from most of the other leaf components, has not been measured. Siren (13) has published an extensive series of amino analyses on leaves from several tree species which closely resembles analyses on forage crops. This is not surprising, for Byers (14) found that proteins from different leaf species were very similar. The annual yield of protein from poplar growing in ideal conditions in southern Sweden could reach seven tons per hectare (12). In more normal conditions, the annual yield of leaf (dry matter) is only 2-4 tons. Nevertheless, there seems to be a case for studying the yield and composition of tree leaves and their potentialities as sources of human and animal food.
Young leaves from many trees (e.g., :horse-radish or drumstick tree in India and hawthorn in the UK) are, or were, traditional foods. In some Indian villages the drumstick trees are so defoliated that their survival is threatened.
Similarly, the leaves of certain plants that make good hedges, notably chaya in the tropics of Central America and beech in Europe, are or have been eaten. There is scope for much more research here. It is absurd that so much effort is expanded in the suburbs on clipping hedges and disposing of the clippings, when, by suitable choice of species, a section of the hedge could be clipped every few days for the pot. Many useful books have been published recently that advocate the use of wild plants as food, though some of the statements made in them are questionable. It would be helpful if authors stated clearly the amount of leaf that they regularly eat, as danger lurks in every type of food. Many familiar vegetables such as cabbage, kale and spinach are beneficial when eaten in moderation, but harmful if eaten in bulk daily. Although there are no precise figures, the quantity dividing "bulk" from "moderation" is approximately 150 g (fresh weight). This is an argument for using more rather than fewer species because there are different, possibly harmful, substances in different foods. In spite of the consequent loss of vitamin C, cooking water should often be discarded: toxicity is a quantitative matter.
A TROPICAL FOREST IN ASIA nutrients waiting to be harvested by research workers
It is easy to collect tree leaves for domestic use but bulk collection raises difficulties. Leaves that fall in the autumn (e.g., oak leaves) usually contain little protein. Certain chemicals make leaves fall at a time of year when they contain more protein. The use of these would be worth investigation, although growth would probably be restricted as much by coppicing the trees every few years. The straight and unbranched habit of coppiced trees simplifies mechanical leaf strip ping. Coppiced trees are already grown extensively as sources of paper pulp and firewood. Half the trees that are now being felled worldwide are used as firewood, and several countries have plans to cultivate coppiced "energy plantations" for industrial energy. When choosing species and varieties for these plantations, it would be sensible to pay some attention to the usefulness of one of the by-products-leaves.
Leaves and small twigs collected during conventional forestry are dried, ground, fractionated in an air stream, and used as cattle fodder in the USSR (15). Acacia leaf meal, presumably collected in a similar manner, appeared on the market in the Netherlands mislabelled "lucerne" (16).
The advantages of making products primarily intended as ruminant fodder have been stressed in Romania and Yugoslavia (17). It is obviously better to use leaves and conifer needles in this way rather than wasting them, but it would be even better to make food for humans, if this were possible, and avoid the 80-90% loss that occurs when protein in a fodder is con versed to edible protein in an animal. The ample evidence that found leaf protein to be useful food is surveyed elsewhere (18).
Protein has been extracted from pine needles by procedures that would be feasible only in a laboratory: Gezelius (19) gives several references. Preliminary attempts at Rothamsted to extract protein from the leaves of several species of broad-leaved trees suggest that extraction is not as easy as from the familiar forage crops. Protein can be easily extracted from elder bushes and from Gliricidia (a tropical tree): Carlsson (unpublished) has had success with some other species. More species and varieties should be examined, and variations in the technique of extraction should be tried out in the hope of either finding species that extract readily by existing methods, or methods suitable for use on a large scale that are effective on the more recalcitrant species. Work along these lines will not start until foresters and those who control the funding of research have been convinced that we are literally throwing away a potentially valuable byproduct that is likely to become even more abundant in future.
At present, the elements in fallen leaves fertilize the growing trees and maintain a more or less closed cycle. The timber that is carted away contains mainly carbon, hydrogen and oxygen which are replenished from: air and rain. Foresters have been. curiously reluctant to realize that this state of near-equilibrium may not be ideal and that trees, like other crops, are likely to grow more quickly if adequately fertilized. Foresters have become reconciled to the idea that tree nurseries should be fertilized, but the only time that I have been barracked by my chairman, a forester, was when I said at a public meeting -.hat adult trees would also benefit from proper nutrition.
Early experiments (20, 21) showed that fertilization could increase timber production fourfold. More :recent experiments are described by Miller (22) and Matthew (23). It is reasonable to assume that the response to fertilizers will be even greater if leaves are being carted away regularly for use elsewhere, instead of being allowed to decay on the land on which they were grown. In spite of a few romantic attempts to get something for nothing, there is now general agreement that, if elements are steadily withdrawn from soil, they ultimately have to be put back if any type of crop is to grow properly. A tree in a garden from which leaves are being collected will need regular fertilization.
1. BLAXTER, K.L.; KAY, R.N.B.; SHARMAN, G.A.M.; CUNNINGHAM, J.M.M. & HAMILTON, W.K. Farming the red deer. 1974. Department of Agriculture and Fisheries, Scotland Edinburgh, HMSO.
2. JACKSON, J.E.; CHAPMAN, D.I. & DANSIE, O.A. Note on the food of Muntjac deer (Muntiacus reevesi)). 1977. J. Zool. (Lond.), 183: 546.
3. HOPPE, P.P.; QVORTRUP, S.A. & WOODFORD, M.H. Rumen fermentation and food selection in East African Zebu cattle, wildebeest, Coke's hartebeest and topi. 1977. J. Zool. (Lond.), 191: 1.
4. MCLEOD, M.N. The digestibility and the nitrogen, phosphorus and ash contents of leaves of some Australian trees and shrubs. 1973. Aust. J. exp. Agr. Animal Husbandry, 13: 245.
5. REES, W.A. Preliminary studies into bush utilization by cattle in Zambia. 1974. J. appl. Ecol., 11: 207.
6. FINZI, A. & GIULINI, P. Le foraggere della boscaglia della regione del Benadir (Somalia); raccolta composizione botanica e chimica. 1975. Riv. Agric. Subtropicale Trop., 69: 129.
7. MCKeLL, C.M. Shrubs - a neglected resource of arid lands. 1975. Science, 187: 803.
8. NIGH, H. The use of olive leaves as roughage for dairy cattle. 1977. Appropriate Technology, 4(2): 11.
9. ROBEL, E J. & FROBISH, L.T. Air-dry tree leaves for use in a restricted dietary regime. 1977. Poultry Sci., 56: 715.
10. ANDERSON, C.K. & LOZANO, E.A. Pine needle toxicity in pregnant mice. 1977. Cornell Vet., 67: 229.
11. NATIONAL ACADEMY OF SCIENCES. Leucaea: promising forage and tree crop for the tropics. 1977. Washington.
12. NEDORIZESCU, M. Producerea unor fainuri furajere din diverse resurse ale fondului forestier. 1972. Revista de Zootehnie de medicina Veterinara, 22(12): 31.
13. SIREN, G.; BLOMBACK, B. & ALDEN, T. Proteins in forest tree leaves. 1970. Royal College of Forestry, Stockholm, Publ. 28.
14. BYERS, M. The amino acid composition and in vitro digestibility of some protein fractions from three species of leaves of various ages. 1971. J. Sci. Fd. Agric., 22: 242.
15. YOUNG, H.E. Muka: a good Russian idea. 1976. J. Forestry, 74: 160.
16. ANON. 1974. J. Flour & Animal Feed Milling, 156(7): 45.
17. NADAZDIN, M.; DZINIC, M.; TERZIC, D.; BUGARSKI, D. & BUKOJEVIC, J. Possibility of using vitamin-mineral conifer meals in feeding sheep. 1974. Quoted from Nutr. Abs. Rev., 46: 188. 1976.
18. PIRIE, N.W. Leaf protein and other aspects of fodder fractionization. 1978. Cambridge University Press, London.
19. GEZELIUS, K. Extraction and some characteristics of ribulose 1, 5-diphosphate carboxylase from Pinus Silvestris. 1975. Photosynthetica, 9: 192.
20. LAURIE, M.V. The place of fertilizers in forestry. 1960. J. Sci. Fd. Agric., 11: 1.
21. STOECKLER, J.H. & ARNEMAN, H.F. Fertilizers in forestry. 1960. Adv. Agrom., 12: 127.
22. MILLER, H.G. Nitrogen nutrition of pines on the sands of Culbin Forest, Morayshire. 1969. J. Sci. Fd. Agric., 20: 417.
23. MATTHEWS, J.D. Prospects for improvement by site amelioration, breeding and protection. 1975. Phil. Trans. R. Soc., 271: 115.
