Teak (Tectona grandis) is by far the most popular tree species used in taungya and is planted as stumps or seedlings. Gmelina arborea is also widely used. Apart from the growth of tree crops for the production of timber and other traditional wood products, the taungya system may be used to raise each crops such as cashew (Anacardiun occidentale). This species has been successfully introduced on poor sites in the savannas of southern Guinea, sown at a spacing of 2 × 4 m with maize, or sometimes cotton, between the row of trees. The cashew begins fruit production about the fifth year; the average production in the Ivory Coast is 400 kg/ha at the age of 15 years, which provides the farmers with a very good return.
There is some evidence that where tree crops are planted with agricultural crops, a wider espacement of the tree crops reduces mortality, increases the rate of growth and, at the came time, the presence of suitable agricultural species effectively reduces soil exposure. In addition, the farmer obtains higher yields per unit of plantation. Closer tree espacement frequently reduces early tree growth because of the increased competition, and necessitates earlier silvicultural tending.
The agricultural crops which are grown in conjunction with the forest trees are generally chosen because of the agricultural and feeding habits of the cultivator. The moat commonly cultivated are bajara (Pennisetum typhones), barley (Hordeum vulgare), beans (Phaseolus spp, Vigna spp), bhajee (Amaranthus spp), brinjal (Solanum melangena), cabbage (Brassica spp), castor (Ricinus communis), chilli peppers (Capsicum spp), coco yam (Colocasia antiquarum), cotton (Gossypum spp), cucumber (Cucumis sativus), dasheen (Colocasia esculenta), dhal (Cajanus spp), ginger (Zinzibar officinale), groundnut (Arachis hypogaea), ladys fingers (Anthyllis vulneraria), linseed (Linum usitatissimum), lucerne (Medicago sativa), melon (Citrullus vulgaris - Cucumis melo), millet (Pennisetum spp, Panicum spp), mustard (Brassica spp), oats (Avena sativa), ochra (Hibiscus esculentus), papaya (Carica papaya), pineapple (Ananas comosus), potato (Solanum tuberosum), pumpkin (Cucurbita maxima), rye (Secale cereale), sesame (Sesamum indicum), sorrel (Hibiscus sabdariffa), soyabean (Glycina soja), sweet potato (Ipomoea batatas), tannia (Xanthosoma sagittifolium), tomato (Lycopersicon esculentum), tumeric (Curcumalonga spp), wheat (Triticum spp).
There are several agricultural species which are controversial and are excluded in plantations in some countries, such as bananas and plantains (Musa spp), cassava (Manihot utilissima), maize (Zea mays), rice (Oryza sativa), sugar cane (Saccharum officinarium), tobacco (Nicotina tabacum) and yam (Dioscorea spp). Limitation or exclusion of bananas and plantains is due to various reasons, among which are: to avoid human interference in the plantations (since the farmers are reluctant to cut or abandon a plant which continues to produce foodstuff), to conserve soil fertility, and to prevent young trees from being deformed. However, at Mayumbe, Congo Brazzaville, bananas combined with Terminalia superba seedlings are exploited during four or five years in state sylvo-bananiers, the spacing of the trees being 12 × 4 m with bananas in two or three intervening rows. Cassava is excluded in Dahomey and Uganda because it exhausts the soil, it has a long life and it attains a height of 2-3 m rapidly, thereby retarding the development of the tree crop. The same reason, fast growth, is given for the exclusion of maize in Malawi, Mauritius and Senegal. However, maize has had no significant effect on the mortality of teak (from stumps and seedlings) in plantations made in Gambari in Nigeria, but may have an effect on height growth according to the type of planting stock used. Tobacco may be excluded because it has a deleterious effect on soil nutrients and because of its inability to provide adequate soil cover and therefore the liability of the land on which it is planted to erode.
Hill rice is grown with tree crops particularly in Malaysia, Senegal, Assam and Kerala, the growth of trees being enhanced because rice suppresses the weeds. However, in Sri Lanka it is felt that the returns from rice are so high that farmers are likely to exert their influence to convert the land to single use agriculture. Sugar cane is generally excluded because it is a long-growing crop, because of fear of soil depletion and because it casts a heavy shade. Nevertheless, where it has been cultivated with considerable success in Assam, India and in Burma, the presence of the cane led to increased height growth of the tree seedlings.
In China intercropping is generally applied in forestry. There are examples of agricultural crops being planted between rows of poplars, Cunninghamia lanceolata and Pinus spp (massoniana, taeda or elliottii) for a period of two years. In some plantations, particularly pine, tung oil trees (Aleurites spp) are interplanted concurrently with the agricultural crops; they yield oil from the fourth to the tenth year after which they are felled leaving the pine as the final tree crop. Intercropping is not only regarded as a tending operation to replace weeding but also a multiple land-use practice for joint production of wood and food. Depending on soil quality, crops may be sweet potatoes, soyabeans, peanuts, watermelons, or maize. In general, intercropping with legumes is preferred as it enriches the soil, provides green manure and also feed for animals. Depending on crops and, on the management skill, intercropping may yield 1.4-4.0 tons of food per hectare. In some places, it may yield 20 tons/ha of green leaves which are used as feed for animals (pigs) or as manure. The effect on tree growth has been observed as very favourable. Survival rate of Cunninghamia is 5 percent higher than that of non-intercropped plantations and plant height is 33 percent greater.
A further example of mixed cropping can be taken from the southern Pacific coast of Colombia where Cordia alliodora and Cedrela odorata are planted on small landholdings concurrently with the traditional crops of plantain, maize and cocoa (Theobroma cacao).
Although mixed cropping may be contrary to the thought of many foresters, accustomed to the tidy and regular appearance of their plantations, this system is practiced not only for traditional reasons, but because it suite the environment, maintains soil fertility and combats erosion and leaching. There is also an economic justification, since more production may be achieved from mixtures of crops, thus making full use of the space available. Good management is an important factor in intercropping with strict enforcement of any rules that may be laid down. In cultivating and in harvesting the agricultural crops, and particularly tubers, great care must be exercised in order not to damage the roots of the tree crop. If creeping species are used, against the general rule, the farmers should provide bean-stakes or poles (in the case of yams) to prevent strangulation of the tree seedlings. It is important to emphasize that growth and yield of the agricultural crops are directly influenced by the spacing and density of the tree crop. Concommitant with these two factors are the rate of growth and relative crown size of the tree species.
The taungya system is a way to reduce the costs of forest plantations, and at the same time to contribute to solving social problems. In Campeche, Mexico, where Cedrela mexicana, Swietenia macrophylla and Cordia ciricote were the main species planted, the net costs per hectare for planting and tending during 5 years, with 2 weedings per year, were reduced to as much as 27 percent (to US$ 58.4) of the normal costs because of the revenues from the maize harvest. If mechanization is used, the costs drop to 18 percent (to US$ 34.3/ha).