Inter-cropping has been paracticed for many years on both smallholdings and estates to maintain incomes, especially during the relatively long period of immaturity. Obviously, some crops are more appropriate than others in particular locations due to local climates, tastes, customs, markets, etc. Animal husbandry and the harvesting of honey are relevant in some areas.

Crops include upland rice, mungbean, soybean, corn, vegetables, banana, pineapple, cassava, castorbean, jute, etc. The recommended planting distance of the intercrop from the rubber row depends on the kind of intercrop. Smaller crops are planted about one metre from the rubber rows, but for larger types of biennial crops this is increased to about 2 metres. After harvesting intercrops, it is strongly recommended that the residue is mulched to conserve moisture and to improve the soil's physical properties to promote the growth of young rubber.

Sugarcane and guinea grass + centrosema are not recommended as an intercrop in Thailand. Sugarcane needs a large quantity of nutrients, especially potassium. Besides, these crops are a fire hazard in the dry period. In the case of guinea grass + centros the experiment showed that the growth of rubber is not improved and also delayed tapping for about 1 year.

In a trial performed during 1981-86, rubber was planted with a spacing of 7 m between rows and 3 m between trees within the rows and was then intercropped with a legume cover crop mixture including Calopogonium mucunoides, Centrosema pubescens and Pueraria phaseoloides, or with mung beans (Vigna radiata), soyabeans, groundnuts, rice, maize, bananas or pineapples. Mung beans and groundnuts gave average yields of 0.59 and 1.15 t/ha, respectively, but soyabeans gave only low yields of 1.21 t/ha. Rice given 40 kg N and 21.5 kg P/ha yielded 2.14 t/ha, and maize given 93.8 kg N, 40.2 kg P and 75 kg K/ha yielded 2.96 t grain/ha. Bananas planted at 477 plants/ha began to yield early in the 2nd year and gave 2082 bunches/ha annually. Pineapples also began to yield in the 2nd year, and at 17 000 plants/ha gave 11 572 fruits/ha annually. The legume cover crop established rapidly and inter-row weeding was not needed.

A study on cropping systems suitable for young rubber plantations in the South-East Region of Vietnam was conducted for six years (1991 - 1996) by the Institute of Agricultural Science of South Vietnam. The following results were obtained: 1. Intercropping peanut (Arachis hypogaea L.) in young rubber plantations an advantageous technology, suitable for the agro-ecological conditions and the economic situation of farmer households in SER.

The new technology contributed to improving the effectiveness of land-use, increasing farmers' income and enhancing soil protection and weed control as well as maintaining soil fertility in the rubber plantations. 2. Peanut variety HL25 (original ICRISAT's name - ICGS E 56 - introduced IRRI in the Asian Rice Farming Systems Network) was a new peanut variety suitable for intercropping with young rubber and other farming systems in SER. 3. Application of inorganic fertilizer (NPK) and lime as well as the spra of foliar fertilizers significantly improved the economic effectiveness of intercropping peanut and young rubber. The optimum fertilizer recommendation was 40 - 60 N + 80 - 100 P2O5 + 60 - 80 K 2O + 300 - 1000 lime (kg/ha) + 3.5 litres of Bio-top/ha. 4.

The use of the new peanut variety HL25 with the application of intensi cultural practices might obtain a profit of 1.2 to 3.9 million VND/ha, whilst traditional cultural practices (using local variety Giay and low fertilizer application) only obtained a profit of 0.2 to 1.9 million VND/ha. The new technology could be widely expanded to young rubber plantations in SER. Intercropping with annual and perennial crops in rubber fields has been practised for a long time by both estates and smallholdings in Vietnam and contributes to the improvement of rubber growers' income and effective use of land. Modified systems with hedgerow planting techniques have also been adopted to improve the effectiveness of intercropping in rubber plantations. The social and economic aspects of intercropping with Hevea are discussed.

Mixed farming has been practised in rubber plantations in China since the late 1950s when China began, with great effort, to expand its rubber plantation industry. Many forms of mixed farming have been tried during the last four decades or more. Rubber trees have been intercropped with tea bushes, sugar cane, pineapple, pepper, cinnamon etc apart from various cover crops. The mixed farming system is desirable in the context of economic returns and environmental effects, although it varies depending on the crop combinations and market demands as well as geographical locations and terrain. In general, mixed farming or intercropping in rubber plantations could fetch an average net economic return at around RMB¥1500/hectare in China.

Intercropping in rubber plantations can increase the land use capacity by 30 - 50% and create more job opportunities. The ecological and environmental effects are more pronounced from such mixed farming systems. This can be seen from reduced soil erosion, enhancement of soil nutrient status, better biodiversity compared with arable farming, decreased temperature by 1.2 - 7.0oC and increased humidity by 2 - 10% to create a situation similar to secondary tropical forests. Some interactions between rubber and intercrops and the new ecosystems in terms of fauna and flora are discussed. It is concluded that correct intercropping in rubber plantations is a sustainable farming system. Some existing intercropping problems in China are dealt with and recommendations for their solution are made.

Abstracts of papers delivered at IRRDB Symposia held at Beruwela, Sri Lanka, Nov 1996 Symposium No 2 - Farming system aspects of cultivation Paper 21

Two intercropping systems involving rubber/cinnamon and rubber/forage grasses/tree legumes were investigated. Cinnamon reached harvesting age 2½ years after establishment under immature rubber. Cinnamon also performed satisfactorily under mature rubber where light availability for cinnamon was ca. 53% daylight. The yields under immature rubber were a little low, but comparable to that of monocrop Cinnamon. Among the two species of grasses, the dry matter (DM) yield of Panicum ma was almost double of that of Brachiaria brizantha during the first two years but the DM yields of both species declined with the growth of rubber trees.

The growth and yield of both species of tree legumes were poor in all treatment combinations. The growth of rubber was not affected adversely by these intercrops during the period of study, ie 3 and 5 years with Cinnamon and grass respectively. An ample number of studies have been carried out to answer the problems of Hevea-based intercropping systems in different rubber producing countries. These studies generally aimed at finding the types of intercrop which are technically suitable and economically advantageous as intercrops. However, information regarding the details of agro-physiological processes which occur in Hevea-based intercropping systems is still lacking.

Research carried out at Sembawa Research Station, Indonesia, (on-station research) and at farms (on-farm trial) around the station (latitude 3o 8' and longitude 104o 18') and commenced in 1993. The objective of the on-station research was to analyze quantitatively the effect of intercrops on the growth of rubber. The treatments were comprised of Rubber +: clean weeded interrows (A), Legume Cover Crops (LCC) (B), Upland rice-fallow (C); pineapple (D); pineapple + banana (E) and alang-alang (Imperata cylindrica) (F).

The on-farm trial was carried out to cover the variability in growth of rubber and its intercrops and to analyze the dominant factors which influenced that variability at the farmers' level. The factors considered in choosing 16 farmer participants were the origin of the land; the farmers' profession, the accessibility to farmers' land and labour availability; upland rice was grown for at least the first year as the rubber intercrop. Two types of plot were made: A farmers' plot, where the farmers were free to decide what they did with their interrows, and A clean weeded plot, where the plot was kept free from weeds throughout the research period. Station results showed that the growth of rubber depended on the types of intercrop. The stem diameter of rubber (until 30 months) of the treatment rubber +: the clean weeded soil (A), pineapple (D), pineapple + banana (E), were comparable, but there was a tendency for the rubber growth of these last two treatments to be slower than in treatment A from 15 months.

The slowest growth of rubber was found in the alang-alang treatment (F). The two other treatments, Upland rice - fallow (C) and LCC (B), were intermediate. The growth in diameter of treatments F, C, B and D and E were 10.5, 7, 5 and 3.3 months, respectively, slower than that of treatment A. The stem growth variation was principally due to soil moisture rather than due to mineral nutrient factors. On farm research results showed that many farmers stopped planting upland after the first harvest. Maintenance of other intercrops, such as banana, pineapple, katu and perennial fruit trees was continued with. Chilli was the best intercrop in terms of both agronomic and economic aspects. After two years, when the intensity of intercropping decreased, the intensity of weeds increased and rubber growth was affected. The management of interrows was very important in that period to avoid water competition.