Mixed cropping is not an unknown subject in plantations, although various terminologies such as intercropping, multicropping, supplementary cropping etc., are used. It is a type of agro-forestry system where two or more crops are grown simultaneously on the same land. This concept had been introduced and developed to maximise land productivity and to minimize the risks involved in growing a monocrop.
When properly planned and managed, this has added advantages such as enhanced net returns, favourable economic, social and environmental benefits and increased employment. But, still it remains an under utilized option to boost productivity in plantations.
Tea , rubber, and coconut are perennial tree crops which have a long immaturity period. It is therefore difficult to regulate supply on a short-term basis according to changing market conditions. This is one of the major reasons for prices to remain rather volatile and create uncertainty among both growers and consumers particularly the small businesses. These crops are already grown on smallholder lines covering extents in the region of 65% to 70% of the total extent which are in the region of 180,000, 115,000 and 440,000 hectares in Tea, Rubber and Coconut, respectively. Even a short period pf fluctuating produce prices could be difficult to tide over. This is because small holders have little access to credit or cash reserves to meet such a contingency.
Crops selected for this system in plantations are of two types, on the basis of their adaptability to the different plantation development stages.
For example, in rubber, the first type includes those fit for immature rubber plantations where there is an adequate solar radiation e.g. pineapple, passion fruit, banana etc. The second type includes those tolerant to limited or higher degrees of shade nd can therefore be grown during both the immature and mature phase. Example, tea, coffee, cocoa, pepper, cinnamon, timber species, is as boundary trees etc.
Economic and Ecological benefits
Economic returns include those from secondary crops via harvesting and from increased output from the main crop as well as cost savings due to mixed cropping. The economic returns of the secondary crops depends on market demand, It can be as high as 1 to7 times that of the value of the main crop output when the market is high or negligible or even a deficit when the market is low.
Systemic mixed cropping has been found to increase the growth rate and yield of the main crop whilst also reducing the upkeep cost and fertilizer applications. Generally, a saving of 45-60 labour days/ha/year can be expected during the immature period of properly mixed cropped rubber stands.
Mixed cropping provides an integrated approach system in the use of agricultural resources. Correct cropping pattern can help to establish as stable artificial ecological community in plantations to form a favourable recycling system and improve the ecological environment. This makes it possible for satisfactory sustained growth and production from both the main crop and the secondary crops.
Higher biomass production is demonstrated by higher total yield from the main crops and the secondary crops due to normal growth of both crops. In addition, a large amount of litter is a common phenomenon in some mixed cropping combinations such as rubber/pepper ( creeping) which contribute litter of 7545-9200 kg/ha/year which is higher than that of rubber legume cover ( Pueraria 5350 kg/ha/year).
This system would provide innovative approaches for either shortening or even avoiding the period of soil reconditioning after uprooting tea. Thus eliminating the cost of this operation and the 2-year unproductive period.
Higher output and land use capacity
The commodity output rate per unit area in properly mixed cropped plantations is higher than that of the monoculture system, generally 05- 1 time higher.
This is due to faster growth and higher yield of trees, assisted by certain produce from the secondary crops. Ecologically, this is due to the favourable environment of the new ecosystem formed.
Generally, intercrops do not require the planting density to be changed drastically when planted between the trees or in the inter rows. This facilitates a higher land use capacity per unit area. Land use capacity can be increased by 30 – 50% with a maximum of in the region of 75%.
Analysis of soil nutrient contents of some mixed cropping combinations indicates higher soils organic matter content, N,P and K with rubber/tea, rubber/pepper and rubber/sugarcane combinations. In some cases, micronutrients are also brought into circulation.
The thicker canopy of mixed cropped plantations reduces the airflow rate while increasing the solar absorption and reflection rates. Therefore, less sunlight reaches the ground and there is weaker air turbulence at ground level and a lower wind velocity. Hence, evaporation is decreased but the relative humidity and the soil moisture content is increased. This is especially marked during high temperature period when the under-canopy temperature is lower to form a favourable microclimate for growth and yield. This is also true for the soil to maintain a steady micro environment.
Reduced soil erosion
It has been shown that properly mixed cropped plantation can reduce surface runoff and soil erosion due to the better raindrop interception capacity of the thicker canopy, thicker litter coverage and better soil structure.
However, not all mixed cropped plantations can reduce soil erosion. Soil erosion can occur in plantations on steeper slopes where soil tillage is required or down slope planting is practiced. This is true even in those plantations with good mixed cropping combinations, if no mulching or cover crop establishment is made in the early stage of the plantation.
Mixed cropping can produce more products and goods for the market and increase the income of growers. Plantations are sometimes in peril of natural calamities such as wind damage, drought stress etc. Mixed cropping can help to reduce the damage in terms of economic returns.
More job opportunities are created. For example a field worker can take care of about 1.67-2.0 ha. in conventional immature rubber plantations, but in mixed cropped plantation each field worker can take care of only about 0.27 – 0.67 ha. thus increasing job opportunities by 3-6 times.
In mature monoculture rubber plantations, each field worker can be responsible for about 1.0 ha. on the conventional tapping system but each field worker can take care of only about 0.32 – 0.33 ha. in mixed cropped stands thus increasing job opportunities by 1-2 times.
Mixed cropping policies
Adoption of mixed cropping policies in plantations should be therefore considered as an exercise to;
• Utilize the inter-row space in plantations to generate an early income from the land during the unproductive period.
• Generate an income even during unflavourable weather conditions when harv esting may not be possible from the main crop and thereby to provide a steady stream of income for the grower.
• Generate an income during adverse trading conditions for the main crop.
• Increase the productivity per unit area of land.
• Develop agricultural soils, degraded due to mismanagement and continuous adoption of monocultural cropping systems.
• Shorten or eliminate the period of reconditioning of tea soils prior to replanting.
• Convert marginal / uneconomical areas into better use with a view to make such units more profitable and economically viable.
• Generate or reduce employment ( depending on the type of previous crop) which would enable estates to utilize the available labour resources more efficiently and effectively.
Rubber and Tea
Cropping of tea and rubber together is considered feasible in the agro-climate regions; Low Country Wet Zone ( WL1, WL2, WL3 and Wl4), Mid Country Wet Zone ( WM2 and WM3), Low Country Intermediate Zone ( IL1 and IL2), Mid Country Intermediate Zone ( IM3)and Up Country Intermediate Zone ( IU2 and IU3), where conditions are favourable for the cultivation of both these crops.
Guidelines provided for this system indicates that it is possible to have 70 percent tea stand and 70 percent rubber stand as per individual mono crop stand or in another system where it is possible to have 70 percent tea stand and 65 – 70 percent rubber stand as per respective monocrop stands. In both systems, the land use efficiency is in the region of 140 – 150 percent.
Tea based systems
It has been established that Tea and Coconut can be grown together under three systems of cultivation; Intercropping tea and coconut simultaneously, Intercropping coconut in tea lands and Intercropping tea in coconut lands . These are feasible in the mid and low country in the agro ecological regions where both tea and coconut can be cultivated.
Among the export crops, pepper ( piper nigrum) is the most compatible with tea of all types. The pepper vines are easily trained to grow on shade trees such a Gravellia rebousta and Gliricidia maculalata in tea. As coffee is more suitable for seedling lands, where tea stands are average to poor, Robusta could be considered for mid elevations and Arabica for high elevations.
It has been reported that it is also possible to intercrop fruits tree species such as citrus, mango, avocado and rambuttan with tea in fields with a low tea plant density.
Coconut based systems
It has been recognized that coconut based farming systems provides the best option for optimizing the productivity and augmenting the economic viability of low productivity coconut lands, particularly on land suitability classes S3,S4 and S5.
A range of acceptable crop/farm models for coconut, have been proposed. The models include integrated livestock farming with forage grasses for zero- grazing and legumes for stall feeding, secondary crops such as banana, pineapple, cashew, cocoa, coffee, tea, vanilla, ginger, black pepper, cinnamon, betel etc. Establishment of honey bee colonies in such mixed cropped areas in the wet zone is also possible.
Gliricidia sepium is known to be an ideal perennial tree crop to be grown either as a monocrop plantation or as a mixed crop in coconut, tea and rubber plantings. Although gliricidia had been grown historically as a boundary fence and subsequently introduced as a shade tree in plantations, but it is now popularly grown as a green manure crop in plantations and more recently for biomass power generation.
As a green manure crop, it provides nutrients in the region of 12,000 kg of N per hectare per year ( equivalent to 300kg of urea). Other benefits include, alleopathy, a natural phenomena of repelling pests and suppressing weed growth and the foliage is also used as cattle fodder . All these promote successful integrated farming systems in plantations, known to be an effective technology in coconut. Bio gas units can also be established in such systems with cattle / goat dung as raw materials.
As a biomass power generation crop, it provides wood yield in the region of 30 mt/ha/year which could partly meet the energy requirement of some of the domestic, industrial and commercial sector industries. It has been identified as the most suitable source for thermal / electrical wood-based power generation.
Mixed cropping models
More, technically feasible, economically viable and socially acceptable crop / plantation models based on Tea/ Rubber / Coconut / Gliricidia, that are adaptable to diverse identified agro-climatic considerations, growth stages of plantation crops and platation sectorial divisions should be developed to meet the emerging needs of the plantation industry in particular the smallholder sector who are more vulnerable to changes than the corporate sector.
This becomes more relevant to the current scenario when attempts are being made to extend plantation crop planting into non-traditional areas as smallholder farming models and out-grower systems.
The area under mixed cropping in Sri Lanka is still rather small, constrained by available resources and knowledge. Research and development and extension services on mixed cropping in plantations are also very limited and superficial.
As market mechanism will play more and more important role in regulating and maintaining the sustainability of mixed cropping, the selection of annual or perennial crops for the use as mixed crops in plantations must take market demands into consideration quite apart from the technical know-how availability based on research findings.
Some traditional mixed cropping models can be continued with but some new intercrops with high market potentials must be introduced into the cropping scheme, for this system to be more attractive and profitable.
Environmental concern must be a further factor for consideration in mixed croping. Final aspect of importance is that of rural social development and political commitment. Integration and not proliferation of subjects / departments / ministries, will help in this regard.