By Dr. Roshan Rajadurai
Since the spraying of herbicides was introduced as a form of weed management in the 1960s with paraquat, plantations have used pre-emergent, residual, post-emergent, broad spectrum, systemic, translocated, and contact herbicides for weed management, depending on bush cover, rainfall pattern, growth stage of weeds, and tea bush. Plantations have not exclusively depended on herbicides for weed management but have used preventive, cultural, manual, biological, ecological chemical, mechanical, and genetic weed management methods to control weeds found in the various stages of tea cultivation.
Weeds were treated based on length of lifecycle of weeds (annuals, bi-annuals, or perennials), habitat (upland or lowland), morphology of weeds (broad leaved, grasses, sedges, shrubs, or trees), harmfulness (soft or hard), and stem tenderness (woody or herbaceous). Herbicide usage has followed Tea Research Institute of Sri Lanka (TRI) recommendations regarding selectivity (selective or non-selective herbicides), mode of action (contact or systemic), placement site (foliage applied or soil applied), and time of application (pre-emergent or post-emergent).
According to the TRI, chemical weeding is the most convenient and cost-effective method among various techniques under integrated weed management (IWM). Herbicide spraying creates a mulch of dead weeds on the surface and improves water retention, adds organic matter, and recycles nutrients removed by weeds. When manual weeding is done, weeds physically removed from the field carry away 32 kg of nitrogen, 40 kg of phosphorus, and 320 kg of carbon from the field, reducing the organic carbon and nutrients.
TRI has confirmed that without regular weeding, mature tea will have a 20% yield loss after six months and if manual weeding is resorted to, yield loss after four months will be 15%. Furthermore, when manual weeding is done, soil fertility and water holding capacity of soil is lost due to erosion and 30-45 cm of topsoil is lost, leading to 30-50% loss of crop productivity. Currently, appropriate and recommended IWM systems are being undertaken, of which, use of herbicides is a component. Currently two applications of herbicides are undertaken at a cost of around Rs. 12,000 per hectare per annum using around four workers per hectare for spraying. However, if manual weeding is done, 30-40 workers per hectare has to be used every three months; 120-160 workers per hectare at a cost of around Rs. 180,000.
This is completely unaffordable; as the cost of manual weeding would increase the cost of production (COP) for tea by Rs. 130. This is not practical and the industry does not have the human resource capacity to undertake such a labour intensive exercise. Already, the tea industry is plagued with serious labour shortage and are unable to even pluck minimum required four rounds in the existing tea fields resulting in crop and quality loss. Herbicide usage is universally accepted as a convenient, cost effective, good agronomic practice with benefit of timeliness, replicability, specificity, predictability, and availability of required manpower and materials.
Weeds in the tea fields cause crop loss through competition for space, soil nutrients, and soil water. Weeds compete and remove nutrients applied to the tea bushes. Weeds also have an allelopathy effect on tea plants (illuk [Imperata cylindrica] and exudation of noxious substances from roots), host parasites (lygus bug, rosselina, loranthus, and cuscuta), and cause soil loss through erosion when manually weeded and interfere in the agricultural and cultural operations. The presence of weeds significantly reduces harvester productivity when weed foliage reaches the plucking table; interfering in the harvesting operation, reducing harvester output and income, leading to reduced quality of life.
Harvester safety and wellbeing at work is compromised with presence of leeches and reptiles in the fields because of profuse weed growth. Weeds reduce the quality of made tea due to poor standard of harvested leaf and contamination of noxious weed foliage in made tea, leading to many unwarranted issues in terms of crop loss, quality loss, and worker health, safety, earnings, and livelihood-related issues.
Before the advent of herbicides, weeding in the plantations was done using an implement named “Sorandi/Scraper” which led to massive and irreversible soil erosion. Conservation of soil in which the plants grow and the maintenance of soil fertility are two basic requirements for any crop, more so especially, for a perennial vegetative harvest crop such as tea and without fertile soil or depleted soil, plant life cannot be economically sustained. TRI, after intensive research, concluded that in the first year clonal tea fields, more than 52 tonnes of soil per hectare per annum is lost due to erosion and more than 40 tonnes of soil per hectare per annum was lost due to erosion in seedling tea fields when “Scraper” weeding was done.
This level of soil loss affects directly in reducing soil fertility, soil water, soil microorganisms, soil nutrients, rooting zone for plants, soil permeability, and creates a hard pan. The use of the “Scraper” led to such a scale of soil erosion that the government introduced a “Soil Conservation Act” in 1958. The Central Highlands of the country which boasted of a rich agricultural heritage suffered as a result of the reduction of soil fertility and many agricultural holdings were abandoned leading to unemployment and poverty while cultivated tea lands reduced by about 50% in the Central Province as a result of poor land productivity.
It is common knowledge that 300 to 1,000 years are taken to build up one inch of soil. Manual weeding is not a recommended form of total weed management the world over and a well managed clonal tea field using herbicides will lose only 330 kg of soil per hectare. Manual weeding will also disturb the soil and encourage even more profuse growth of weeds in the field after soil disturbance, as the number of weed seeds in the top 15 cm of topsoil is found to be between 52 to 108 million according to research by the TRI.
After the introduction of herbicides, plantation management as a routine practice relied on IWM for the control of weeds. IWM preventive methods, cultural methods, agronomic practices, manual weeding, chemical weeding, biological, ecological, and mechanical methods of weed management. Even in chemical weed management, until the recent past, the plantations had the benefit of a choice of a variety of appropriate TRI-recommended herbicides that act in different methods to manage weeds; for example, paraquat as a contact herbicide, simazine as a pre-emergent herbicide, karmax/diuron as a residual soil applied herbicide, MCPA for broad leaved weeds, fernoxone as a plant growth regulating hormonal herbicide, oxyfluorfen as a residual herbicide, glyphosate as a translocated herbicide, and glufosinate as a systemic herbicide, amongst others. The plantations had the benefit of using target-specific weedicides for the many varieties of weeds found on the estates.
The government banned the universally-used herbicide paraquat in 2004 in Sri Lanka, solely based on the illogical reasoning that farmers ingested paraquat to commit suicide. All other tea growing economies competing with Sri Lanka still use paraquat. The government also banned the universally used herbicide glyphosate in 2015 and restored its usage in 2019, again illogically on the unproven, unscientific, false premise and hypothesis that glyphosate was linked to chronic kidney disease of unknown etiology (CKDu).
This has been proved wrong beyond any reasonable doubt by findings of the World Health Organisation (WHO), several government-appointed committees, tree crop research institutes, universities, Department of Agriculture, scientists, agriculture researchers, medical scientists, and nephrologists, in addition to expert and experiential knowledge of agriculture professionals and practitioners. Currently, Sri Lanka has only one recommended herbicide i.e. glyphosate and this too is not available since June 2021.
However, presently plantations have been compelled to use only glyphosate as a systemic herbicide and this is not a practical method to control a wide range of weeds found in the plantations. The gramoxone ban in 2004 and the glyphosate ban in 2015 made an irrecoverable impact on the plantation industry. Since 2016, Sri Lankan tea crop has come down to below 300 million kg per year from the previous 340 million kg level, recording 279 million kg in 2020. This was clearly predicted through many communications and newspaper articles in 2015 itself.
About 10% of total tea extent was reduced from 222,000 hectares in 2013 to 200,000 hectares in 2020, as large extents of tea land was covered with weeds and was abandoned due to the lack of weedicides after the glyphosate ban. The direct loss from the crop reduction was about Rs. 15 billion per annum as a result.
TRI over a half a century of research has confirmed that without regular weeding, within six months a yield loss of 20% is expected and if manually weeded, yield loss will be 15% within four months. The COP of Ceylon tea which is the highest in the world will increase to an exorbitant amount with a yield loss and Ceylon tea will not be competitive in global markets. Once weeds invade and cover the surface of the plucking table, it will interfere with the plucking operations and quality of leaf harvested will be poorer, which will reduce our current sale average.
Loss of traditional quality expected from Ceylon tea will drive away traditional buyers to other competing teas and we will lose our hard won markets which have been with us for over 100 years. Weed foliage will get mixed with tea leaves and this will invariably contribute to further loss of quality and rejection of our tea, as taste, chemical content, and the appearance of tea will vary significantly along with appearance of made tea. Harvesters will find it extremely difficult to clean the plucking surface covered in weeds which will result in lower crops and lower output. Low crops will result in reduced earnings, number of days of work, leading to reduced income.
Reduced earnings will severely impact on the health, sustenance, and quality of life of those involved in the tea industry, leading to poverty and starvation which will eventually lead to massive social unrest. Without weedicides, there will be profuse growth of weeds in fields which will attract leeches, small insects, rodent animal life, and dangerous reptiles, which will attract larger predators as experienced in the last five years during glyphosate ban and this is a definite danger to workers in fields. Such danger from predators and reptiles will keep pluckers away from fields and reduce attendance at work which will reduce tea workers’ income and lead to abandonment of vast areas of tea lands as experienced, leading to the eventual loss of tea crop.
Pests and diseases in tea
Plantation agriculture is spread in 46 agro-ecological regions in three rainfall zones and in three elevations. Each geographic region exhibits its own distinctive ecological and environmental characteristics and pest fauna. Pests like insects, mites, and nematodes are organisms that cause economic damage to tea plants and cause diseases in leaves, stems, branches, and roots of tea bushes, resulting in conditions that interfere with normal physiological and crop generating functions.
There are many perennial, seasonal, potential, occasional, and secondary pests and diseases caused by biotic, pathological, and parasitic agents as fungus, bacteria, virus, viroides, phytoplasmas, parasitic seeds, and parasitic plants in tea cultivation. There are also physiological and non-parasitic diseases caused by sun scorch, cold, light, temperature, humidity, water logging, soil pH, and mineral deficiencies. In tea cultivation, all pests and disease treatments are specific, need-based, and targeted to protect the crop or the tea bushes and if there are no pests, no pesticides are used.
Tea is subject to many pests and diseases from nursery to maturity, whatever the age of growth stage the plant is in; for example, yellow mite, tea tortrix, nematodes, and fungal diseases in the nursery and in new clearings and mature fields. Tea is subject to pests; tea tortrix, nettle grub, red mite, scarlet mite, spider mite, purple mite, pink mite, lobster caterpillar, looper caterpillar, red slug, lygus bug, upcountry and low country live wood termite, and shot hole borer are some.
Apart from these, different parts of the bushes are subjected to specific diseases. Leaves are subject to blister blight, brown blight, grey blight, black blight, red rust, oil spot, and phloem necrosis disease. stems are subject to wood rot, collar, stem and branch canker, and hypoxylon. Roots are subject to red, black, white, brown, violet, and charcoal root disease.
Pest and disease attacks result in economic loss due to loss of crop and loss of quality, and incur additional costs for control. It reduces land productivity, user efficiency, and has a detrimental effect on soil, water, air, and environment, reducing the effectiveness of resources and inputs of land, labour, and human resources and minimises worker attitude and efficiency. Pests and diseases have a significant impact on the appearance, taste, and quality of made tea.
Sri Lanka has an annual average rainfall of 3,800 mm in the upcountry catchment areas and with a mean temperature of around 16.5⁰C and humidity over 84% in areas like Nuwara Eliya, tea bushes easily succumb to pests and diseases. TRI has confirmed that, if relative humidity is more than 80% and if the temperature is below 25⁰C, tea is strongly susceptible to blister blight fungal disease which reduces the crop by 30-40%. The susceptibility of tea plants to nematodes such as Pratylenchus loosi which invade roots within 48 hours after planting and dry weather pests like tea tortrix and fungal diseases like blister blight during high humidity seasons are significant challenges in plantations.
Currently, only very limited and few contact and systemic fungicides are permitted to be used for control of pests and diseases because of the maximum residue limit (MRL) requirement by buyers. However, well distributed annual monsoonal rainfall patterns, accompanied by high humidity, tropical high temperature, and humid climatic regime easily pre-dispose tea plants to many pests and diseases and provides a very conducive ecological platform for pests and disease attacks on tea.
Climate change impact
Vapour pressure deficit (VPD) determines productivity of tea as well. During dry months and low VPD with low relative humidity, which results in high transpiration and desiccation; pre-drought foliar spraying of potassium sulphate is required to protect the tea bushes and maintain crop physiology. High VPD periods with high relative humidity, encourage rapid spread of fungal diseases that require prophylactic spraying of copper fungicides.
(The writer is the Hayleys PLC Plantation Sector Managing Director, which comprises Kelani Valley Plantations, Talawakelle Tea Estates, and Horana Plantations. A former Chairman of the Planters’ Association of Ceylon, he has 36 years of experience in the plantation sector)
The views and opinions expressed in this column are those of the author, and do not necessarily reflect those of this publication.
| Probable financial loss: Unavailability of agrochemicals | |||
| Non-availability | Crop loss | Revenue loss (Rs. billion) | Revenue loss ($ million) |
| Weedicides | 20% | 52 | 257 |
| Fungicides | 25% | 65 | 322 |
| Pests and diseases | 5% | 13 | 64 |
| Climate effects | 10% | 26 | 128 |
| Total earnings from tea – Rs. 260 billion or $ 1.3 billion Impact of glyphosate ban – Rs. 54 billion direct loss and Rs. 16 billion indirect loss | |||