|Thursday, 24 June 2004|
'Be cautious in using glyphosate for weed control in tea'
by Dr. Kapila Prematilake
(Senior Research Officer), Low Country Station, Tea Research Institute, Ratnapura and B.P. Ekanayake (Officer-in-Charge), Mid Country Station, Tea Research Institute, Hantana, Kandy.
Glyphosate herbicide, when sprayed to the weed foliage gets translocated to all plant tissues and effectively kills the entire plant including rhizomes of perennial weeds. Ability of killing wide range of weeds makes Glyphosate one of the most versatile herbicides.
Thus, most of the tea estates and smallholders heavily depend on glyphosate for weed control. Glyphosate (36% a.i.) is marketed in Sri Lanka under different trade names.
However, indiscriminate use of glyphosate on tea lands has adversely affected health and productivity of the tea bush in the recent years. Furthermore, increasing levels of glyphosate residues have been reported in our tea exported to other countries.
Glphosate was first introduced to tea cultivation in 1980s for the control of problem weeds such as Couch and Illuk grasses. Higher dosages of glyphosate (36%) at 11 and 5.5 litres in 600 of water per hectare were recommended to control Couch and Illuk grasses, respectively.
Later in 1995, the lower rates of glyphosate i.e. 1.4-2.8 1/ha (0.25.5%) were recommended to control other weeds as well. From the inception TRI has recommended series of precautionary measures to be adopted when using glyphosate for weed control in tea.
However, symptoms of discolouration, browning of leaves, wilting, twisting or curling of leaves have been observed as phtotoxic symptoms of glyphosate. Higher dosages of glyphosate could result in leaf fall, and sometimes death of the plant.
In mature tea, damages are generally observed on peripheral shoots, which grow laterally at a lower height almost at the ground level. Symptoms of deformed leaves, development of multiple buds and formation of rosettes may appear about 4-5 weeks after application.
Thus, the ultimate result of regular spraying of glyphosate is debilitation of the bush and decline in yield. Recent field investigations have also confirmed the debilitation and yield decline due to regular use of glyphosate (i.e 3.4 rounds per year) over a period of 4-year cycle (Table 1).
Furthermore, regular use of glyphosate could result in glyphosate residues in made tea. According to the World Health Organization (WHO) the Maximum Residual Level (MRL) of glyphosate allowed in Black tea is 0.5 parts per million (ppm), i.e. 0.5 mg in one kg of black tea.
When glyphosate is used in excess of 0.5% (>2.7 l/ha) on tea lands glyphosate residues have been detected until 14 days after application, whereas, glyphosate residues have not been detected in tea seven days after application when the rates are below 0.5%.
Moreover, it is also well known that the use of a single herbicide can lead to development of resistance in weeds.
In spite of the cautionary notes issued by the TRI, some estates and tea small holders are using glyphosate at rates higher than 0.5% i.e. about 3-4 l/ha/round 4-5 times a year.
As a result of such deviation from the TRI recommendations for the use of glyphosate, stipulated in Circulars and Guidelines, a serious situation is being emerged in the tea industry. This may be attributed to lack of knowledge and awareness on safe use of herbicides, particularly of glyphosate.
The majority of smallholders used to get improper advices from the salesmen of pesticide outlets.
Even in the estate sector field staff and workers have not been properly educated on safe and effective methods of herbicide use.
Therefore, it is necessary that the field staff and workers, who are directly engaged in herbicide application be educated in all aspects of chemical weed control, which covers phytotoxicity, mode of action, residue levels, persistency, dosages and rates of application of all recommended herbicides used for weed control in tea.
Further, they should be educated on proper use of spray equipments, correct nozzles, spray guards and correct pressure of application.
In order to minimize adverse effects and build up of residues in made tea it is recommended that number applications of glyphosate should be limited to two rounds per year for mature tea and it should not be used on young tea (up to first pruning) and first year after pruning.
Though, Glyphosate is a total weed killer, some weeds such as Commelina, Hedyotis, Cyperus spp, Wedelia and Morning Glory are tolerant to Glyphosate.
Therefore, it is not advisable to use glyphosate on these weeds and they should be controlled by cocktail mixtures of herbicides or by other methods. Today, the estates are compelled to practise chemical weeding due to the acute shortage of labour. The use of glyphosate in the estate sector is alarmingly high.
Some estates are undergoing economic crisis due to lower NSA and lower profit margin with a low capital investment. As a result those estates are compelled to use a single herbicide like glyphosate to reduce cost of weeding.
However, the adverse effects of improper use of this weedicide on tea fields overweigh the short term benefits such as reduction of cost of weeding.
Therefore, it is extremely necessary to adhere to the recommendations of the TRI given in the form of advisory circulars, guidelines and precautionary notes.
In this context, a lower dose of glyphosate could be used with a wetting agent or a surfactant available in the market or with Ammonium Sulphate, Urea of Kaolin for weed control in tea.
Weeds that are not killed by glyphosate should be selectively controlled by other means of managing weeds such as manual or cultural method or with a cocktail mixture of weedicides as recommended by the TRI.
Otherwise these weeds could become dominant and eventually a threat in tea cultivation. Finally, for an effective and sustainable weed management in tea, resorting to a minimum number of rounds of the same herbicide particularly glyphosate within a year in combination with other herbicides and practice of other manual, cultural and biological methods in rotation should meticulously be followed.
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