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Weeds have been a persistent menace for farmers since the advent of
agriculture. Among several methods adopted for controlling weeds, chemical control is
the widely used and most effective. However, the extensive use of herbicides has led to
the generation of a wide range of problems including development of herbicide resistant
weeds. As a result, extensive research is being done to exploit non chemical methods
of weed management. The mechanism of allelopathy has been suggested as a potential
biorational method towards this goal. Hence the present study entitled “Allelopathy for
weed management in field crops” was conducted during February-October 2021 in the
Department of Agronomy, College of Agriculture, Vellanikkara, Kerala Agricultural
University, Thrissur.
The study consisted of two parts viz., screening Andrographis paniculata,
Plectranthus ambonicus and Tagetes minuta for their allelopathic potential, and
evaluating alleloapathic effect of these plant extracts on weeds and the test crops rice,
cowpea and green gram. Cold water, hot water and methanol extracts of these plants
were prepared at six concentrations viz., 5 %, 10 %, 15 %, 20 %, 25 % and 30 % in a
completely randomized design (CRD) in factorial arrangement with three replications.
In the first experiment plants were screened for their allelopathic potential
against upland weeds in 165 plastic trays (25 cm x 20 cm x 5 cm) that were filled to
three-quarters with uniform quantity of soil (1.5 kg) collected from an open area. Best
10 treatment combinations from this experiment and a control with distilled water were
carried over to the second experiment. The second experiment consisted of two parts;
in petri plates to test the phytotoxic activity on test crops (cowpea, green gram and rice)
and in pot culture study with test crops and weeds. Both studies were carried out in
completely randomized design (CRD) in factorial arrangement with two factors and
three replications. Factor A consisted of time of application (on the day of sowing and
6th day after sowing). Factor B consisted of the best 10 allelopathic treatments from
experiment 1.
Experiment on screening allelopathic plants for their potential to control upland
weeds revealed the significant influence of the plants, the method of extraction and the
concentration of extract on weed growth parameters such as weed density and weed dry
weight. Broad leaved weeds were more sensitive to allelopathic extracts than grass
weeds. Reduction in weed germination count and dry weight after one month of
application was noticed with methanol extract of Tagetes minuta at 30 per cent
concentration and was 6.67 nos./m2 and 21. 33 g/m2
respectively as compared to control
(68.33 nos./m2
and 54. 25 g/m2
). Pre mergence application of Andrographis paniculata
methanol extract at 30 per cent concentration was the next best treatment. Allelopathic
effect of plants was significant only for a short period of time i.e. up to one week after
application, indicating absence of residual action.
Germination indices and seedling growth parameters of test crops were
adversely affected by the application of allelopathic extracts. Among test crops, cowpea
and green gram were more sensitive to allelopathic extracts than rice. A notable delay
in germination of test crops, in shoot and root length, and in fresh and dry weights were
observed by the application of allelopathic treatments. Phytotoxicity symptoms were
observed on test crops both under laboratory condition and in pot culture. Root decay
and reduction in number of rootlets were observed by the pre emergence application of
30 and 25 per cent methanol extract of Tagetes minuta. When Tagetes minuta methanol
extract was applied at 30 or 25 per cent as pre mergence treatment, germination of
cowpea and green gram started on the 4th and 5th day after sowing as compared to the
2nd day in control. Root length reduction was more pronounced than shoot length
reduction. The reduction was 43.29 and 41.46 per cent in cowpea, and 41.56 and 37.05
per cent in green gram, due to pre emergence application of 30 and 25 per cent methanol
extract of Tagetes minuta in petri plates. In pot culture study, root reduction of 70.85
and 71.01 per cent in cowpea and green gram by the pre emergence application of 30
per cent methanol extract of Tagetes minuta as was observed. A notable delay in weed
germination and weed dry weight reduction were observed by 30 and 25 per cent
Tagetes minuta methanol extract applied as pre emergence treatment and their residual
inhibitory effect in the soil persisted up to one week.
Based on the results of the study, pre emergence application (on the day of
sowing) of methanol and cold water extracts of Tagetes minuta and Andrographis
paniculata at 30 per cent concentration can be recommended for control of broad leaved
weeds in uplands.