June 2002
In Search of a Natural Alternative to Herbicides in the Highlands
By
S.J.H. Rizvi, Dariush Bazzazi, Mozaffar Roustii and Habib Ketata
CWANA Highlands and Mountains > In Search of a Natural Alternative to Herbicides in the Highlands
The use of agrochemicals in fragile highland environments can have far-reaching, harmful effects. Herbicides, including those of the bipyridinum and organophosphorous groups, are particularly harmful to humans and domestic animals. They also pollute the environment, and become ineffective when weeds develop herbicide resistance. Scientists in the highlands of Iran are investigating ways to make use of the natural growth inhibiting ability of some plants to reduce weed competition, boost yields, and reduce the need for herbicides.
The wheat cultivar on the left was able to inhibit weed growth, while the other cultivars could not.
The observation that some plants have the ability to depress the growth of neighboring plants led to the discovery of allelochemicals —naturally occurring growth-inhibiting chemicals released into the environment by living plants, through volatilization or exudation, or by decomposing plant residues. These chemicals only temporarily suppress plant growth,

and, therefore, regulate species diversity without causing species extinction. Unlike synthetic herbicides, they rapidly degrade into non-toxic compounds and exert less toxic or pollutive effects on non-target species and the environment.
     In the past few decades, the allelopathic suppression of weeds has received considerable attention as a possible alternative to weed control based on synthetic chemicals. The allelopathic properties of crop plants might be exploited by: growing specific crop varieties, mixing certain crop residues with the soil, or by using isolated allelochemicals as natural herbicides.
     Apart from allelopathic properties, certain plants are better able to compete for available growth resources, such as moisture, nutrients, or light, than their neighbors.
     Researchers in lowlands have found that certain rice cultivars (e.g., ‘Taichung Native 1,’ ‘Johna 349,’ ‘Masrai,’) are able to suppress weeds by 60% to 90%. Recent research by ICARDA and Iranian scientists has shown that specific wheat cultivars can effectively control weeds under field conditions in highlands of western Iran. The level of weed control is about 50% under rainfed conditions and 40% under irrigation. Among the identified weed-suppressing wheat genotypes are the cultivars ‘Azadi,’ ‘Karaj 1,’ ‘Alborz,’ and ‘Sabalan.’
     Although this weed-suppression ability might be due to both the allelopathic and competitive properties of those cultivars, it remains, nonetheless, a genotypic characteristic that might be used for effective weed control in wheat fields.
     Other results by ICARDA and Iranian scientists show that residues of certain crops, such as sunflower and barley, when mixed adequately with soil can control weeds in chickpea crops to the same level achieved by proven chemical herbicides.
     Several allelochemicals (e.g., ajoene, caffeine, citral, citronellol, geraniol, and lawsone) have been found to be effective in controlling plant pathogenic fungi, such as Aspergillus flavus, Fusarium oxysporum, Phytophtora cajani, and others. In our experiments, we tested a gum collected from the plant Asafoetida and found it to control the chickpea disease Ascochyta blight by 70% compared to a check.
     There is indeed a real potential for the use of certain crop species or varieties to control weeds or diseases without jeopardizing the ecosystems or the natural resources in the fragile environments of the highlands of Central and West Asia and North Africa. Such an alternative approach would be environment-friendly and cost effective.

Dr S.J.H. Rizvi is Plant Physiologist, ICARDA, Iran; Mr Dariush Bazzazi is Weed Scientist, Dryland Agricultural Research Institute (DARI), Iran; Mr Mozaffar Roustii is Head, Cereal Department, DARI, Iran; Dr Habib Ketata is Cereal Breeder and Coordinator of the ICARDA/Iran Project, Iran.