1369 AJCS 7(9):1369-1374 (2013) ISSN:1835-2707 Allelopathic potential of five Labiatae plant species on barnyard grass (Echinochloa crus- galli) A K M Mominul Islam 1,2,3, * and Hisashi Kato-Noguchi 1,2 1 Department of Applied Biological Science, Faculty of Agriculture, Kagawa University, Miki, Kagawa 761-0795, JAPAN 2 The United Graduate School of Agricultural Sciences, Ehime University, 3–5–7 Tarumi, Matsuyama, Ehime 790–8566, JAPAN 3 Department of Agronomy, Bangladesh Agricultural University, Mymensingh–2202, BANGLADESH *Corresponding author: bulbulbau@gmail.com Abstract Uninterrupted application of synthetic herbicides to control barnyard grass (Echinochloa crus-galli (L.) P. Beauv. ) makes them resistance against many herbicides with different mode of action. To overcome this problem, many researchers are interested in searching new novel natural compounds in medicinal plants to develop natural herbicides. Plants belongs to the Labiatae family attracted the attention of many researchers in pharmacological interest because of their toxic potential and medicinal properties. However, there is very limited information available for the allelopathy of this family. To explore the allelopathic potential of the aqueous methanol extract of five Labiatae plants: Leucas aspera L., Leonurus sibiricus L., Ocimum tenuiflorum L., Mentha sylvestris L. and Hyptis suaveolens L. were tested against barnyard grass at four different concentrations (3, 10, 30 and 100 mg DW equivalent extract/mL). The root growth was more sensitive to the plant extracts than the coleoptile growth and the inhibitory activities were concentration dependent. At the concentration of 100 mg DW equivalent extract/mL, L. aspera and H. suaveolens plant extracts strongly inhibited the seedling growth of barnyard grass. However, at the same concentration the lowest inhibition was observed in case of O. tenuiflorum plant extract. Among the plant extracts, the seedling growth of barnyard grass was most susceptible to L. aspera plant followed by H. suaveolens to confirm 50% coleoptile and root growth inhibition (defined as I 50 ), whereas that of barnyard grass was less susceptible to O. tenuiflorum. These results suggest that L. aspera and H. suaveolens possess strong allelopathic potential and therefore, could be used as the good candidates for isolation and identification of allelochemicals to develop environment friendly new natural herbicides to control barnyard grass. Keywords: Allelochemicals; Hyptis suaveolens; Leonurus sibiricus; Leucas aspera; medicinal plants; Mentha sylvestris; natural herbicides; paddy weed; Ocimum tenuiflorum; sustainable agriculture. Abbreviation: DW – dry weight, I 50 – concentration required for 50% inhibition, sig. – significance, df – degrees of freedom. Introduction Weeds, the primary constrain for rice production can reduce the rice yields from 44 to 96% in one crop season if the weeds are not control from the crop fields (Ampong-Nyarko and De Datta, 1991). Moreover, on an average 13-30% of crop reproduce is actually lost in the farmers’ fields even after adopting conventional weed control techniques due to the weeds that grow after weed control (Swarbrick and Mercado, 1987; Mamun, 1990), though it may vary from country to country. Among the major weeds of rice, barnyard grass (Echinochloa crus-galli (L.) P. Beauv.) is the most notorious one (Holm et al., 1977). The C 4 photosynthetic system of barnyard grass gives it greatest advantage under hot, arid and high light conditions (Patterson, 1985; Vidotto et al., 2007) as well as higher water and nitrogen use efficiency than C 3 plants like rice (Ampong-Nyarko and De Datta, 1991). Another special characteristic, ‘mimicry’ with rice seedlings helps it to escape manual weeding (Barrett, 1983; Gibson et al., 2002). These two special characteristics make barnyard grass more competitive against rice and can reduce rice yields up to 100% (Ampong-Nyarko and De Datta, 1991). To control this notorious weed from the rice fields on an average three million tonnes of herbicide has been used every year in almost all agricultural systems by the farmers (Stephenson, 2000). This over and uninterrupted use of synthetic herbicides in the rice field makes them resistance to some herbicides (Juliano et al., 2010; Beltran et al., 2012) and in the same time creates environmental hazards. To avoid these detrimental effects of synthetic herbicide, research on novel natural plant products have moved from the fringe to the mainstream for the development of ecologically acceptable, environment friendly, cost-effective and relatively safe natural herbicides. Many researchers around the world show their keen interest on medicinal plants for searching new novel compounds as it may provide the clues to new and safe herbicide chemistry (Duke, 1986; Nimbal et al., 1996; Bhowmik and Inderjit, 2003; Li et al., 2009). In addition, screening of medicinal plants for new natural compounds is easier than other plants (Fujii et al., 2003), possibly due to their existed certain metabolic compounds which was used for curing many diseases of human being. In order to compete with neighboring plant species for light, nutrient and moisture, a number of plants have been reported to release allelochemicals to the surrounding environment through volatilization from the leaves (Petrova, 1977; Oleszek, 1987), leaching from the above ground parts by precipitation (Overland, 1966), decomposition of leaf litter or sloughed root tissues (Guenzi et al., 1967; Hedge and Miller,