## 1 Introduction  
Weed interference is one of the most common abiotic threats to global crop production, as weeds can contaminate crops by spreading their seeds, thereby perpetuating the problem for subsequent growing seasons [1]. Weeds compete with crops, resulting in an annual reduction of approximately 10% in worldwide agricultural production [2]. Uncontrolled weeds can even lead to a complete loss of crop yields. Globally, there are approximately 30,000 species of weeds with detrimental effects on crop yield and quality [3; 4]. In the United States, it is estimated that weeds cost USD 33 billion in lost crop production annually, and farmers spend over USD 6.2 billion each year to control weeds [5; 6]. In Australia, the overall cost of weeds was about AUD 3.3 billion per year, with yield losses of 2.7 million tons of grains at a national level [6]. Over the last six decades since the discovery of 2,4-D, 2,4,5-T, and other synthetic herbicides, the use of herbicides has successfully decreased labor costs spent on weed control and increased global crop production [7]. In Vietnam, the use of synthetic herbicides for weed management in rice only significantly increased from the early 1990 and strikingly increased to 42,000 tons/year in 2013, equivalent to USD 300 million [8], and tremendously intensified in recent years [9]. In fact, modern agricultural systems have effectively employed synthetic herbicides to control weeds and minimized the need for weeding labor in agriculture. However, indiscriminate use or overuse of synthetic herbicides results in unwelcome side effects, such as environmental contamination, unsafe agricultural products, and human health concerns, as well as the development of herbicidal resistance in weeds [10; 11]. To the best of our knowledge, there have been approximately 380 herbicide-resistant weed biotypes reported in 98 crops in over 72 countries globally [12; 13]. This fact requires either increasing doses of applied synthetic herbicides in practice or developing new synthetic herbicides, which leads to further severe problems and threatens the sustainability of agricultural systems. Alleplopathy is simply described as natural chemical interaction(s) between plants that can inhibit or enhance the growth of surrounding plant species [14; 15]. In nature, green plants suppress other plants in the vicinity by releasing phytotoxic compounds (allelo-chemicals) into the environment from root exudates, aerial parts, and residues, etc. [16]. Therefore, the allelopathic traits of plants may be an effective tool for biological weed management in agricultural production [10]. Numerous plants in nature exert significant weed-suppressing ability; some of them have allelopathic potential to incorporate into paddy fields to manage weeds and significantly enhance rice yield. Examples include passion flower (_Passiffon adults_), alfalfa (_Medicago sativa_ L.), fragrant thoroughwort (_Eutaparium cannabium_), buckwheat (_Fagoyrum esculentum_), chinaberry (_Melia azedarach_), kava (_Pipret methylicum_), galacia (_Galacia perthula_), neem (_Azadirachita indica_), billy goat weed (_Ageratum conyzoides_), and white lead-tree (_Leucaea leuccephala_) [17]. Some common allelochemicals found in plants are generally secondary metabolites, including phenolics, alkaloids, terpenoids, coumarins, flavonoids, steroids, etc. [18; 19]. Allelopathic additives acquired from some plant biomass and their constituents may boost crop output while concurrently minimizing weed growth via mulch, cover crops, and green manures [11; 17]. In recent decades, the allelopathic traits of plants have gained much attention from many researchers, since plants were identified to self-regulate density and distribution in nature with defense mechanisms by means of allelopathic interactions. Some allelochemicals are non-halgenated and water-soluble, and express short half-lives relative to synthetic herbicides; they are, therefore, considered safe for agricultural products and the environment.