Sunflower allelopathy for weed control in agriculture systems

Recent developments in weed science and allied aspects have involved several interdisciplinary approaches. In this context, indiscriminate use of herbicides for weed control has become a questionable subject, which besides controlling the weeds, the chemical herbicides are harmful in many ways to soil, crops, other plants and the environment as a whole. Taking into consideration ecologically sound weed management, in modern days the reliance on chemical herbicides has decreased and a shift towards naturally occurring biological herbicides has received great attention throughout the world. Sunflower is an annual dicotyledonous plant, herbaceous, erect, and a native of North America. It is thermo and photo-insensitive, hence it can be grown year round in sub-tropical and tropical countries. Only two spp. Helianthus annuus L. and Helianthus tuberosum are cultivated for food, the remaining spp., are ornamentals weeds and wild plants. However, H. annuus is allelopathic and inhibits the growth and development of other plants thus reducing their productivity. Sunflower is a major oil-yielding crop in India and its cultivation in northwest India started 25 to 30 years ago in areas located in the plains. In this region, rice-wheat rotation became very popular owing to its high yields; however, these crops are highly infested by weeds, thus farmers use herbicides for their control. Hence, this rotation consumes a maximum quantity of herbicides in this region, which has resulted in several problems viz., environmental pollution, human health hazards, and development of herbicide resistance in weeds. Thus, serious ecological questions about the reliance on herbicides for weed control in this rotation have been raised. One of the alternatives to overcome these problems is with the use of allelopathic strategies, including the use of weed-smothering crops for weed management and for the sustainability of agriculture. The field, pot culture, and laboratory studies have shown that inclusion of sunflower crops in rotation and intercropping considerably reduced the weed population in the current and succeeding crops. Rhizosphere soil of sunflower drastically smothered the weed germination, population, and biomass. The residual suppression effect of sunflower also persisted in the next crop up to 75 days. Thus, it is conceptualized that the inclusion of such oilseed crops before the rice crop in the rice-wheat rotation may provide satisfactory weed control in the succeeding rice crops and may minimize the use of herbicides. Likewise, the replacement of sorghum by summer sunflower oilseed crops may also help in the control of summer as well as winter weeds. More studies in this direction may provide avenues for satisfactory weed management in agro-ecosystems and may help to minimize the use of herbicides and thereby pave the way to develop sustainable agricultural practices for biodiversity conservation and enhancing biological integrity.