Vol. 10 No. 2
Weedy rice invasion in rice fields of India and management options
Author(s): C. T. Abraham and N. Jose
Abstract: Weedy rice are complex of Oryza morphotypes widely distributed in the commercial rice fields in more than 50 countries of Asia, Africa and Latin America, especially in areas where farmers have switched to direct seeding due to labour shortage and high cost. Wild and weedy forms of different species coming under the genus Oryza are also known as wild rice, red rice, padi angin, windy rice, air rice, varinellu, wild rice etc. Southeast Asia has been identified as the centre of origin of rice and many wild and weedy relatives are present in different rice growing areas of this region. Indian weedy rice is identified as Oryza sativa f. spontanea which belongs to the indica group. They are major threat to irrigated rice production systems in Southeast Asian countries because of their great adaptability to emerge at low temperature and from deep soil or water depths. Weedy rice is problematic, especially in Asian rice bowls. The spread of wild rice takes place in West Bengal, Andhra Pradesh, Assam, Bihar, Karnataka, Madhya Pradesh, Orissa, Tamil Nadu and Uttar Pradesh. Wild and weedy forms are problematic in Eastern India (Eastern U. P., Bihar, Orissa, Manipur, and West Bengal) and Southern India (Kerala). In North Western states like Punjab and Haryana, weedy rice is rarely seen. Heavy infestation of weedy rice in rice fields during the recent years is forcing farmers to abandon the crop due to huge reduction in crop yield (around 40-70 per cent) depending on the severity of infestation (5-15 or even more mature weedy plants per square meter). Genetic and morphological similarity of weedy rice to the cultivated species makes it difficult to control and chemical methods alone are unlikely to be effective.Origin and spread of weedy riceCultivated rice is included in the genus Oryza of the grass family (Poaceae). This genus includes two cultivated species (Asian rice - Oryza s ativa, and African rice - O. glaberrima) and more than 20 wild species with ten different genome types, i.e., AA, BB, CC, BBCC, CCDD, EE, FF, GG, JJHH, and JJKK. The wild relatives of rice with different genome types usually have significant reproductive isolation, making them unlikely to hybridize under natural conditions. The AA genome weedy and wild relatives are highly compatible sexually with cultivated rice. Their interspecific F1 hybrids could form complete chromosome pairing in meiosis and have relatively high pollen and seed fertility to produce viable offspring. It is widely hypothesized that weedy rice has a variety of origin.Weedy plants are adapted to a wide range of environmental conditions. The spread is generally favoured by the planting of commercial rice seeds that contain grains of the weed. The spread of weedy rice is likely to be accomplished by several means, including water, cattle, farm machinery, and as contaminants of new varieties. Disk cultivation also helped to move the seeds downward to the 3 - 15 cm soil layer resulting in serious infestations and difficulties in control....
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