A study of the abundance and diversity of rice weeds of Thailand, with special emphasis on their variation with different methods of rice cultivation

This is the first comprehensive study of weeds of rice throughout Thailand. Information on the current and past perceptions of problematic weeds of rice was obtained from the results of questionnaires returned from 368 farmers. Data was gathered directly in the field from 56 rice farms throughout north (N). north-east (NE) and central (C) Thailand A total of 145 species of weeds were recorded, of which 24 represented new weed records for Thailand. In farmers' perceptions, major broad-leaved weeds appear to be increasing in severity. while grass weeds are under control. Weed frequency and occurrence varied markedly between regions and farms, with most species occurring at low frequency and low mean abundance. Variation in rice cultivation techniques was associated with weed diversity and geographical location. Weed diversity between farms was compared using three different indices of biodiversity assessment and also ordination analysis. For all different methods of analysis, NE farms were shown to have the greatest diversity of weeds. Direct dry seeding is practiced in NE farms and not in other regions and analysis of the distribution of the species most strongly associated with NE farms shows that they are generally worldwide weeds and are not confined to the NE of Thailand. It was therefore concluded that cultivation technique is more likely to be the underlying cause of the higher diversity of weed species found in NE Thailand, rather than geographical location. Species had higher densities at the edge, rather than in the centre, of rice fields, in agreement with several other studies and indicating that the edge may act as the source for the establishment and spread of weed species. Weeds on rice farms are currently in decline because of the introduction of herbicides in the last decade and recent improvements in weed management practice by water management.EThOS - Electronic Theses Online ServiceGBUnited Kingdo