Water, tillage and weed interactions in lowland tropical rice: A review

Interactions among water, tillage and weed management practices are complex, and are further complicated by soil and climatic variabilities and heterogeneities. Studies from the tropical regions on possible effects of tillage and water control on weed emergence and growth in the presence and absence of herbicides have yielded conflicting results due to site specificity. Surface pending of water in rice (Oryza sativa L.) reduces weed emergence and growth with variable degrees of success depending upon water depth, nature of weed species and time of pending. Most studies, however, indicate that shallow pending for the first few weeks after planting can effectively suppress weeds. Integration of herbicide in a weed control program makes proper water management more critical. However, good water control is still lacking in most parts of tropical Asia. Investigations of various tillage intensities have revealed that invariably zero tilled soils have more weed population compared with those conventionally tilled in the absence of herbicides, Tillage may bury some weed seeds and expose others that were once deeply buried. Also, repeated tillage will uproot and bury the already germinated weeds. In a diverse weed community situation weed control is effectively achieved if tillage is combined with herbicide application, because tillage is known to enhance herbicide effectiveness, There are reports of identical rice yields being obtained under saturated and flooded water regimes, and zero and conventionally tilled soils. However, effective weed control is required for obtaining such results. Investigations of interactions of tillage intensities and water regimes with weed populations have not been adequately addressed, as most studies have been confined to quantifying competition factors in terms of critical weed population thresholds. Some studies have attempted to explain the nature of competition and its mechanisms. Of the many weed species reported to grow in rice fields, only few actually compete with rice. Generally there are only three to four weed species which are economically important for rice farmers in the tropics, It is these weeds which should be essentially controlled, although total weed control is preferred by most rice farmers. Continuous use of the same control measure in some areas may contribute to a buildup of some tolerant weed species which are difficult to control. Therefore, for effective and sustained weed control, integrated weed management with proper tillage and water control is needed.