WATER, TILLAGE AND WEED INTERACTIONS IN LOWLAND TROPICAL RICE - A REVIEW

Interactions among water, tillage and weed management practices are co mplex, and are further complicated by soil and climatic variabilities and heterogeneities. Studies from the tropical regions on possible eff ects of tillage and water control on weed emergence and growth in the presence and absence of herbicides have yielded conflicting results du e 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 pendin g. Most studies, however, indicate that shallow pending for the first few weeks after planting can effectively suppress weeds. Integration o f herbicide in a weed control program makes proper water management mo re critical. However, good water control is still lacking in most part s 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 de eply buried. Also, repeated tillage will uproot and bury the already g erminated weeds. In a diverse weed community situation weed control is effectively achieved if tillage is combined with herbicide applicatio n, because tillage is known to enhance herbicide effectiveness, There are reports of identical rice yields being obtained under saturated an d flooded water regimes, and zero and conventionally tilled soils. How ever, effective weed control is required for obtaining such results. I nvestigations of interactions of tillage intensities and water regimes with weed populations have not been adequately addressed, as most stu dies have been confined to quantifying competition factors in terms of critical weed population thresholds. Some studies have attempted to e xplain the nature of competition and its mechanisms. Of the many weed species reported to grow in rice fields, only few actually compete wit h rice. Generally there are only three to four weed species which are economically important for rice farmers in the tropics, It is these we eds which should be essentially controlled, although total weed contro l is preferred by most rice farmers. Continuous use of the same contro l measure in some areas may contribute to a buildup of some tolerant w eed species which are difficult to control. Therefore, for effective a nd sustained weed control, integrated weed management with proper till age and water control is needed.