Contents & Foreword, Characterizing And ... - IRRI books

The relatively high densities of weeds and the diversity of weed species presentmay simply reflect the fact that farmers in the region perceive little economic benefitin weed removal during the later stage of crop development, particularly as furthermanual weeding may damage the crop. Moody (1993), however, has pointed out thatadditional weeding close to rice panicle initiation will often increase final yield. Clearly,without more detailed experimentation, it is only possible to hypothesize as to likelyyield reductions resulting from these observed levels of weed infestation. The followingcomments are in consequence purely speculative. Field observation that mostweeds were below canopy height and the anticipated limited growth periodpostweeding suggests that weed biomass per unit area may be low and that interferencewith photosynthetically active radiation capture by rice will be limited. Weedspecies capable of strong plasticity in growth and late development, however, such asAmmannia baccifera, may pose a competitive threat to yield during grain filling.Anecdotally, this species is reported to increase in height and biomass as the cropmatures and as fields drain. Competition for nutrients during late crop developmentmay also occur from high densities of weeds lower in the canopy. Soils in this regionare characteristically phosphate- and potassium-limited (Clough et al, this volume)and all sites surveyed in this study were nutrient-poor. While underlying heterogeneityin soil pH may restrict the distribution of weed species, Figure 7 suggests thatthere may be differential species preferences for phosphate. This, in turn, raises thehypothesis that competition for this nutrient may occur particularly in low toposequencepositions in which weeds of irrigated rice persist. Monochoria vaginalis, Marseliacrenata, and Scirpus juncoides are all species that have been reported to competestrongly with rice for nutrients (Soerjani et al 1987) as has Eclipta alba in upland rice(Lee and Moody 1989). The extent to which Lindernia spp. may compete for nutrientswith rice remains open to question (not least because of the lack of formal identity).However, the distribution of this group of species was correlated with acidic,nutrient-poor soils in which rice yields themselves are likely to be low. The extent towhich overall productivity governs the nature and intensity of competitive interactionsfor nutrients deserves further study.Implications for weed managementThe wide diversity of weed species present within farm localities strongly indicatesthe potential for transient temporal and longer-term shifts in relative abundance ofweed species in relation to changes in agronomy and water and weed management.Studies of soil fertility in rainfed lowlands have clearly pointed to the importance offertilizer use in increasing rice yield. Improvement in crop nutrition and deploymentof new varieties may result in suppression of many weed species simply throughenhanced crop vigor by improved agronomic practices. Many authors (e.g., Cooperet al 1999), however, have pointed to the difficulty of breeding widely adapted ricecultivars for rainfed environments tolerant of abiotic stresses throughout the life ofthe crop.The presence of a wide spectrum of weeds encompassing species common tointensive irrigated production systems as well as rainfed and upland environmentsWeed communities of gogorancah rice and reflections on management 283