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

Characterizing biotic constraintsto production of Cambodianrainfed lowland rice: limitationsto statistical techniquesG.C. Jahn, Pheng Sophea, Pol Chanthy, and Khiev BunnarithBecause rainfed lowland rice (RLR) accounts for 86% of the rice-growing areaof Cambodia, it is important to investigate the biotic constraints to rice inthis ecosystem. Over 3 years, a total of 73 RLR fields in Cambodia werestudied to determine the effect of pests on destabilizing Cambodian RLRyields, which pests and pest combinations affect RLR yields, and how croppingpractices affect pest levels. Pests included insects, weeds, and diseasesof rice. Pesticides were not used in any of the fields in this study.Information on crop characteristics and biotic constraints was gathered atfour crop development stages: tillering, booting, milk, and maturity. The incidenceof damage caused by insects and diseases was recorded from 10hills chosen randomly from each field. Weed infestation was recorded as thepercentage weed cover in three 1-m 2 areas. The rice yield of each field wasestimated from an average of three randomly selected 10-m 2 areas. Correspondenceanalysis was used to characterize the patterns of cropping practices,pest infestations, environmental conditions, and yields. The results ofthis analysis generated testable hypotheses about the factors contributingto pest problems. Fields of early duration rice tended to have low levels ofhispa and high levels of Pentatomids, while late-duration fields had high hispaand low Pentatomid levels. Fields without standing water had higher thanaverage levels of weeds, cutworm, hispa, and Pentatomids. Brown spot andnarrow brown spot were the only diseases observed frequently enough tomake inferences about their relation to cropping practices.As a component of systems research in rice plant protection, this studyassisted in predicting the effects of cropping practices on pest infestations.These techniques have several limitations, however: the danger of drawingfalse conclusions, difficulties in interpreting results, insufficient attention tothe soil type and relative rates of fertilizer, the inability to capture time adequatelyas a variable, the lack of information on the relative contribution ofpests to variation in yield data, the incomplete coverage of pests, and thefallibility of observers. Each limitation is discussed in detail. The results ofthis study contributed to an assessment of relative pest importance andthereby helped prioritize research to develop integrated pest managementrecommendations.Characterizing biotic constraints to production . . . 247