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

ResultsRelations among pests and cropping practicesLate-duration rice fields tended to have high levels of hispa and low levels ofPentatomids, whereas early duration fields were the opposite. Fields without standingwater at the four key crop stages had higher than average levels of weeds, cutworm,hispa, and Pentatomids. Brown spot and narrow brown spot were the only diseasesobserved frequently enough in the RLR production systems to make inferences abouttheir relation to cropping practices. Other diseases, such as sheath blight, are associatedwith intensive and high-input production systems (Cu et al 1996). Fields receivingmore than 150 kg fertilizer ha –1 had lower than average levels of brown spot andnarrow brown spot (Table 4). The fertilizer associations would depend, of course, onthe type of fertilizer used, not only the rates. Later experiments addressed the associationbetween type of fertilizer and pest levels (CIAP 1998, 1999).Correspondence analysisThe first and second axes of the graphical display of the correspondence analysis(Fig. 8) accounted for 79.7% and 11.6% of total inertia, respectively. Therefore, thefirst two axes provided a good overall view of the numerical output of the correspondenceanalysis (Table 8), as 91.3% of the total inertia was represented. Axis 1 representsthe gradient of decreasing levels of rice yields. It involves the contributions ofY1, Y4, CP1, CP2, PE2, and PE4 (Tables 5, 6, and 7). Axis 2 involves the contributionsof Y2, Y3, CP3, CP4, PE1, and PE3. Four domains were generated from Figure8—domain A (Y1, CP1, PE4), domain B (Y2, CP4, PE3), domain C (Y3, CP3, PE1),and domain D (Y4, CP2, PE2)—by correspondence analysis using STAT-ITCF (Savaryet al 1996). Table 9 summarizes the characteristics of each domain.Correspondence analysis of Table 7 indicates that CP1 (Table 5) tends to havelower yields, whereas CP2 tends to have medium to higher yields. Fields with a pestprofile matching PE2 (Table 6) are more frequently associated with high yields, butfields matching PE4 more frequently have low yields (Table 8, Fig. 8).DiscussionThe effects of cropping practices on the levels of pest infestation (Table 4) are basedon chi-square tests of independence. While this type of analysis indicates whether aparticular practice (or condition) tends to be associated with high or low levels ofparticular pests, it does not measure the relative strength of that association (or possiblyeffect). The interactions between combinations of cropping practices, multiplepests, and yields (Fig. 8, Table 9) are based on correspondence analysis. In this case,the relative strength of these relationships is indicated by the distance between clustersin Figure 8.Characterizing biotic constraints to production . . . 257