Toposequential effects on water balance and productivity in rainfed lowland rice ecosystem in Southern Laos

In the Mekong region, most paddies in rainfed lowland rice (Oryza sativa L.) lie in a sequence on gentle sloping land, and grain yield (GY) often depends on the toposequence position. There is, however, lack of information on toposequential effects on field water supply in rainfed lowland rice and how that influences GY. A total of eight field experiments were carried out on sandy, coarse-textured soils in Southern Laos (Champassak Province and Savannakhet Province) over three wet seasons (2000-2002). Components of the water balance, including downward and lateral water movement (D and L, respectively), were quantified at three different positions along toposequences (top, middle and bottom). GY, days-to-flower (DTF) and rainfall were measured, and the water productivity (WP) was determined. In most experiments, standing water disappeared first in the top position and gradually in lower positions. This was associated with the observation that when there was standing water in the field, the higher position had larger D in both the provinces and also larger L in Champassak Province. However, in one experiment, water loss appeared later in the higher position, as the result of lower L, apparently due to some water inputs other than rainfall occurring at this position. Despite larger D plus L at the top position, seasonal sum of D and L were not much affected by the toposequence position, as the daily rate of D plus L became minimal when the standing water was lost earlier in the top position. Lower GY was associated with earlier disappearance of standing water from the field. Relatively low GY was expected in the top toposequence position. This was clearly shown in the toposequence of Phonthong, Champassak Province, as the timing of standing water disappearance relative to flowering was earlier in the top position. Variation in GY across the toposequence positions was coupled with the WP variation, and both GY and WP tended to decline with increased DTF. Therefore, variation in productivity of rainfed lowland rice across toposequence positions depends mainly on the field water status around flowering time. (c) 2005 Elsevier B.V. All rights reserved

Genotypic performance under fertilised and non-fertilised conditions in rainfed lowland rice

The rate of fertiliser application to rainfed lowland rice is commonly low while selection in breeding programs is conducted under well-fertilised conditions. The appropriateness of applying fertiliser in a rainfed lowland rice breeding program at a rate above the average farmers' rate was examined in Laos. Experiments were conducted at three locations in 2 years using 72-60 genotypes most of which were typical materials used in the Lao breeding program. The fertiliser rates were nil and 60N : 13P : 16K kg ha(-1). The effect of fertiliser application was significant in most experiments, but the response to fertiliser depended on location, There was often significant genotype-by-environment interaction for grain yield in individual experiments. There was also significant correlation for grain yield between fertilised and non-fertilised conditions, with coefficients of determination (R-2) of 0.45 and 0.54 in the 2 years, for combined results of three locations. Combined analyses of grain yield from three locations indicate that for either year the variance component for the genotype-by-fertiliser interaction was smaller than for genotype. The coefficient of variation for grain yield was often smaller under fertilised compared with non-fertilised conditions. These findings as well as the rapidly increasing use of fertiliser justify the level of fertiliser currently applied in the Lao breeding program, The significant genotype-by-location interaction requires that for an effective breeding program, locations be selected to represent farmers' fields. (C) 2000 Elsevier Science B.V. All rights reserved

Genotype differences in nutrient uptake and utilisation for grain yield production of rainfed lowland rice under fertilised and non-fertilised conditions

A large number of rainfed lowland rice genotypes were grown under two fertiliser conditions to identify whether a genotype's ability to extract more nutrients or to use absorbed nutrients more efficiently to produce grain yield was more important in determining genotypic variation in grain yield. From the yield responses of lines to fertiliser application, 16 contrasting lines were selected, and dry matter and nutrient (N, P and K) contents were determined for grain and straw separately for crops grown under non-fertilised and fertilised (60-13-16 N-P-K kg ha(-1)) conditions at three locations in Laos. There were significant effects of both genotype and genotype-by-fertiliser interaction for grain yield, which were closely associated with total N and P content at maturity. There was, however, also significant genotypic variation in nutrient-use efficiency (grain yield per unit nutrient absorbed), and this also contributed to the genotypic variation for grain yield. There was a large effect of genotype and a smaller effect of genotype-by-location interaction for nitrogen-use efficiency for grain yield. Nitrogen-use efficiency was negatively correlated with grain N concentration at two of the three locations. Both N- and P-use efficiency were consistent across fertiliser levels, and hence are likely to be used as selection criteria. The work also indicates that genotypes with high harvest index (HI) are likely to perform well in different fertility conditions. (C) 2000 Elsevier Science B.V. All rights reserved