Food security in Lao PDR, a small nation in southeast Asia categorized as low-income food-deficit, is strongly dependent on the success of the rainfed lowland rice crop in the southern provinces. Soils there are sandy in texture, low in water-holding capacity, and low in pH and available nutrients. Farmers are reluctant to apply recommended fertilizer doses, as rainfall and crop responses are unreliable. This paper examines fertilizer response, and considers opportunities to improve the risk profile. The effects of 12 fertilizer-water-genotype treatment combinations were examined across 60 locations, and combined analysis of variance and pattern analysis was used to examine crop response and its relationship to soil fertility, genotype, and climatic variability. The results showed there was little relationship between soil test and grain yield in unfertilized plots, suggesting nutrient release and capture were more important than soil nutrient concentration, so integrative measures of soil nutrient release should be more promising. Despite low yield in the absence of applied nutrients (0.89–2.66, mean 2.18 t ha-1), NPK alone increased yields by 1.0 t ha-1 on average (1.48–3.86, mean 3.12 t ha-1), while water and genotype together with NPK increased yields by up to a further 1.0 t ha-1 (1.95–4.76, mean 3.42 t ha-1). Fertilizer responses were greater and more reliable when soil nutrient buffering capacity was greater, which, together with seasonal expectations, could be used to better inform decisions on fertilizer application in relation to risk. A longer-term and moderate input strategy was proposed in order to gradually improve fine-fraction SOC, soil nutrient buffering, and soil microbial biomass, in order to improve soil nutrient and water retention and release characteristics. This strategy should be evaluated in future research.

Improving nutrition-sensitive food security is important in low-income, food-deficit nations like Lao PDR, where subsistence farmers rely on a single rainfed lowland rice crop in the wet season. Access to resource conservation technologies, small on-farm water storages and electricity are starting to allow farmers to consider post-rice crops. With water availability still limited, short-duration pulse crops may be viable. This paper examined grain yield and water use during grain filling of eight short-duration mungbean genotypes after rainfed lowland rice at Champhone and Pakse in southern Lao PDR in the 2013 dry season. Two supplementary watering regimes were used, watered every 15 days (at 15, 30 and 45 days), and watered as needed at the first sign of visible wilting (at 23 and 45 days), with three replicates. Soil volumetric water content was measured by time-domain reflectometer. Over two drying cycles, VC1913A, VC3890A and VC7118A used significantly less water than other genotypes at both sites. Genotypes VC7118A and NM94 were higher yielding at Champhone, and VC6310 was higher yielding at Pakse. The relationship between grain yield and water use during grain filling was less straight forward, as it applied within apparent water-use efficiency categories. Overall, VC7118A was superior, due to its effective combination of higher apparent water-use efficiency during grain filling, lower water use in grain filling, and higher grain yield. NM94 also performed well, but required more water for similar yield performance. Drill sowing would allow timely establishment of the post-rice crop to utilize residual soil water in the dry season. These results suggest that mungbeans have a role as a post-rice crop with supplementary irrigation in Lao PDR and comparable environments, to improve nutrition-sensitive food security.

SUMMARY Genotype by environment (G x E) interactions for grain yield were investigated in 13 perennial rice (Oryza sativa L./Oryza longistaminata) derivatives over three sites and 2 years in Lao PDR. Genotype accounted for 29.0% of the total sum of squares, with environment and the G x E interaction responsible for 60.2 and 10.8%, respectively. Cluster analysis identified three environment and six genotype groups, which accounted for 49.7, 98.0 and 42.8% of the E, G and G x E sums of squares, respectively. Principal component axes 1, 2 and 3 accounted for 54.0, 30.6 and 11.7% of the G x E sum of squares, respectively, with PCA1 indicating yield potential, PCA2 timing of cessation of rainfall in the 2011 wet season, and PCA3 environmental stresses affecting regrowth in the 2012 wet season. Genotype groups differed in adaptation to these contrasting conditions. G6 (Line 213, 240 and RD23) was widely adapted to all environments, with G5 (Line 248) being especially adapted to the 2012 environments. G3 and G4 were neutral, though G3 (Line 53) showed some preference for the Na Pok environments. G1 and G2 were poorly adapted everywhere, with the tall and late G1 (Line 11) being especially poor at Na Pok 2011 in a dry finish. While yields were lower in 2012, all derivatives survived the dry season with access to life-saving irrigation. This is promising, as the annual rice RD23 was unable to ratoon under these conditions, and had to be re-sown. Importantly, Line 213, 240 and 248 yielded comparably to RD23 from regrowth in 2012. Development of perennial rice should target rainfed and especially upland environments.

Genotype by environment (G×E) interactions for grain yield were investigated in 14 rice genotypes across eight rainfed lowland field environments in Lao PDR, in order to identify stable adapted cultivars for improved farmer livelihood and food security. G×E accounted for 20.3% of the total variance, with three vectors from ordination analysis accounting for 75.1% of the G×E-SS, in 6 genotype × 6 environment groups. PCA1 indicated water-limited yield potential, PCA2 pre-flowering stress and PCA3 post-flowering stress. Genotype groups (G1–G6) differed in adaptation to these environments. G5 (VT450-2 and TSN9) were widely adapted and high-yielding. G6 (TDK11 and TDK37) were also high-yielding, topping the rankings in three environment groups, but yielded less in Phalanxay 2012 and Phalanxay 2011, where their phenology was unstable under stress. Other genotype groups showed specific adaptations, but failed to exceed yields of G5 and G6. Hence, VT450-2 and TSN9 (G5) were the preferred genotypes for rainfed lowland in southern Lao PDR, due to their high and stable grain yields. Stability in flowering time and high yield in rainfall deficit were desirable traits for improved farmer livelihood and food security.