Soil Quality and Region Influence Performance and Ranking of Switchgrass Genotypes

Development of switchgrass (Panicum virgatum L.) as a dedicated bioenergy feedstock requires intensive and extensive breeding programs that include careful and thoughtful consideration of appropriate target populations of environments (TPEs). The purpose of this study was to evaluate region (climate), soil quality, and N fertilization level as potential factors influencing the choice of TPE. A total of 45 switchgrass genotypes were evaluated in uniform field studies at six field sites defined as prime or marginal soils in New Jersey, South Dakota, and Wisconsin. Region and soil quality had strong interactions with genotype, but N fertilization had little impact on genetic variation or ranking of genotypes. Lowland genotypes were considerably more sensitive than upland genotypes to interactions with environmental factors, probably due to these field sites being outside of the traditional lowland adaptation zones. Genotype rankings were highly inconsistent across regions and soil types, indicating that breeding programs that target marginal soils should be located on soils that represent the appropriate TPE. Furthermore, interactions across the three regions suggest that breeding programs for the lowland ecotype should be subdivided into different sets of TPEs, which are largely a function of hardiness zone and annual precipitation. Lastly, even with negligible interactions involving N fertilization level, future definitions of TPEs should be based on minimal or no N fertilizer applications to allow breeders to select plants with greater N-use efficiency, N-scavenging ability, and N-recycling efficiency

Developing selection protocols for weed competitiveness in aerobic rice

Aerobic rice production systems, wherein rice is dry-sown in non-puddled soil and grown as an upland crop, offer large water savings but are subject to severe weed infestation. Weed-competitive cultivars will be critical to the adoption of aerobic rice production by farmers. Breeding weed-competitive cultivars requires an easily used selection protocol, preferably based on traits that can be measured under weed-free conditions. To develop such an indirect selection index for weed competitiveness, 40 rice cultivars were evaluated in aerobic soil conditions in a weed-free environment in 2003 and in weedy environments over 3 years (2001¿2003). Broad-sense heritabilities (H) of vegetative and harvest traits and their genetic correlation with weed biomass and yield under weed competition were estimated. All the traits measured under weed-free conditions were closely correlated with the same traits measured under weedy conditions. Crop vigor ratings at 2, 4, and 6 weeks after sowing (WAS), canopy ground cover at 6 WAS, height at 3 and 4 WAS, tillers per plant at 4 and 8 WAS, vegetative crop biomass at 4 and 9 WAS and plant erectness at 3 WAS under weed-free conditions in 2003 were all positively correlated with means for yield under weed competition and negatively with means for weed biomass across three years. In general, traits associated with rapid seedling biomass accumulation were also strongly associated with weed suppression and yield under weed competition. Regression analysis revealed that yield and early vigor under weed-free conditions in a single three-replicate trial could be used together in an indirect selection index, explaining 89% and 48% of variation for yield under weed competition and weed biomass, respectively. The predicted indirect selection efficiencies of weed-free yield and vigor ratings as selection criteria for yield under weed competition and weed biomass were high. Visual vigor rating at 4 WAS is the best vegetative trait as an indirect selection criterion for use together with weed-free yield, but it could be replaced by plant height at 4 WAS without loss in selection effectivenes

Interaction of genotype x management on vegetative growth and weed suppression of aerobic rice

Water shortage in drought-prone rice-growing areas of the world is threatening conventional irrigated rice production systems, in which rice is transplanted into fields where standing water is maintained until harvest. Aerobic rice production systems, in which rice is grown as a direct-seeded upland crop without flooding, require less water than conventional systems, but the transition to aerobic rice systems is impeded by severe weed infestation. An environmentally friendly and less labor-intensive weed control method needs to be introduced to aerobic rice farmers. A study was conducted at the International Rice Research Institute in the 2003 wet season and 2004 dry season to evaluate the effects of genotype, seeding rate, seed priming and their interactions on vegetative growth, yield and weed suppression. Three contrasting aerobic rice genotypes differing in yield and weed-suppressive ability (WSA) were grown at three seeding rates (100, 300 and 500 viable seeds m¿2) with or without seed priming under two weed management treatments (weed-free and weedy) in a split-plot design. In 2004, the overall weed pressure was higher than in 2003, and consequently treatment effects in this year were more distinct than in 2003. No significant interactions among the experimental factors were found for crop yield, weed biomass, leaf area index, tiller number and vegetative crop biomass. Raising seeding rate from 100 to 300 viable seeds m¿2 resulted in a significant increase in yield and a decrease in weed biomass, whereas a further increase from 300 to 500 viable seeds m¿2 did not result in a further improvement in yield and weed suppression. The stronger WSA of genotype Apo than that of genotypes IR60080-46A and IRAT 216 related to a stronger competitive ability of individual plants and a faster canopy closure (0.5¿6 days earlier). The WSA of weakly competitive genotypes was partially compensated for by a higher seeding rate. Seed priming, which was only evaluated in 2003, accelerated emergence by 2 days and slightly enhanced early crop growth, but had no significant effect on yield and weed suppression. The present study suggests that combining a weed-suppressive genotype with an optimum seeding rate can serve as a tool to manage weeds

Cultivar weed-competitiveness in aerobic rice : heritability, correlated traits, and the potential for indirect selection in weed-free environments

Forty rice (Oryza sativa L.) cultivars and breeding lines used in the International Rice Research Institute (IRRI) upland rice breeding program were evaluated in adjacent weed-free and weedy trials in aerobic soil conditions during the wet seasons of 2001, 2002, and 2003. The objectives of this study were to investigate genetic variability in weed suppression and yield and to identify traits that could be used as selection criteria for improved weed competitiveness. Correlations among and heritability (H) of agronomic traits and early vigor were estimated in weedy and weed-free trials. Regression analysis was performed to predict weedy yield and weed biomass. Cultivars differed widely in the growth of weed biomass they permitted (126¿296 g m¿2) and in yield under competition (0.5¿2.5 Mg ha¿1). Cultivar yield, duration, biomass, harvest index, height, and vegetative vigor under weed-free conditions were closely correlated with the same traits measured under weedy conditions. Weedy yield and weed biomass were both moderately heritable (H = 0.55 and 0.38 for means estimated from single-year, three replicate trial, respectively) and genetically correlated with each other (r = ¿0.84). Weed-free yield and vigor at two weeks after seeding (WAS) were moderately heritable (H = 0.68 and 0.38 for means estimated from a single-year, three replicate trial, respectively) and were highly genetically correlated with weedy yield (r = 1.00 and 0.88, respectively) and weed biomass (r = ¿0.89 and ¿0.67, respectively). Vegetative vigor at two WAS and grain yield measured under weed-free conditions explained a combined 87% of cultivar variation in weedy yield and 40% in weed biomass. Indirect selection on these two traits was predicted to be efficient for improving yield under weed competition and weed-suppressive ability of aerobic rice

A QTL for rice grain yield in aerobic environments with large effects in three genetic backgrounds

A large-effect QTL associated with grain yield in aerobic environments was identified in three genetic backgrounds, Apo/2*Swarna, Apo/2*IR72, and Vandana/2*IR72, using bulk-segregant analysis (BSA). Apo and Vandana are drought-tolerant aerobic-adapted varieties, while Swarna and IR72 are important lowland rice varieties grown on millions of hectares in Asia but perform poorly in aerobic conditions. Two closely linked rice microsatellite (RM) markers, RM510 and RM19367, located on chromosome 6, were found to be associated with yield under aerobic soil conditions in all three backgrounds. The QTL linked to this marker, qDTY6.1 (DTY, grain yield under drought), was mapped to a 2.2 cM region between RM19367 and RM3805 at a peak LOD score of 32 in the Apo/2*Swarna population. The effect of qDTY6.1 was tested in a total of 20 hydrological environments over a period of five seasons and in five populations in the three genetic backgrounds. In the Apo/2*Swarna population, qDTY6.1 had a large effect on grain yield under favorable aerobic (R 2 ≤ 66%) and irrigated lowland (R 2 < 39%) conditions but not under drought stress; Apo contributed the favorable allele in all the conditions where an effect was observed. In the Apo/IR72 cross, Apo contributed the favorable allele in almost all the aerobic environments in RIL and BC1-derived populations. In the Vandana/IR72 RIL and BC1-derived populations, qDTY6.1 had a strong effect on yield in aerobic drought stress, aerobic non-stress, and irrigated lowland conditions; the Vandana allele was favorable in aerobic environments and the IR72 allele was favorable in irrigated lowland environments. We conclude that qDTY6.1 is a large-effect QTL for rice grain yield under aerobic environments and could potentially be used in molecular breeding of rice for aerobic environment

Comparing rice germplasm groups for growth, grain yield and weed-suppressive ability under aerobic soil conditions

Germplasm and cultivars need to be selected as parents for breeding weed-competitive aerobic rice in the tropics. Forty rice (Oryza sativa) cultivars belonging to the aus, indica and tropical japonica germplasm groups, or derived from crosses among them, were evaluated in adjacent weed-free and weedy trials in aerobic soil conditions during the wet seasons of 2001¿2003. The objectives of this study were to assess vegetative growth, grain yield under weed-free (YF) and weedy (YW) conditions, and weed-suppressive ability (WSA) of different germplasm groups. In the first 4 weeks after sowing, indica cultivars had faster growth in height, tillering and crop biomass than other groups. They also had high YF, YW and strong WSA. Aus cultivars were similar to the indica types in early growth and WSA, but were poor in YF. Tropical japonica groups and the group derived from indica/tropical japonica crosses were generally inferior to aus and indica groups in early growth and WSA. Both of their YF and YW were lower than that of the indica group. Therefore, indica germplasm seemed to be most suitable for breeding high-yielding and weed-suppressive aerobic rice for the tropics. The relationship of WSA with various traits within tropical japonica germplasm revealed that fast early growth, rather than plant erectness, is crucial to WSA