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 effectiveness. (c) 2005 Elsevier B.V. All rights reserved.