Résumé
Weed-competitive upland rices with an acceptable yield potential are needed for labor-limited systems in Africa, particularly where shortened fallow periods have increased weed pressure. Crosses between weed-competitive but low-yielding African rice,
Oryza glaberrima, and improved
Oryza sativa tropical-japonica rices, might reduce tradeoffs between competitiveness and yield potential. Parallel field studies under moist upland conditions were conducted during the 1996 and 1997 wet seasons at Mbe in Cote d'Ivoire to (1) characterize canopy properties, growth vigor and yield for
O.
glaberrima,
O.
sativa and interspecific progenies under monoculture, and (2) determine their competitiveness when grown in single rows in competition with natural weed growth, maize (removed 50 days after sowing [DAS]), the highly weed-competitive
O.
glaberrima IG10, and with themselves. In the monoculture study with 21 lines, dry matter, leaf area index (LAI), PAR extinction coefficient (
K
df), mean tip elevation angle (MTA) and specific leaf area (SLA) were measured 31, 46 and 64 DAS. Across lines, LAI was positively correlated with SLA, dry matter partitioning to leaves (31 and 46 DAS) and
K
df (46 DAS); and negatively with MTA (46 and 64 DAS). Plant height was negatively correlated with tiller number. In the competition study with 16 lines, the
O.
glaberrima landraces had superior relative yield (yield under interspecific competition/yield under intraspecific competition). Some breeding lines were competitive with specific competitors. Correlations between canopy characteristics under monoculture and competition indicated that LAI, SLA and tillering ability were predictive of competitiveness regardless of the competing species, whereas partitioning,
K
df and MTA were correlated with competitiveness only for specific growth stages and/or competitors. Competitiveness was negatively but weakly correlated with yield potential, and positively, with crop duration. The authors conclude that SLA and tillering ability, which are major determinants of vegetative vigor, and crop duration, which affects the ability to recover from early competition, are useful traits in the selection of weed-competitive rices, particularly in breeding programs that use
O.
glaberrima. The traits are compatible with high yield potential if cultivars have large SLA during early developmental stages and small SLA during advanced stages. Major knowledge gaps remain on weed competitiveness under drought- and flood-prone conditions, which are frequently associated with weed problems.