A recent breakthrough in inter-specific hybridization between African indigenous rice, O. glaberrima, and Asian rice, O. sativa, has helped developing highly weed competitive upland rice cultivars with a yield potential greater than that of the currently cultivated improved and traditional rices in West Africa. Tradeoffs between weed competitiveness and yield potential were reduced by selecting for dynamic plant types that resemble the weed competitive O. glaberrima parents during the vegetative growth phase, and the O. sativa parents during the reproductive phase. Field trials during 1995 and 1996 showed that O. glaberrima cultivars, such as CG14 and IG10, produce between two and three times the leaf area of improved O. sativa japonica upland rices, such as WAB56-104 or IDSA 6, under both high and low input conditions. Ground cover was also achieved significantly earlier in O. glaberrima than in O. sativa cultivars, mainly owing to a higher specific leaf area (SLA; area produced per dry wt), which permitted the production of more leaf area with a given amount of assimilate. Detailed growth analyses were conducted during the early vegetative stage on potted plants in the screenhouse. CG14 (O. glaberrima) had higher leaf appearance rates (phyllochron) and tillering rates than WAB 56-104 (0. sativa), but similar extension rates and shorter growth durations for individual leaves. The greater overall leaf area CG14 was therefore due to a rapid multiplication of the number of leaves through tillering and the phyllochron, and not to growth characteristics of individual leaves. The greater SLA of CG14 resulted in significantly lower photosynthetic rates at the leaf level, but this was more than compensated by gains in light interception during vegetative growth. The new weed competitive, interspecific genotypes are characterized by high seedling vigor, resulting from (i) high specific leaf area (SLA), (ii) a high light extinction coefficient (Kdf), and (iii) increased part