Weed competitiveness of the lowland rice varieties of NERICA in the southern Guinea Savanna.

Weed competition is a major constraint to lowland rice production in West Africa. Interspecific rice varieties named New Rice for Africa (NERICA) may have superior weed competitiveness and could as such play an important role in integrated weed management. The NERICA varieties were developed from the wide cross between high-yielding Oryza sativa (L.) and weed competitive and disease resilient Oryza glaberrima (Steud.). In this study weed competitiveness of all 60 lowland varieties of NERICA (NERICA-L) was compared with their most frequently used parents [IR64 (O. sativa) and TOG5681 (O. glaberrima)], the weed competitive variety Jaya (O. sativa) and the O. glaberrima upland NERICA parent CG14. During the 2006 and 2007 rainy seasons these varieties were grown under weed-free and weedy conditions in a lowland farmers’ field with partially controlled irrigation in south-east Benin. Weedy plots included single hand weeding at 28 days after sowing, whereas weed-free plots were weekly weeded. Most important weed species encountered in this study were Basilicum polystachyon, Alternanthera sessilis, Echinochloa colona, Sorghum arundinaceum, Cyperus halpan and Cyperus rotundus. Average weed-inflicted yield loss across varieties was 39% in 2006 and 8% in 2007. In both years varieties differed significantly in grain yields under weed-free and weedy conditions and in the growth of weed biomass they permitted under weedy conditions, as observed at harvest. None of the lowland varieties of NERICA consistently had stronger weed suppressive ability than TOG5681 across 2 years. Nine varieties of NERICA-L (-6, -32, -35, -37, -42, -53, -55, -58 and -60) were identified with high yields under both weed-free and weedy conditions. These nine NERICA-L varieties, together with Jaya, out-yielded the other 51 NERICA-L varieties as well as IR64 and the two O. glaberrima varieties. Weed-free yield, crop growth duration, and weed biomass at harvest significantly correlated with weedy yield in both years. Interspecific breeding using O. glaberrima appears to be an effective approach for improving yield potential and weed competitiveness of semi-dwarf O. sativa and as such for widening the range of useful varieties for lowland rice farmers in Africa.