Yield Gap as Occurring in Lowland Rice Cropping under Guinea Savanna Ecology: Spatial and Temporal Diagnosis for Fixing Research Priority

Two lowlands under continuous rice-rice cropping in the centre of Côte d'Ivoire were surveyed in 2013 and 2014 considering 31 rice fields (farmers) for each location. Soil samples (93) were taken in 0 – 20 cm depth systematically (50 m along × 20 m across) extending the hydromorphic zone (HZ), the fringe valley (FV) and the valley bottom (VB) along the upper stream (US), median (MS) and downstream (DS) positions respectively. Highest yields of 6.09 – 6.16 tha-1 were observed in DS and MS while limited chances of 24% to 30.17% were characterizing the yield over 5 tha-1 (yield class I) in dry season against 18% - 46.31% in wet season. Weeding and irrigation were the most significant agricultural practices for yield improvement during wet and dry seasons respectively meanwhile, soil content of silt and humification rate (K2) were reversibly accounting for 65.85% contributions of the yield class I in addition to Ca and Mg effects. Of course, the recommended NPK fertilizer (150 and 200 kgha-1) should contained Mg in basal application (transplanting) when, 75 and 150 kgha-1 of urea were required at the tillering stage in dry and wet seasons respectively. However, applying NPK fertilizer at heading stage could further increase the yield during the wet season. Definitively, the yield gap was defined as absolute and relative for well understanding and research priority was identified as relevant matters for dry season and hydromorphic zone during further study.