Rice Yield Gap due to Iron Toxicity in West Africa

Iron toxicity is a widespread nutrient disorder in lowland rice, notably in West Africa. It occurs in irrigated or rain-fed rice crops when the soil contains excessive amounts of iron. Associated with leaf discoloration symptoms (bronzing), this excessive iron uptake causes poor growth and tillering and leads to severe yield reductions. Field experiments were carried out in West Africa from 1994 to 1998 at two sites with high iron toxicity and one non-toxic site to assess the effects of iron toxicity on rice cropping and evaluate the tolerance of promising rice cultivars available in West Africa. To estimate yield losses caused by iron toxicity, the yield potential was simulated using the ORYZA-S rice growth and yield model. Based on the potential yield, the yield loss in an iron-toxic site is the combination of the yield gap caused by unknown site factors and the yield gap caused by iron toxicity. Compared to the referential yield obtained in a non-iron-toxic site, iron toxicity reduced rice yields by 16-78 % (mean 43 %). The extent of the yield loss depended on rice cultivar, iron toxicity intensity and crop management strategy (water control and mineral fertilisation). A strong correlation obtained between yield and the iron toxicity score, based on visual symptoms indicated an approx. 400 kg ha(-1) yield loss for each visual score point increase. The high genetic variability in iron toxicity tolerance and close correlation between leaf symptom score and grain yield between rice genotypes provide a good basis for breeding varieties that can produce higher yields under iron-toxic conditions.