Foliar urea spraying in rice (Oryza sativa L). Effects of time of application on grain yield and protein content

It is known that rice response to N fertilizers (mainly in granulated form) is lower than in other crops due to the greater complexity of the N cycle under flooding. A field trial was carried out to study the effect of foliar urea spraying on biomass production, harvest index, grain yield and grain protein content on three rice cultivars. Nitrogen (30 kg/ha), was applied as foliar urea spraying at the end of tillering, heading or postanthesis. Spraying N at heading increased grain yield due to higher grain number/m(2) and a more efficient dry matter partition to the grain (harvest index), without changes in the biomass production. Independently of the time of application, urea sprayings did not modify head number or grain weight. The effect of urea sprayings on grain N content and protein percentage varied according to the cultivars and time of application (positive interaction cultivar x time). Both grain N content and grain protein percentage increased significantly with postanthesis spraying in two of the three cultivars studied but it had no effect on the grain yield. The efficiency of N fertilization for grain yield was higher when applied at heading. No apparent N recovery in the grain was observed when urea spraying was done at tillering; but it rises to 70% when applied at heading and to 47% when applied at postanthesis. In all cultivars N spraying at heading increased grain protein production per ha due to an increase of both grain yield and grain protein percentage. It has been concluded that N spraying in rice, even at low doses, could be effective to increase grain yield and grain protein content depending on rice cultivars and time of application.