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Nitrogen doses and timing of topdressing application affects the sensibility of rice plants to low temperature in reproductive stage. For these reasons it were installed two experiments with the objective of determining the effect of nitrogen topdressing application doses and timing on the susceptibility of rice plants to low temperature in the reproductive phase. The experiments were sowed in 2007/08 growing season in a greenhouse at Instituto Nacional de Investigação Agropecuária (INIA), INIA Treinta y Tres, Uruguay, with INIA Olimar cultivar. In both experiments, a factorial experiment was carried out in a completely randomized design, with four replications (factor A: stages or rates of N application and factor B: temperature treatment, with or without cold). In the experiment 1 the treatments of factor A were N application of 30 kg ha-1 in V4, V5, V6, V7, V8, R0, R1+4 days and an untreated check without N in topdressing. In experiment 2, the factor A was doses of N: 0, 23, 46 and 69kg ha-1 in V4 stage and the same levels of N in R0 stage plus a check without N in topdressing. The plants were treated with 12.5 C during seven hours for four nights when the collar of the flag leaf was at the same height of the previous leaf. The following aspects were evaluated: grain yield; spikelet sterility; SPAD index at three, ten and sixteen days of low temperature treatment; number of panicles per plot and weight of 1000 grains. Spikelet sterility was not affected by the application of N in different stages or doses, by cold treatment or their interaction, but the cold treatment affected temporally the SPAD index with delaying panicle emerge. It is not necessary to modify the N applications for this variety in Uruguay. [ABSTRACT FROM AUTHOR]

Urea is the main source of N used in agriculture, but has large losses in irrigated rice. The objective of this study was to evaluate the efficiency of N sources alternative to urea for use by rice applied at different stages of plant development through the quantification of N-NH3 and agronomic parameters. The experiment was conducted in the field in the agricultural years 2007/08 and 2008/09. The experimental design was randomized blocks with four replications. The treatments consisted of different N sources applied at seeding, tillering and at the stage of floral differentiation: control without N, urea, urea + urease inhibitor, two sources of liquid N (N-28 and N-30) ammonium sulfate and urea + sulfur. In both seasons, the volatilization of N-NH3 at seeding was almost negligible, whereas in the V4 stage, the source that was less volatilized was the ammonium sulfate. Yield response varied among years, probably due to the availability of N in the soil and climatic aspects. When economically advantageous, the use of urease inhibitor and ammonium sulfate provide yields similar to urea supply. [ABSTRACT FROM AUTHOR]