Grain Yield and Nitrogen Use Efficiency of a Small-Grain-Weight Variety Grown Under Different Nitrogen and Seeding Rates in Direct-Seeded Rice in South China

Small-grain-weight rice varieties are popular in South China. However, to maintain grain yields comparable to medium-grain-weight varieties, adjustments in nitrogen (N) and seeding rate management may be necessary. In this study, the small-grain-weight variety Mabayinzhan was direct-seeded in a three-season field experiment conducted from 2022 to 2023 in Qujiang County, Guangdong Province, South China, using four N rates and three seeding rates. The results showed that grain yield and grain weight ranged from 3.45 to 7.95 t ha⁻¹ and from 11.4 to 14.1 mg, respectively, across all treatments with N application. The variation in grain yield was primarily attributed to differences in spikelets per m², biomass production, and grain weight. Grain yield generally increased with higher N rates, but no significant difference was observed across the range of seeding rates. The grain yield responded more strongly to N rate than to seeding rate. The decrease in grain yield resulting from lower N rates could not be compensated by increasing seeding rate. While dense planting with reduced N application could improve N use efficiency (NUE), the increase in NUE was due to the reduced N rate rather than the seeding rate. Our findings indicate that high seeding rates are not essential for achieving high yields with a small-grain-weight rice variety. Moreover, increasing planting density while reducing N application may not be optimal for small-grain-weight inbred rice production under direct seeding conditions in South China.