On-farm agronomic manipulations to improve rice (Oryza sativa L.) production in the saline coastal zone of the Red River Delta in Vietnam

Sporadic salinity caused by seawater intrusion is a serious concern for rice (Oryza sativa L.) production in the Red River Delta (RRD) of Vietnam, and guidance for on-farm agronomic management is needed. Three sets of on-farm experiments – namely, trials of (1) nitrogen (N) fertilizer use (replicated trials and surveys of farmer fields, including plots receiving zero N), (2) resistant varieties, and (3) shallow irrigation management – were conducted from 2015 to 2017 in three communes in Nam Dinh province in the RRD, to assess technologies adaptable by farmers. Yield response to N fertilizer in the farmer fields was generally low and greatly variable, associated with variable N recovery efficiency and internal efficiency, particularly in fields at risk of high salinity in one commune during summer 2015. In the replicated trials, N fertilizer applied at 120–180 kg N ha⁻¹ in spring and 100–150 kg N ha⁻¹ in summer generally produced better yields with higher N use efficiencies than the current high N application rates (219 and 189 kg N ha⁻¹, respectively). Recently developed salinity-resistant varieties M2 and M14 produced almost 50% higher yield than the conventional varieties in very saline at-risk fields. Shallow water-depth management (<5 cm) resulted in grain yields similar to the higher conventional-depth management in the less saline fields, but significantly reduced yields in the more saline fields. These results indicate potential resource-use efficiencies in rice production to better cope with coastal salinity in the RRD.