UC Davis

Rice production relies heavily on tillage for seedbed preparation, crop establishment, and residue management. However, reducing tillage may decrease production costs, conserve soil moisture, and provide greater flexibility in the timing of field operations. Despite these potential benefits, the effects of reduced tillage on rice productivity remain uncertain, particularly in flooded rice systems. This dissertation evaluated the agronomic effects of reduced tillage in rice at both global and regional scales and examined alternative tillage and establishment systems for California rice production.The first study used a global meta-analysis of 115 studies and 579 paired observations to quantify rice yield responses to no-till (NT) relative to conventional tillage (CT) and identify conditions associated with improved NT performance. Across studies, NT reduced rice yield by 4.8%. Soil organic carbon, N fertilizer rate, soil pH, and clay content were among the most important factors associated with NT performance. Higher N rates and soils with greater organic carbon, moderate pH, and greater clay content reduced the NT yield penalty, although NT generally did not exceed CT yield. The second study evaluated a summer stale seedbed (SSB) as an alternative approach for reducing spring tillage in California water-seeded rice. On-farm experiments conducted across four location-years and a replicated experiment at the Rice Experiment Station compared SSB, in which fields were prepared during a preceding summer fallow and planted the following spring without additional tillage, with conventional spring tillage. Grain yield and N response were similar between SSB and CT. SSB also conserved more preplant soil water, while crop establishment, disease severity, and seed midge abundance were generally similar between tillage systems. These results demonstrate that shifting tillage from spring to the preceding summer can reduce spring soil disturbance without compromising productivity. The third study expanded this work by comparing conventional tillage water-seeded rice (CT-WS) with summer stale seedbed drill-seeded rice (SSB-DS) and two continuous no-till drill-seeded systems (NTDS) differing in rice straw management. SSB-DS maintained similar or greater yield potential than CT-WS, whereas the NTDS systems had lower yield potential overall. Drill-seeded systems required greater N rates to reach maximum modeled yield than CT-WS. Differences in emergence and final stand density did not explain the contrasting yield responses among drill-seeded systems, and reduced tillage did not increase stem rot or aggregate sheath spot severity. Collectively, these studies demonstrate that the agronomic effects of reducing tillage in rice depend on how reduced tillage is implemented. Continuous NT presents a greater risk of yield loss, whereas a summer SSB can substantially reduce spring tillage while maintaining rice productivity. In California, shifting seedbed preparation to fallow periods and combining summer SSB management with either water or drill seeding provides a promising approach for increasing flexibility in rice production while reducing reliance on conventional spring tillage.