Water use and plant response in two rice irrigation methods

Pin-Point (PP) irrigation is being used by rice producers in the southern US to suppress red rice (Oryza sativa), the major weed in rice production. In PP irrigation, germinated seed is dropped into the floodwater. After 24 h the field is drained, remains moist for 3 to 5 days, then reflooded until near harvest. Little is known about water use of the PP method in comparison to the conventional Flush-Flood (FF) method in which germinated seed is also dropped into the floodwater and the field is drained after 24 h, similar to PP, or the field is drill-seeded, flooded and drained, but permanent flood is delayed for 30 to 35 days. During this 30 to 35 days non-flood period, flushing (periodic irrigation) is used to maintain seedling contact with the soil and prevent water stress. Water use of PP and FF irrigated rice was studied during 1994 and 1995 growing seasons near Beaumont, TX. Three plots were subjected to the PP irrigation technique and three to the FF method. A flow meter measured irrigation water applied, and lysimeters measured evapotranspiration, transpiration, and evaporation. The FF method required an average of 113 mm more irrigation water than the PP method, due to flushing of FF plots during the non-flood period. Evapotranspiration (ET) was higher for PP plots during the period when FF plots were dry. During the flood period, ET in FF plots was higher than in PP plots in 1994, due to mainly a lower leaf area index and a more open canopy which led to greater evaporation from the water surface. No differences in ET were found in 1995. Stomatal conductance was lower in FF plots during the non-flood period, indicating some degree of water stress. Irrigation method did not affect yield, but the PP method reduced time to 100% heading by 5 to 7 days. These results suggest that the PP method can be useful not only to suppress red rice, but also to save water and produce an earlier maturing crop. Early maturity is particularly important in areas where ratoon cropping is practiced.