Development and Validation of Alternate Wetting and Drying (AWD) Water Management for Rice Grown on Clay Soils in Mississippi.

Rice irrigation currently accounts for the greatest amount of irrigation water applied per hectare over corn, soybeans, and cotton in the mid-southern US. The alluvial aquifer serves as the major source of irrigation water for rice production in Mississippi; however, it is declining at a rate of 37,000 hectare meters yr^-1. An experiment was conducted at the in Stoneville, MS to evaluate the yield and physiological response of rice to several alternate wetting and drying (AWD) irrigation regimes. Three rice cultivars were evaluated in six different rice irrigation treatments. Irrigation treatments included: a continuous flood, allowing the flood to recede to the soil surface, 10 cm below, 20 cm below, 30 cm below, and 40 cm below the soil surface. Water level in each paddy was monitored and irrigation events were triggered at each respective threshold back to a 10 cm flood, then allowed to subside until threshold was reached. Rice grain yield response of two AWD treatments were equal to rice grown with a continuous flood. A 200 kg ha^-1 grain yield increase was observed when the flood within a paddy was allowed to recede to the soil surface compared to a continuous flood. Grain yield for continuous flood was equal to rice grown with flood receding to 10 cm below soil surface. Reduction of grain yield was observed when the flood receded past 20 cm below the soil surface as compared to a continuous flood.   Control of barnyardgrass (Echinochloa crusgalli) was evaluated for conventional and Clearfield herbicide programs in an alternate wetting and drying (AWD) irrigation regime as compared to a continuous flood. Experimental plots were over-seeded with barnyardgrass and evaluated for barnyardgrass control. Control of barnyardgrass and rice grain yield in experimental plots were not different for rice grown under continuous flood compared to AWD (20 cm below soil surface). Barnyardgrass control for Clearfield® herbicide program was 93% pooled over all herbicide treatments, and was not different for continuous and AWD irrigation. For conventional rice, barnyardgrass control (pooled over all herbicide treatments) for continuous irrigation averaged 74% control, while AWD irrigation averaged 82% control. Data from this experiment in 2015 suggest that allowing flood to subside to 10 cm below the soil surface does not result in yield loss compared to a continuous flooded system. Water management practices that reduce groundwater withdrawals are a viable option for rice producers in the mid-south.