Application of mulch under reduced water input to increase yield and water productivity of sweet corn in a lowland rice system

•Rice straw mulch increased sweet corn yield, but more so with reduced water input.•Reducing water input with bare soil did not affect water productivity and reduced gross margin.•Mulch reduced soil evaporation by 114–163mm, but only 0–57% of this was partitioned to extra transpiration.•Adding mulch and also reducing water input maintained or increased transpiration.•Hence, it greatly increased water productivity and gross margin per hectare and per m3. While straw mulch usually reduces soil evaporation and stabilizes soil temperature, hence increasing yield, this effect may depend on the irrigation water input conditions. Two experiments in lowland rice paddies in Lao PDR tested the effect of rice straw mulch under various water input—standard farmer condition to reduced input condition by using either drip irrigation or lower furrow irrigation water input (WI) by increasing the furrow irrigation interval before flowering—on growth and yield of sweet corn. The time course of water balance components was determined to elucidate the mechanisms of mulch and water input interaction for row planted maize after rice harvesting. The experiments found that adding straw mulch reduced estimated soil evaporation by 114–163mm, but only some of this was partitioned into extra transpiration, so the non-transpiration flux (the difference between water input and transpiration) changed little. Only when mulch was added and water input also reduced did it maintain or increase transpiration, reduce the non-transpiration flux and hence substantially increase water productivity (WP). Most if not all of the non-transpiration flux occurred in the first 60 days; the opportunity to apply treatments to increase water productivity arose mostly in the first 60 days. Mulch had a greater effect with extended furrow irrigation intervals before flowering than with standard intervals, but there was no effect under drip irrigation. In Experiment 1, mulch increased fresh ear yield and water productivity to water input (irrigation plus rainfall) (WP) by 42% with Low WI, but had no effect with High WI or with drip irrigation. The combination of mulch and reducing water input from High WI to Low WI increased gross margin (GM) per hectare by 20% and GM per m3 water input by 66% due to increased yield and reduced water and labour costs. In Experiment 2, mulch increased fresh ear yield, WP by 93% and consequent GM with low WI, but also increased fresh ear yield and WP by 60% and GM with High WI.