The state of the art on deficit irrigation in soybean

Deficit irrigation (DI) could be an important strategy to achieve the goal of reducing irrigation water consumption. This review aims to identify the impact of different DI strategies on grain yield, water use efficiency (WUE) and oil and protein content in soybean seeds. A total of 25 articles were considered and then divided into DI throughout the whole cycle (standard deficit irrigation, StDI) and DI only at certain stages of the cycle (regulated deficit irrigation, RDI). In StDI, yield reductions were approximately 20% when the replacement of the crop water requirement was between 70% and 90%. For RDI, yield reductions ranging from 9% to 30% were observed depending on the phenological stage at which the crop evapotranspiration (ET_c) deficit was imposed. StDI always increased WUE compared to full irrigation, whereas for RDI, the response in terms of WUE changed considering the stressed phenological stage. Few studies have reported the effects on oil and protein content, showing high variability and contrasting results. In general, the application of a reduced amount of water led to a decrease in yield and an increase in WUE, with a magnitude significantly influenced by the stage at which the stress was imposed.