Water Stress Indices as Indicators of Silage Soybean [Glycine max (L.) Merr.] Productivity Under Drought Conditions

Silage soybeans subjected to water stress show symptoms as if it had been subjected to drought stress due to lack of rainfall. Understanding how water stress impacts crop yield is crucial for developing effective irrigation strategies in drought-inclined areas. This study investigates the relationship between silage soybean forage yield and water stress indices, specifically the Crop Water Stress Index (CWSI) and the Water Deficit Index (WDI). In addition, water-yield relations were also determined. The study was carried out in a randomised complete block design with four irrigation levels (S100, S75, S50, S25), three replications, and a period of 2 years. At the end of the study, crop evapotranspiration (ET), irrigation (I), water use efficiency (WUE) and forage yield decreased as irrigation water levels were reduced from S100 to S25. ET was found to be between 501 and 755 mm, WUE was found to be between 2.61 and 3.58 kg m-3, irrigation water use efficiency was found to be between 2.53 and 2.97 kg m-3, forage yield was found to be between 15.4 and 26.2 t ha-1, WDI varied between 0.16 and 0.5, and CWSI ranged between 0.34 and 0.90. The results revealed that silage soybean should be irrigated when the average CWSI value is nearly 0.36 and the average WDI value is nearly 0.26 for high yield. The rate of decrease in forage yield from S100 to S25 was compared with the rate of increase in WDI and CWSI. In the first year of the study, there was a 45% decrease in forage yield between S100 and S25, while CWSI increased by 65%. However, this situation was 40% in WDI. In the second year of the study, there was a 38% decrease in forage yield from S100 to S25, while CWSI increased by 33% and WDI increased by 26%. The relationship between forage yield and stress indices is inversely proportional. The fact that the values in CWSI were proportionally higher than WDI suggested that the calculations were made without taking into account the soil temperatures during the measurement. Water stress indices were correlated with forage yield and regression analysis was performed. Although relations of forage yield and water stress indices showed slightly higher correlations with WDI, this could not strongly suggest that WDI was more accurate than CWSI. In order to obtain clearer results in the comparison studies of water stress indices, it is recommended to carry out studies with different varieties for more than 2 years.