Evaluation of yield, water productivity and drought tolerance of forage sorghum [Sorghum bicolor (L.) Moench] cultivars under different irrigation methods and regimes

The lack of irrigation water resources in the country in recent years has reduced the yield of crops and threatened food security; Therefore, it is necessary to think of ways to produce crops while saving water. One of the ways to increase production with limited irrigation water resources is the cultivation of drought-tolerant crops with high water use efficiency. Sorghum is known as one of the most important crops in arid and semi-arid regions of the world with its unique characteristics (including tolerance to environmental stresses, especially drought and heat, deep and extensive root system, C4 photosynthetic pathway, etc.). The use of appropriate irrigation methods is also known as another way to increase the production of crops with limited water resources. In this regard, the irrigation methods of alternate furrow irrigation and drip irrigation have been used as new irrigation management solutions in the country with favorable results. This study aimed to evaluate the effect of different irrigation regimes and methods on yield, morphological characteristics, water productivity, and drought tolerance of open-pollinated and hybrid forage sorghum cultivars.

This experiment was conducted as a factorial split-plot based on a randomized complete block design with three replications at the Seed and Plant Improvement Institute, Karaj, Iran, during the 2019 and 2020 cropping seasons. The factorial combination of four irrigation methods (CFI, conventional furrow irrigation; FFI, fixed alternate furrow irrigation; AFI, variable alternate furrow irrigation; and drip irrigation) and three levels of drought stress (full irrigation, moderate stress, and severe stress; including the supply 100, 75, and 50% of soil moisture deficit, respectively) as the main factor and two sorghum cultivars (Speedfeed and Pegah) as sub-factor were investigated. In this study, traits including silage yield, dry matter yield and percentage, irrigation water use efficiency, benefit per drop, net benefit per drop, leaf-to-stem ratio, plant height, and drought stress tolerance indices were evaluated. Due to the homogeneity of the variances of experimental errors over two years, the data were subjected to combined analysis of variance using SAS 9.1 statistical software, and Tukey's test (P≤0.05) was used to compare the means. In addition, IPASTIC online software was used to calculate stress tolerance indices and principal component analysis (PCA) based on these indices.
Research

The results showed that drought stress in all investigated irrigation methods decreased the silage yield, dry matter yield, and plant height and increased water use efficiency for silage and dry matter production, dry matter percentage, and leaf-to-stem ratio. The highest silage and dry matter yields (79.11 and 17.19 ton ha-1, respectively), and the maximum net benefit per drop (18854 Rials m-3) were obtained by cv. Pegah under full irrigation and drip method, whereas the maximum water use efficiency for silage and dry matter production (34.53 and 7.88 kg m-3, respectively) and the highest benefit per drop (60542 Rials m-3) were recorded in hybrid Speedfeed under severe stress and drip method. The highest leaf-to-stem ratio (0.715) was obtained in the FFI method under severe stress and the amount of this trait in cv. Pegah was about 8.6% higher than that of hybrid Speedfeed. Based on the stress tolerance indices, cv. Pegah under the CFI and drip methods and hybrid Speedfeed under the FFI method had more drought tolerance. In contrast, there was no significant difference between the cultivars under the FFI method.

In order to save water while achieving suitable forage yield and water productivity, the drip irrigation method and supplying 75% of soil moisture deficiency is recommended. If it is impossible to implement the drip irrigation system, the variable alternate furrow irrigation method can produce suitable forage yield while reducing the amount of water consumed.