A meta-analysis of drought stress impact on the yield of some important forage-grain crops in Iran

Climate change drastically reduces the amount of water available to plants and increases the severity and frequency of droughts. By the end of this century, it is anticipated that droughts will continue to increase significantly. Drought stress, as one of the major climate events, induces physiological and morphological alterations, which can restrict crop growth, yield, and quality. In recent years, separate research projects and studies in the country have evaluated the effects of drought stress on grain maize, forage maize, grain sorghum, forage sorghum, grain millet, alfalfa, and clover yields, as well as the effects of drought stress on their yield gap. However, if the results of these separate studies were evaluated together, it would have been possible to compare the drought sensitivity and tolerance of various crops. Consequently, the optimal crop to plant under drought-stress conditions in the nation's agroecosystems would be determined. The meta-analysis is a statistical analysis that can assess the aggregate effects of numerous studies conducted under varying conditions. In other words, meta-analysis is a statistical technique for integrating the findings of multiple studies in order to comprehend a problem. The purpose of this meta-analysis was to determine the impact of different drought stress levels on the yields of grain maize, forage maize, grain sorghum, forage sorghum, grain millet, alfalfa, and clover.

In this study, the data obtained from 145 final reports of the Agricultural Research, Education, and Extension Organization (AREEO) and published articles over the past decade were analyzed using the meta-analysis method. This included grain sorghum (18 studies), forage sorghum (21 studies), grain maize (28 studies), forage maize (23 studies), grain millet (18 studies), alfalfa (17 studies), and clover (20 studies). In addition to the difference in quantitative results between studies, the effect size variance was utilized in this method. Consequently, the means, standard deviations, and sample sizes (replications in each study) for the control and evaluated treatments were determined. After classifying the data, the reaction ratio (R) and its natural logarithm were calculated. Subsequently, the method determined which treatments have additive or decreasing effects on yield under drought stress conditions, as well as which treatments had no positive or negative impact on yield. Utilizing the T-test, a group comparison was conducted between the mean yield under drought stress conditions and the mean yield under normal irrigation conditions in order to determine the significance level of yield changes caused by drought stress. The statistical analysis and preparation of the figures were performed by Excel.

According to the findings of the meta-analysis, the overall effect of drought stress on the yield of all seven crops was significant for grain sorghum (-16.30%) (P<0.05), forage sorghum (-18.27%) (P<0.05), grain maize (-31.75%) (P<0.01), forage maize (-33.24%) (P<0.01), grain millet (-25.66%) (P<0.01), alfalfa (-30.79%) (P<0.01), and clover (-32.14%) (P<0.01). The yield reductions in grain maize, forage maize, alfalfa, and clover were significant under all levels of drought stress, whereas grain sorghum, forage sorghum, and grain millet yield reductions were significant only under severe drought stress. The yields of grain sorghum, forage sorghum, grain maize, forage maize, grain millet, alfalfa, and clover decreased by 10.03, 11.84, 18.21, 20.27, 14.13, 19.74, and 20.37%, respectively, under moderate drought stress, and 21.98, 25.41, 39.09, 41.37, 32.55, 40.13, and 42.08%, respectively, under severe drought stress.

As a result, it is impossible to cultivate and develop grain maize, forage maize, alfalfa, and clover in areas with limited water resources without significantly reducing yield. According to the findings of this meta-analysis, grain sorghum, forage sorghum, and grain millet with a moderate drought stress tolerance threshold could be recommended for cultivation in regions where drought stress conditions are likely to occur.