long term evaluation of yield stability trend for cereal crops in iran

during the last few decades cereals yield have increased drastically at the national level however, information about yield stability and its resistance to annual environmental variability are scare. in this study long term stability of grin yield of wheat, barley, rice, corn and overall cereals in iran were evaluated during a 40-year period (1971-2011). stability analysis was conducted using two different methods. in the first method the residuals of regression between crop yield and time (years) were calculated as stability index. for this different segmented regression models including linear, bi-linear and tri-linear were fitted to yield trend data and the best model for each crop was selected based on statistical measures. absolute residuals (the difference between actual and predicted yields for each year) as well as relative residuals (absolute residuals as percent of predicted yield) were estimated. in the second method yield stability was estimated from the slope of the regression line between average annual yield of all cereals (environmental index) and the yield of each crop in the same year. results indicted that in wheat and barley absolute and relative residuals were increased during the study period leading to reduction of stability despite considerable yield increment. however, for rice and corn residuals followed a decreasing trend and therefore yield stability of these crops was increased during the last 40 years. the same result was obtained with the environmental index but in this method reduction of yield stability in barley was lower than wheat. based on the results, yield and yield stability of cereals crops in iran increased during the last 40 years. however, the percentage increase in stability is lower than that of yield. application of nitrogen fertilizers was led to reduction in stability. yield stability of wheat, barley, rice, corn and overall cereals was improved with increasing their cultivated area.