Changes in yield and yield stability in wheat during the 20th century

An analysis of trends in yield and yield stability throughout the century was made for 21 countries (Algeria, Argentina, Australia, Canada, Chile, Egypt, France, Germany, India, Italy, Japan, Mexico, New Zealand, South Africa, Spain, Sweden, Tunisia, UK, Uruguay, USA and the former USSR). Regressions (linear, bi-linear or tri-linear fitted with an optimisation technique) were used to evaluate the trends in yield during the century. Residuals and relative residuals of these regressions were used to evaluate in absolute and relative terms, respectively, trends in yield stability. Countries varied greatly in their yields and yield gains as well as in changes in harvested area. But almost all of them showed a remarkable lack of yield gain during the initial 3 to 5 decades of this century, followed by noticeable increases in yield. Yield trends for relatively young agricultural wheat-exporting countries, such as Argentina, Australia, Canada and USA, reveal an important breakpoint ca. 2 decades earlier than European countries with longer tradition in wheat production. Ln addition, yield gains in many countries have apparently been levelling off during the last decade. Trends in yield residuals during the present century revealed a decrease in yield stability in 14 of the 21 countries analysed, but the increase in yield residuals was relatively small (less than or equal to 0.3 Mg ha(-1)) compared with increases in yield. Therefore, relative yield residuals indicated that yield stability, as a percentage of yield, increased or at least did not change for most of the analysed countries. Moreover, it is suggested that wheat production systems have been, in general, highly successful in increasing yield while maintaining or increasing relative yield stability with respect to that existing at the beginning of the century. Finally, no relationship was found between variations in yield stability, both in absolute and relative terms, and the increase in yield comparing the present values and those at the beginning of the century. (C) 1998 Elsevier Science B.V. All rights reserved.