CHANGES IN YIELD AND YIELD STABILITY IN WHEAT DURING THE 20TH-CENTURY

An analysis of trends in yield and yield stability throughout the cent ury was made for 21 countries (Algeria, Argentina, Australia, Canada, Chile, Egypt, France, Germany, India, Italy, Japan, Mexico, New Zealan d, South Africa, Spain, Sweden, Tunisia, UK, Uruguay, USA and the form er USSR). Regressions (linear, bi-linear or tri-linear fitted with an optimisation technique) were used to evaluate the trends in yield duri ng the century. Residuals and relative residuals of these regressions were used to evaluate in absolute and relative terms, respectively, tr ends 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. Y ield trends for relatively young agricultural wheat-exporting countrie s, such as Argentina, Australia, Canada and USA, reveal an important b reakpoint ca. 2 decades earlier than European countries with longer tr adition in wheat production. Ln addition, yield gains in many countrie s have apparently been levelling off during the last decade. Trends in yield residuals during the present century revealed a decrease in yie ld stability in 14 of the 21 countries analysed, but the increase in y ield residuals was relatively small (less than or equal to 0.3 Mg ha(- 1)) compared with increases in yield. Therefore, relative yield residu als indicated that yield stability, as a percentage of yield, increase d or at least did not change for most of the analysed countries. Moreo ver, it is suggested that wheat production systems have been, in gener al, highly successful in increasing yield while maintaining or increas ing relative yield stability with respect to that existing at the begi nning of the century. Finally, no relationship was found between varia tions in yield stability, both in absolute and relative terms, and the increase in yield comparing the present values and those at the begin ning of the century. (C) 1998 Elsevier Science B.V. All rights reserve d.