Have changes in yield (1900-1992) been accompanied by a decreased yield stability in Australian cereal production?

Abstract

Regression analyses of the relationships between yield and time over the last century were conducted for wheat, barley, oats, maize, and rice at a national level and those for the three winter cereals in New South Wales (NSW), Victoria (Vic), South Australia (SA), Western Australia (WA), and Queensland (Qld). The fitted function accounts for the impact of agronomic improvement on yield and the residuals stand for the variability of yield beyond this impact. Another measure of the stability was obtained by considering the residuals as a proportion of actual yield ('relative residuals'). Crops varied greatly in their yields but there was no relationship between the coefficient of variation and average yield amongst these cereals. As expected, yields of all five crops are presently higher than early this century. The residuals and relative residuals for each crop were plotted against years. Residuals of wheat yields over time showed no increase or decrease during the century, but the relative residuals exhibited a clear trend towards an increased yield stability. Barley and oats showed sirnilar patterns to those of wheat. Residuals of maize yields increased with time, but relative residuals remained unchanged. Rice yields exhibited the largest residuals, but no trends with time. Yield stability, measured as a proportion of yield, has been clearly improved during this century. Yield trends for the winter cereals at a State level confirmed the results observed at a national level. In conclusion, yields of newer cultivars grown under modern management were higher and more stable, or at least not less stable, than those of crops grown early in the century. Across States and crops, there was no relationship between yield stability and yield.