Yield improvement and associated characteristics of some Australian spring wheat cultivars introduced between 1860 and 1982

Twenty-eight Australian wheat (Triticum aestivum L. em. Thell.) cultivars representing a series from the 1860s to 1982, were grown in 20 field trials over four years in the wheatbelt of Western Australia. The cultivars included introductions and selections made before 1900, plus important cultivars bred or grown in Western Australia up to 1982. Five of the latter group were from crosses including semidwarf cultivars as parents. Grain yields were measured on all trials, and six trials were also sampled for biomass and yield components.Based on the regression of mean grain yield versus the number of years elapsed since 1884, yields have increased from 1022 kg ha-1 in 1884 to 1588 kg ha-1 in 1982. This represents a rate of increase of 5.8 kg ha-1 year-1 or 0.57% per year. Regression of cultivar yield on site mean yield gave values of b, the slope of the regression, from 0.66 to 1.24, and these were higher for modern than for old cultivars.In six trials sampled for yield components, above-ground biomass appeared to have increased slightly when comparing early selections and their derivatives with later cultivars, but over 80% of the overall increase in grain yield was due to increase in harvest index. Grains per car and grains m-2 were strongly and positively correlated with grain yield, but there were weak negative correlations between 1000-grain weight and yield, and between 1000 grain weight and years since 1884. Cultivars with a semi-dwarf background had equal biomass, but higher yield, harvest index, ear number m-2 and grains ear-2 than modern tall cultivars. The results show that genetic improvement has substantially increased yield potential in this environment and that this has been achieved through substantial increases in grain number m-2 associated with an improvement in harvest index.