THE AREA UNDER THE FUNCTION - AN INDEX FOR SELECTING DESIRABLE GENOTYPES

The linear regression approach has been widely used for selecting high -yielding and stable genotypes targeted to several environments. The g enotype mean yield and the regression coefficient of a genotype' s per formance on an index of environmental productivity are the two main st ability parameters. Using both can often complicate the breeder's deci sion when comparing high-yielding, less-stable genotypes with low-yiel ding, stable genotypes. This study proposes to combine the mean yield and regression coefficient into a unified desirability index (D(i)). T hus, D(i) is defined as the area under the linear regression function divided by the difference between the two extreme environmental indexe s. D(i) is equal to the mean of the i(th) genotype across all environm ents plus its slope multiplied by the mean of the environmental indexe s of the two extreme environments (symmetry). Desirable genotypes are those with a large D(i). For symmetric trials the desirability index d epends largely on the mean yield of the genotype and for asymmetric tr ials the slope has an important influence on the desirability index. T he use of D(i) was illustrated by a 20-environments maize yield trial and a 25-environments wheat yield trial. Three maize genotypes out of nine showed values of D(i)'s that were significantly larger than a hyp othetical, stable genotype. These were considered desirable, even thou gh two of them had slopes significantly greater than 1.0. The results obtained from ranking wheat genotypes on mean yield differ from a rank ing based on D(i).