Inadequacy of blocking in cultivar yield trials

Abstract

Based on the literature, theoretical considerations and a numerical example on triticale, Complete Randomized Blocks design is shown to be inadequate for cultivar yield trial purposes. Assumptions required for validity and convenience are shown not to be verified throughout most of the published experiments as well as in the present numerical example. It has been referred to the difficulty in forecasting homogeneity within blocks together with heterogeneity between blocks. This is difficult to achieve even in wellknown experimental fields, let alone local fields chosen at random, which leads to unacceptably low correlation between plots from block to block in each trial. Heteroscedasticity, as supported by different regression coefficients in Joint Regression Analysis, does not allow for ANOVA, unless the overall variation of soil fertility level is reduced to an amount comparable with that expected for the unknown errors. In this instance, the loss of degrees of freedom in the two-way ANOVA is known not to be compensated for by block effect deduction. The need to generalize trial results calls attention to the emphasis that should be given to cultivar performance pattern within the area they are to be released. Thus, we advocate the need for precise point evaluations in order to give accurate estimation of that pattern. Genotype-environment interaction, within situations where ecological diversity does not include stress mechanisms of different natures, can be reasonably described through its genotype-fertility level component, since specific instability, related to climatic features, is supposed to be strongly reduced by the screening process of both cultivar production and introduction. Sensitivity of the regression techniques (even through robust methods) requires a broad range of trial fertility levels and, besides an adequate number of degrees of freedom and detection of eventual remaining specific instabilities, demands an experienced evaluation of particular ecological situations; however, randomization is not required except within trials, which should be designed as completely randomized. To carry on trials beyond one year is not an a priori demand and should only be considered when very abnormal seasonal conditions occur.