Phenotyping of complex traits and particularly those associated with canopy development, biomass and yield is challenging when experiments comprise diverse genotypes. This is especially so when confounded with variation in traits such as height and maturity which are known to affect yield. Limited seed and resources may encourage field-assessment in single, spaced rows or smaller, unbordered plots. Competition for water, light and nutrients necessary for canopy growth will be variable and non-definable where immediate neighbours are genetically different, and particularly under stress conditions where competition for survival and thereby productivity is greatest. Response to changes in resource availability will vary among diverse genotypes to alter genotype ranking and reduce heritability. In turn, the relevance of such growing conditions to commercial field-grown plots is at best unclear and at worst dubious.
The planting of multi-row plots and the simple exclusion of plot borders at harvest will increase experimental precision and confidence in genotype response when inferring to performance in target environments. This is demonstrated in a simple study where six wheat genotypes were assessed for spike number at two nitrogen levels: basal and a supplemental 40 kg/ha N predrilled at sowing (Fig. 1). The outside, border rows averaged 56% greater spike number than for centre rows, with variation between genotypes presumably due to their internal tillering propensity. Over all six genotypes, the association for border and centre-row spike number was 0.50ns and 0.74ns for basal and supplemental N treatments, respectively. This lead to the border rows having a 54% yield increase compared to the centre rows, and a poor association (0.48ns) for yield of the border compared to the guarded centre-rows.
Well-planned studies and particularly those focusing on the dynamics of yield formation (canopy-related characteristics) must consider the use of small plots containing multiple rows and with border rows to minimize the effects of interplot competition. Whether these border rows are best excluded, or considered as part of genotype performance will depend on the environment but we recommend they be excluded, especially where stress (eg water deficit) has been applied.
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