In a small target region, it may be possible to exploit local adaptation to increase gains from selection. However, in a large region more extensive testing is usually possible, resulting in more precise estimation of genotype means. A correlated response model was adapted to determine if division of a large target region is likely to increase gains. Genotypic value in a large region and constituent subregions are considered correlated traits. Correlated response in a subregion to indirect selection across the undivided region, relative to direct response to selection within the subregion, is expressed as a function of heritability in the undivided region (H) and in the subregion (H_i ), and of the genotypic correlation between region and subregion means (r_G′). r_G′ depends on the magnitude of the genotype × subregion interaction (σ²_GS) relative to the genotypic variance (σ²_G). σ²_GS is the portion of the genotype × location interaction (σ²_GL) caused by local adaptation, rather than by random site‐to‐site variability in genotype means. Subdivision can increase heritability through the addition of σ²_GS to the numerator of H_i, but this may be offset by reduced replication across locations within the subregion. Modeling using variance estimates from several cereal programs indicated that, unless σ²_GL is large relative to σ²_G and at least 30% of σ²_GL is due to σ²_GS, subdivision is unlikely to increase response. These results help explain the success of breeding programs that test broadly.

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