Crop adaptation to density to optimise grain yield: breeding implications

Competition between crop plants, due to resource limitation, is at the root of a considerable yield limitation, a major problem that future agriculture is faced with. Due to inter-plant variation, intra-crop competition causes plant-to-plant interference and unbalanced use of input which decreases possible profit. Acquired intra-crop competition is a priori present in farming due to spatial heterogeneity. Genetically imposed intra-crop competition is due to the plant-to-plant genetic differences, i.e. the intra-species genetic competition in multi-genotypic varieties grown alone, and both the intra- and inter-species genetic competition in intercropping multi-genotypic varieties. In general, high densities accelerate the acquired plant-to-plant variation and intensify the intra-crop competition. Considering environmental diversity, an additional yield gap element is density-reliance. Density-reliant varieties are inefficient in resource use at the single-plant level and present poor results at low densities, accompanied by variation in optimum density particularly in rain-fed agroecosystems. The remedy relies on breeding of varieties that comprise the ‘weak competitor’ ideotype(s) of improved plant yield efficiency in order to mitigate the intra-crop variation and optimise the resource use across variable conditions. To focus on both, selection at nil -competition (widely spaced plants to preclude interference for inputs) is necessary. Selection among spaced plants is further supported by the negative relationship between genotype competitive and yielding ability. The derived density-independent varieties would be capable of taking advantage of the abundance of resources in favourable seasons at low densities that are suitable for dry seasons, approaching the attainable yield across locations and seasons.