An index of competition reduces statistical variability and improves comparisons between genotypes of Miscanthus

It is important for breeders and producers to be aware of competition effects for variety trials. We aimed at developing an index of competition to reduce statistical variability in field trials and improve comparisons between genotypes of Miscanthus. Twenty-one clones belonging to four species of Miscanthus (M. x giganteus, M. floridulus, M. sinensis, and M. sacchariflorus) planted at the same density were compared at two harvest dates during the second and third crop years. Aboveground volume was shown to be a good predictor of the aboveground biomass of the clones, and was analysed for the competition effect. The best competition index was the ground area occupied by the eight neighbour plants, among the four indices defined as covariates in the statistical models. It reduced the root mean square error of the aboveground volume by as much as 17 %, explaining up to 36 % of the residual error of the model. Our results then concerned the contribution of intra- genotypic competition of Miscanthus to the variability between plants of a same clone during field trials, the relationship between competition ability and plant traits, and the comparison of genotypes regarding this competition. All clones showed negative competition sensitivities depending on harvest date, crop year, and clone. The competition effects lead to reduction in mean aboveground volume by up to 17 %. Competition sensitivities were strongly correlated with aboveground development (height and yield) in both crop years, whatever the harvest dates. In Miscanthus field trials, using a competition index may help to reduce statistical variability and improve comparisons between genotypes