Abstract
Interactions between neighbouring plants in an ecosystem can lead to competition, even in single-species stands. Genetic selection of perennial plants based on the individual values of genotypes does not usually take into account interactions that develop over time. The purpose of this study was to ascertain whether the effects of competition might affect the performance of cacao genotypes tested over long periods, and at what point those effects begin. Competition was studied on cacao trees (Theobroma cacao L) taking into account the diameter of the trees and their yields. The trial design set up in Côte d'Ivoire was a factorial mating design of the main cacao genetic improvement programme. The approach taken was a multivariate model based on 13 years of data gathering, including genetic, spatial and competition effects. The results revealed a gradual onset of competition starting in the early years of production up to the 4 th year, when its effect became significant. It first affected growth then, 2 years later, yields. Depending on the production years, the genetic effect and the spatial effect were the greatest. In years of strong competition, it could affect up to 10 % of the annual production variability, i.e. a quarter of the variability explained by genetics. The most vigorous trees always remained highly competitive and high-yielding. The competition effect will therefore always be substantial with selections of high-yielding individuals. "Group selection" of somewhat average, less competitive individuals would help to maximize yield gains through the combined performance of the group, rather than that of individual trees.