Plant growth, architecture and competition in an intercropped system

Intercropping was once widely practiced in the US and is still practiced traditionally in many parts of Africa, Asia and Latin America. The agronomic importance of intercropping has been extensively studied but the nature of ecological interactions within intercrops has not been investigated in as much detail. I conducted field and greenhouse experiments to evaluate the nature of ecological interactions in a corn-soybean intercropped system. In the field, growth of individual plants when intercropped was not better than growth of plants in monocultures despite Land Equivalent Ratios (LER's) {dollar}>{dollar}1.0. The architectural features of the plants indicated that soybean experienced intense light competition when intercropped with corn while corn was not affected by soybean. Weed community composition was influenced by crop density and by the presence of corn, but not by crop diversity per se. I performed a greenhouse experiment to examine the importance of the components of competition in corn-soybean mixtures. Competition intensity values indicated that root competition was more important in soybean while shoot competition was more important in corn. Biomass measurements relative to plants in isolation showed that competition was due to an interaction of both the root and shoot components. In another greenhouse experiment, I examined the effects of relative time of emergence on the outcome of competition. Relative growth rates of the plants were strongly affected by time of emergence. A greater combined biomass was obtained when soybean was sown earlier than corn. This study as a whole shows that interactions in intercrops are not necessarily positive as suggested in the literature but are determined by the outcome of competitive interactions between the individual crop plants and weeds. A broader knowledge of the ecology of this agronomic practice is imperative to better understand its implications for sustainability.