Studies of the functionality of annual crop and weed diversity in polyculture cropping systems

The goal of this study was to empirically evaluate the ecological and agronomic function of plant diversity in annual cropping systems by conducting greenhouse and field experiments. In the greenhouse experiment, the effects of three different types of plant diversity (crop, barley cultivar and weed species) were compared. Increasing crop richness increased overall crop production, yield stability and weed suppression. Increasing barley cultivar richness produced much weaker effects. Increasing weed diversity resulted in an increase to overall plant (weed and crop) production and production stability. Increased light interception related to greater plant canopy height variation in diverse mixtures of plant types or species could have contributed to increases in productivity. In the field experiment, agronomic effects were investigated for different crop treatments based on three crops (spring wheat, canola and field pea), including the three-crop mixture, all pair combinations of the crops and the sole crops at two sites in Manitoba from 2001 to 2003. The effects of the different crop combinations on weed recruitment and biomass and crop production were studied in the presence and absence of in-crop herbicides. Light, water and N use were measured during the growing season. Some intercrop treatments (e.g., wheat-canola and wheat-canola-pea) tended to produce greater weed suppression compared to sole component crops, indicating synergism among crops within intercrops with regard to weed suppression. Intercrop treatments resulted in land equivalent ratios (LER) > 1 (i.e., overyielding) in both the presence and absence of in-crop herbicides. With herbicide applications, the canola-pea crop treatment produced the greatest grain overyielding (mean LER = 1.22), while without herbicide applications, the wheat-canola-pea crop treatment produced the greatest dry matter overyielding (mean LER = 1.28). There was evidence for temporal partitioning of light use and spatial partitioning of water use between crops. The presence of field pea in crop treatments reduced demands for soil nitrate and tended to result in greater % N in companion plants. In general, this study suggests that there are benefits of production, stability and weed suppression from increasing plant diversity in annual crops within a season and that these are achieved through resource use complementarity.