Compensatory mechanisms associated with the effect of spring wheat seed size on wild oat competition

Crop seed size affects the competitive relationship between spring wheat (Triticum aestivum L.) and wild oat (Avena fatua L.). However, the mechanisms associated with the process are not known. The effect of wheat seed size on wild oat competition was assessed by a mechanistic approach involving yield and its determinants in these species. Wheat plants established from large and small seed were evaluated under different seeding rates and wild oat densities during 1999-2001 near Kalispell, MT. Linear structural model systems based on ontogenic diagrams were constructed for each seed size class. Spikes m(-2) and panicles m(-2) had the greatest positive effect on yield within each species. For wheat, the impact of the two later-formed yield components on yield decreased in an ontogenic manner, whereas for wild oat, their relative contributions were similar in magnitude. Wheat plants derived from large seed had a noticeable negative effect on wild oat via a reduction in panicles m(-2) and seed weight, whereas wheat established from small seed mainly affected wild oat panicles m(-2). Wild oat competition reduced wheat spikes m(-2) and kernels spike(-1) in both seed size classes. However, these reductions were less for plants derived from large seed, which demonstrated enhanced compensatory ability. In summary, nongenetic variations in crop seed size affected the competitive dynamics between these species, where the major crop-weed interference mechanism involved wild oat seed weight.