Quality versus quantity: spring wheat seed size and seeding rate effects on Avena fatua interference, economic returns and economic thresholds

A 3-year field experiment was conducted at Kalispell, Montana, USA, to investigate the effects of spring wheat seed size and seeding rate on wheat yield loss (YL), economic returns and economic thresholds (ETs), as a function of Avena fatua density. Crop competitive ability increased as wheat seeding rate and seed size increased, with the greatest differences among treatment factors being observed at low weed densities. Both treatment factors decreased spring wheat YL, increasing economic returns during all 3 years of the study despite the higher associated seed costs. Averaged over all other factors, adjusted gross returns (AGR) were 477 and 537$ ha⁻¹ for the low and high seeding rates, while values of 453, 521 and 547$ ha⁻¹ were obtained for the small, bulk and large seed size classes respectively. Weed-free yield potential varied yearly. As yield potential increased, A. fatua competitive effects were more evident and ETs decreased. Nonetheless, both treatment factors increased ETs in 2 of 3 years. These results demonstrate that the use of higher seeding rates and larger seed size classes both improve wheat competitive ability towards A. fatua while simultaneously increasing economic returns.