>why not produce plants with, e.g., much bigger seed heads? That would give increased crop yield, plus you'd get negative selection pressure for horizontal gene

>why not produce plants with, e.g., much bigger seed heads? That would give increased crop yield, plus you'd get negative selection pressure for horizontal gene transfer --- the trait would be disadvantageous for weeds and so wouldn't be selected for. The higher energy requirements make crops more susceptible to weeds, which exacerbates the problem you were trying to solve.