Structural equation modeling reveals complex relationships in mixed forage swards

•We use SEM techniques to assess weed-forage relationships in temperate pasture.•Cirsium arvense was a competitor in an upland site despite low infestation levels.•Cirsium arvense had little impact in a mesic lowland despite high weed abundance.•Legumes had marked competitive effects on grasses at both locations.•Broadleaf plant removal led to grass release and net increases in forage production.

Relationships among vegetation components in perennial pastures are complex, particularly those including grasses, weeds, legumes, and other forbs. Where herbicides are used for broadleaf weed control, a trade-off may exist between the benefits of weed removal and legume loss. Few studies have separated the contribution of different vegetation components to total forage yield, either prior to or after spraying. Herein we use Structural Equation Modelling (SEM) to quantify relationships among grasses, legumes (Medicago sativa L. or Trifolium spp.), a common noxious weed (Cirsium arvense (L.) Scop), and other forbs, at two locations within the Parkland region of central Alberta, Canada. After removal of broadleaf vegetation with herbicide, we quantified changes in forage relative yield ratio (RYR) for two years. The SEM approach revealed marked differences in the relationships among sward components between sites. At the more mesic site, abundant thistle biomass had little influence on other sward components and no benefit was observed post-spraying from weed removal. In contrast, even low levels of thistle biomass suppressed grass and legume biomass at the more xeric location, and post-spraying responses revealed benefits from weed removal. Unexpectedly, legumes were found to suppress grass biomass at both sites, suggesting strong interspecific competition between forage types. Subsequent removal of legumes appeared to release grass biomass from competition within sprayed plots, as exemplified by increased forage yields two years after spraying. These results highlight the complexity within temperate perennial pastures, and add clarification to the potential short-term impacts of weeds and legumes to overall sward dynamics and forage production.