Abstract Natural grasslands surrounding cropping fields can act as reservoirs of plant species that influence weed establishment in cropping systems. Understanding these interactions is important for explaining changes in plant community assembly and vegetation composition across agricultural landscapes. This study compared species composition between manured maize ( Zea mays L.) cropping fields and natural grasslands. Additionally, it identified environmental factors that influence the distribution and assembly of vegetation in natural grasslands. The study was conducted in two agroecological regions (Highveld and Middleveld) of Eswatini. Transects in natural grasslands and maize cropping fields were used to sample plant species composition and density. Stacked-species Distribution Models (s-SDMs) were used to predict the assembly of natural grassland vegetation using climatic, soil, and socio-ecological variables. Weed species richness (S), diversity ( H ’), and dominance (D) were greater in manured maize fields than in natural grasslands. Model selection based on the Akaike information criterion (AIC) identified 10 factors explaining patterns in species abundance, richness, diversity, and dominance. Phosphorus influenced all diversity indices. Temperature, soil pH, and elevation influenced vegetation abundance, while soil texture affected vegetation abundance, richness, and diversity. About three-quarters of natural grasslands in the Highveld had the highest potential species richness, whereas more than half in the Middleveld had the lowest potential species richness. A combination of environmental variables influences the assemblage of plant species in grasslands, which, when grazed by cattle and introduced to maize cropping fields through uncured manure, may increase species richness and diversity, challenging weed management.