<p>Forage species that escape from sown pastures can invade natural grasslands, altering their composition and functioning. Native plants and wild herbivores may limit exotic species, whereas domestic herbivores and nutrient enrichment often favour invasion in the short term. However, the long-term interplay of top-down (herbivores) and bottom-up (nutrients) forces in controlling forage invasions remains uncertain. We evaluated how cattle grazing exclusion and nutrient addition influenced the invasion of <i>Festuca arundinacea</i> (tall fescue; syn. <i>Lolium arundinaceum</i>) in the Flooding Pampa (Argentina). Using data collected over almost 30&#xa0;years (1989–2016) from successive phases of a long-term manipulative study, we evaluated (i) the effects of cattle exclusion on plant species composition after invasion and (ii) the combined effects of cattle exclusion and nutrient enrichment (NPK) on tall fescue, plant groups, and diversity. Tall fescue first appeared in the study area in 1995, when cattle exclusion still promoted native grasses. By 2016, it dominated cattle exclosures (&gt; 80% cover) but remained scarce in grazed areas (&lt; 5%). Nutrient addition with exclusion increased the size of tall fescue individuals and reduced native cover and total richness. Declines in richness reflected both a direct negative effect of nutrients and the indirect effect of tall fescue expansion facilitated by cattle absence. In sum, cattle exclusion initially maintained native grass dominance but later increased susceptibility to invasion, ultimately enabling invasive forage dominance, particularly under nutrient enrichment. Long-term interactions between grazing and resource availability strongly shape invasion outcomes. In the face of increasing forage invasions, limited nutrient inputs combined with domestic grazing may help sustain native plant communities.</p>