<p>Grasses represent a major forage resource, which supports livestock productivity and global grazing systems. Despite extensive research on model plants and crop plants, grasses remain understudied under UV-B exposure. Insight into how UV-B affects growth, metabolites and the nutritional quality of grasses is still less explored. So, the present study aims to examine elevated UV-B (ambient + 7.2&#xa0;kJ&#xa0;m⁻² d⁻¹) effects on growth, nutrition, and forage quality of <i>Megathyrsus maximus</i>, a valuable and palatable grass. Despite variations in leaf dry weight, total biomass remains unchanged. Accumulation of primary metabolites (starch, sucrose, soluble and reducing sugar) reaches its peak at the vegetative stage. Protein and tannin content increased by 83% and 92% at the reproductive and vegetative stage, respectively. Forage quality analysis denotes increased dry matter content initially, while crude protein increased only at vegetative stage. Neutral detergent fiber, acid detergent fiber, and acid detergent lignin increased at all stages, whereas ash content reduced at first two stages. Furthermore, plants exhibited a decrease in key nutritional parameters, including dry matter index (DMI), digestible dry matter (DDM), and total digestible nutrient (TDN). Hemicellulose increased initially by 14% but dropped by 10.8% at the reproductive stage, showing a stage-specific response. The present findings indicate that UV-B treatment did not alter growth but stimulated the defense pathway, increasing sugar accumulation. Reduced DMI, DDM, and TDN and increased lignin, suggesting UV-B negatively affects the palatability and nutritional value. So, excessive UV-B radiation not only alters the fodder quality but also the functioning of the grassland ecosystem.</p>