For forage production to be efficient and environmentally sustainable, the extent and timing of nitrogen fertiliser applications should match the uptake and growth capabilities of a grass crop. A two‐year field experiment, comprising four diverse grass cultivars, was conducted to assess the impact of two contrasting N‐application rates on forage and root biomass (RB) production and nitrogen‐use‐efficiencies (NUEs). Replicated field plots of perennial ryegrass, tall fescue and two Festulolium (ryegrass × fescue hybrid) cultivars were compared at Low N (LN) 178 and High N (HN) 356 kg ha−1 over 2 years. HN applications increased dry matter yield (DMY) in Year 1 (p < 0.05) but not in Year 2. Ryegrass outyielded all in Year 1 but in Year 2, fescue had the highest DMY at HN (p < 0.05), but cultivars did not differ in yield at LN. Festulolium yields were consistently intermediate. Root biomass at LN in Yr1 was highest in the Festulolium (Lolium perenne × Festuca arundinacea var glaucescens) (p < 0.05). For all grasses, and in both years, mean RB and nitrogen use efficiency (NUE) and nitrogen utilisation efficiency (NutE) were higher under LN, than HN. Nitrogen uptake efficiency (NupE) was similar in all grasses in Year 1, irrespective of N treatment, but in Year 2, excepting tall fescue, was greater in grasses grown under LN. Increasing RB correlated (p < 0.05) with improved NUE and NutE, but no association was evident for NupE. Grass cultivars differed in their response to nitrogen applications. Whilst relative forage production of ryegrass and fescue contrasted over the 2 years, forage yields of Festulolium cultivars were more consistent. In conclusion, HN application depressed NUE by productive grass cultivars and correlations between RB and NUEs may indicate opportunities to help tailor grass cultivar/fertiliser combinations and achieve sustainable forage and root production.
