Ewe prolificacy potential (PP; predicted number of lambs born per ewe per year) and stocking 314 rate (SR; ewe per ha) are two primary drivers of output in temperate grass-based lamb 315 production systems. The aim of this thesis was to investigate and quantify the effect of ewe 316 PP, SR, and their interaction on animal performance, pasture production and utilisation and 317 the efficiency of lamb production in a grass-based production system. A 2 x 3 factorial design 318 study, consisting of two ewe PP ((medium prolificacy potential (Suffolk X ewes; 1.5 lambs 319 reared per ewe) and high prolificacy potential (Belclare X ewes; 1.7 lambs reared per ewe)) 320 and three SR: low (10 ewes per ha), medium (12 ewes per ha), and high (14 ewes per ha) was 321 conducted. Each treatment was managed in a rotational grazing system. Measurements taken 322 included; ewe body weight, ewe body condition score (BCS), number of lambs born and 323 weaned per ewe and per hectare, lamb growth rate, days to slaughter, lamb carcass traits and 324 output, ewe production efficiency (kg lamb live weight weaned: kg ewe live weight mated), 325 herbage dry matter (DM; kg) production and utilisation, sward quality and morphology, and 326 DM and energy (Unite fourrage laite per kg DM; UFL) consumption. High PP ewes produced 327 more lambs both per ewe and per hectare, with HP lambs achieving a higher average daily 328 gain (ADG) on a per hectare basis and yielded a higher lamb carcass output per hectare 329 compared to MP ewes. The total quantity of DM and UFL consumed per ewe and lamb unit 330 for the full production year did not differ by ewe PP. The HP system required a lower quantity 331 of DM and UFL to produce a kilogram of lamb carcass. The use of higher stocking rates 332 demonstrated the potential to increase lamb carcass output per hectare in a grass-based lamb 333 production, with the LSR and MSR systems achieving similar levels of performance for pre-334 weaning lamb ADG and days to slaughter. Increasing stocking rate increased herbage 335 production, utilisation and sward quality and leaf content. Limitations to increasing stocking 336 rate above 12 ewes per hectare in a grass-based lamb production system due to reductions in 337 individual animal performance and increases in DM and UFL consumption per ewe and lamb 338 unit and per kilogram of lamb carcass produced at the HSR were recorded. The findings from 339 this thesis demonstrate the potential to increase lamb output and the efficiency of lamb 340 production from a temperate grass-based lamb production system through targeted increases 341 in ewe PP and SR levels.