In mid season prime lamb production Keady and Hanrahan (2006) concluded that increasing litter size and good grassland management are the main factors affecting efficiency of production. Nolan (1972) reported lamb carcass outputs of 203 and 301 kg/ha from grass-based systems which received 77 kg N/ha and were stocked with 10 and 15 Galway ewes (mean litter size of 1.4 lambs) per hectare, respectively. Subsequently the Belclare breed was developed (Hanrahan 1997) as a genetic resource for increased prolificacy. Recently there has been interest in either reducing or eliminating the winter indoor feeding period by extended grazing. Recent studies at this Centre have shown that extended grazing in mid, late or throughout pregnancy increased lamb birth and weaning weights relative to progeny from ewes which have been housed unshorn (Keady et al 2007). The primary objective of this study was to evaluate two contrasting grass-based systems for prime lamb production. The systems adopted compared maintaining the stocking rates and levels of fertilizer nitrogen application similar to that of Nolan (1972) but increasing lamb carcass output through improved genetic capacity and either housing the flock during the winter period or maximising the use of grazed grass by year round grazing. Two ewe genotypes were used to examine if there were any interactions between level of prolifacy and the grassland based system of production adopted.