Data from each of 5 commercial, extensive sheep farms in Cumbria, UK were used as parameters in a linear program (LP) representing labour and grazing management in such farming systems. The LP maximised ewe enterprise gross margin subject to constraints dictated by the labour availability and land types on each farm. Under the assumptions used, labour availability and price restricted ewe numbers well below those observed in practice on 2 farms i.e. land resources were adequate for the farming system practiced. On two other farms stocking levels and hence returns were limited by the availability of forage and hence feed input prices relative to output. On one farm, greater grassland productivity was the key determinant of system performance. It was concluded that a holistic systems approach was needed to properly evaluate these farming systems in terms of their potential contribution to animal welfare, land use, profit and hence their sustainability