A study was carried out on six lowland farms in Northern Ireland over 2 years to examine the effect on labour input and lamb output of adopting an outdoor (grass-based) lambing system in comparison to an indoor lambing system. Each farm had four crossbred ewe genotypes – Bluefaced Leicester ✕ Scottish Blackface (BLXB), Texel ✕ Scottish Blackface (TXB), Suffolk ✕ Cheviot (SXCH) and Texel ✕ Cheviot (TXCH) ewes. The ewes were divided into three groups balanced for live weight and condition score and mated with Texel, Suffolk or Beltex rams. Half of the ewes lambed indoors and the other half were turned out to grass three to 6 weeks before lambing and lambed outdoors. BLXB ewes were the most prolific of the four ewe genotypes producing 1·99 lambs per ewe lambed compared with 1·75 for TXB, 1·76 for SXCH and 1·64 for TXCH (P < 0.001). A greater proportion of SXCH ewes lambed without assistance (0·82) compared with BLXB (0.70), TXB (0.71) and TXCH (0.74) ewes (P < 0.01) and subsequently less time was spent lambing SXCH ewes (P < 0.01). This advantage of the SXCH was more marked in the outdoor than in the indoor system (P < 0.01) in terms of lamb output, each of the four ewe genotypes performed similarly in the indoor and outdoor lambing systems. Lambs born in the outdoor system weighed 0.2 kg more at birth compared with those born indoors (P < 0.05). Lamb mortality and lamb output were similar in both systems. Less time was spent catching and moving ewes in the outdoor system compared with the indoor system (P < 0.01). Beltex-sired lambs weighed less at birth (4.9 kg) compared with Texel- (5.1 kg) (P < 0.10) and Suffolk-(5.3 kg) (P < 0.001) sired lambs and had lower live-weight gains from birth to 6 weeks and from birth to weaning (P < 0.001) and lower output of weaned lamb (P < 0.05). The results of this study demonstrate that, compared with indoor lambing systems, similar levels of performance can be achieved through adopting outdoor, grass-based systems. The lack of a significant interaction between ewe or ram genotype and lambing system indicates that all of the genotypes are suitable for incorporating into an outdoor lambing system.