Oat (Avena sativa L.) traditionally has been a major crop for feed and forage in Turkey. The objective of this research was to study hay yield and quality of oat genotypes harvested at the late milk stage. One hundred oat varieties of worldwide origin were compared in field experiments in Samsun (northern Turkey) over two growing seasons (2007-2008 and 2008-2009). Significant differences between the tested oat varieties were observed for the plant height, hay yield, crude protein (CP), acid detergent fibre (ADF), neutral detergent fibre (NDF), total digestible nutrients (TDN), relative feed value (RFV) and macro minerals (Ca, K, P and Mg). Plant height varied from 76.2 to 141.2 cm, hay yield from 6.03 to 11.83 t ha(-1), crude protein from 58.8 to 136.4 g kg(-1) dry matter (DM), acid detergent fibre from 333.2 to 424.8 g kg(-1) DM and neutral detergent fibre from 522.5 to 652.4 g kg(-1) DM. The TDN ranged from 465.1 to 583.3 g kg(-1) and relative feed value from 80.9 to 112%. Cluster analysis grouped the 100 genotypes within 7 clusters, each of which having 17, 21, 13, 12, 20, 12 and 5 genotypes. Sisko, Akiyutaka, Longchamp, Sanova, Flamingslord, Matra and Revisor were identified as the high hay yield potential genotypes. However, quality traits of these genotypes were lower than some of other genotypes. Furthermore, while some macro minerals were insufficient, others were in excess regarding healthy feeding. Hence, some form of commercial mineral supplement would be required to oat-based ration or oat should be grown in mixtures with legumes for feeding productive livestock.