The objective of this study was to investigate the effects of sowing on hay yield and quality of oat genotypes. Sixteen oat genotypes were grown over the consecutive four growing seasons in Samsun, North of Turkey. Hay yield and quality were significantly different between genotypes and sowing dates. Hay yield, Crude Protein (CP), Acid Detergent Fibre (ADF), Neutral Detergent Fibre (NDF), Total Digestible Nutrients (TDN), Relative Feed Value (RFV), Ca, K, P and Mg contents of hay were determined. Hay yield of the oat genotypes in the autumn sowing was higher than in the spring sowing while hay quality (CP, ADF, NDF, TDN, RFV and some elements) in the autumn sowing was lower than in the spring sowing. The hay yield of late-maturing and tall genotypes like Yesilkoy-330, Yesilkoy-1779, Faikbey and Seydisehir (12.1, 12.2, 12.4 and 12.9 t ha(-1), respectively) were higher compared with the other genotypes in sowed in. autumn. But quality of these genotypes in autumn sowing was lower than the other genotypes. In spring sowing, genotypes Samsun and Kupa had the highest hay yield and fairly high quality. Ca, K and P contents of hay were adequate for ruminants in both sowing dates but Mg content was not adequate. This study showed that sowing date had great effect on hay yield and quality potential of oat genotypes.