AbstractContribution of nitrogen (N) and plant breeding to yield formation of oats (Avena sativa L.) was evaluated with two years'field experiments. These consisted of four Finnish oat cultivars released between 1935 and 1988 and one breeding line. Three rates of N application were used (0, 90, and 120 kg N ha−1) and 19 morpho‐physiological traits, including maturity class, plant structure and plant stand structure, were assessed.The modern cultivar out‐yielded the old one under low and high N, indicating that return of old varieties into cultivation under low‐input conditions is unlikely to be feasible. An oat ideotype for low productivity conditions must be characterized. Furthermore, N contributed more than oat breeding to several traits studies. However, in some cases N appeared to affect the traits measured in a negative manner. Use of N increased production of both above‐ground vegetative biomass and panicle weight without changing HI significantly. However, plant breeding slightly reduced the production of vegetative phytomass and increased the grain: straw ratio by 10 %. Moreover, genetic gains in shortening straw length, increasing lodging resistance, and improving BYDV tolerance were partly lost when high rates of N application were used. The contribution of N and oat breeding to other morpho‐physiological traits is discussed.