Context. The effect of long-term gypsum on nitrogen (N) responsiveness in black oat (Avena strigosa Schreb.) forage lacks information.

Aims and methods. This study evaluated interactions between different rates of N (0, 50 and 100 kg N ha^-1) and gypsum (0, 3, 6, 9 and 12 Mg ha^-1) on the yield, nutrition and bromatology of forage black oats in a long-term experiment using a no-till system (NTS) in a Ferralsol in southern Brazil.

Key results. There was an interaction between residual gypsum and N rate on biomass forage yield in 2015, but not for the 2016 growing season. In 2015, the rate of 100 kg N ha^-1 increased yield by 10% in the absence of gypsum and by 30% at low-medium rates of residual-gypsum (3 and 6 Mg ha^-1), but there was no response to high N rate under high gypsum rates (9 and 12 Mg ha^-1). In 2016, a poor-growing season, a gypsum rate of 6 Mg ha^-1 and N rate of 100 kg ha^-1 independently improved forage yield by 5%. Gypsum application produced higher forage contents of N, P, K, Ca and S, and decreased Mg content. N-fertilisation increased the N, K, Ca, and Mg contents, but decreased P content and S concentration. Gypsum had no effect on forage bromatological quality, and even with equivalent forage yield between 50 and 100 kg N ha^-1, only the high N rate improved some of the forage quality attributes in both years.

Conclusions and implications. Neither gypsum nor topdressing of N altered N use by black oat forage.