A significant drop of 2.4-2.9 percentage-points (%-p) in crude protein content (on dry matter basis) in Danish grain crops was observed from around 1990 to 2015. The aim of the analysis was to quantify the effect of the change in N-response of varieties of winter wheat (WW) and spring barley (SB) under farmers’ conditions based on approx. 1000 fertilizer trials with WW and 500 with SB carried out in farmers’ fields from 1987 to 2015. Results indicated that for WW receiving 129-190 kg N/ha, DM yield increased 0.8-0.9 Mg/ha. The amount of harvested N in grain increased 5-11 kg N/ha, and % N in grain dropped by 0.1-0.05 %-points; the largest drop was found for low N-additions. For SB, receiving 93-129 kg N/ha, DM yield increased by 0.9-1.1 Mg/ha. The amount of harvested N in grain increased by 4-6 kg while the N % in grain dropped 0.2-0.24 %-points. For both crops, the effect of plant breeding is likely to be confounded with the effect of increased CO2. Adding up effects of changed N-response and fertilizer rates only explained part of the observed reduction in crude protein content.