Grass and forage maize are important forage crops in ruminant production systems in the temperate regions in northwest Europe. High yields of these crops contribute to farm profitability and local provision of feed, and hence local circularity of biomass and nutrients. Variety choice is an important option to raise potential and actual yields. We analysed 40 years of perennial ryegrass and 25 years of forage maize yield data from Value of Culture and Use (VCU) experiments to determine genetic and non-genetic trends of yields in time. For maize, we calculated an annual genetic trend of +173 kg DM ha−1 and a non-genetic trend of +65 kg DM ha−1. Further analysis of the non-genetic trend showed that maize yields increased with increasing temperature sum during the growing season, and with earlier sowing. The feeding value of forage maize showed a genetic trend of +1.7 feed unit milk (VEM) kg DM−1 year−1. The annual genetic gain of perennial ryegrass was +44 kg DM ha−1. In the grass trials we found opposing non-genetic trends for cutting and grazing. Further analysis of the non-genetic trend showed that drought and the number of days with ground frost during the growing season had a negative effect on yield. We compared the average yields and trends in VCU trials with those of on-farm yields. The on-farm maize yields showed an annual trend of +195 kg DM ha−1. We estimated an average realisation of the genetic gains of 75 % in farming practice, implying a widening gap between genetic potential and on-farm yields. Averaged over the entire period, on-farm maize yields were 4.6 t DM ha−1 (24%) lower than the yields of the VCU trials. The average annual on-farm grass yields did not show any trend, and were 1.6 t DM ha−1 (13%) lower than the yields of the VCU trials. In conclusion, our study revealed significant positive genetic and varying non-genetic trends in DM yields of forage maize and perennial ryegrass, the two dominant forage crops in the Netherlands. On-farm yields showed significant positive trends for forage maize, but no trend for grassland.