Breeding progress of grain and forage maize in long-term variety trials compared to on-farm yield development

Yield gaps between variety trials and on-farm yields show diverging trends for grain versus forage maize in Germany in 1987-2023. The yield gap between variety trials and on-farm reduced for grain yield from 30% (1987) to 23% (2023), while for forage maize dry matter yield increased from 15% (1987) to 32% (2023). In variety trials during the same period starch yield in forage maize showed a moderate increase of 14%, while digestibility and starch content did not change over time. N use efficiency in variety trials was improved by 35%, 27% and 18% for grain, dry matter and starch yield, respectively, while N yield in dry matter did not change over time. Maize cultivation increased significantly in Germany over the past 25 years. With a share of over 20% of arable land, maize has become the second most important crop after wheat, primarily due to the growing demand for biogas production. Based on long-term variety trials for grain and forage maize, we quantified breeding progress applying mixed linear models extended by linear and nonlinear regression terms to estimate time trends between 1987 and 2023. Grain yield increased by 33.4 dt ha^-1 (36.3%) and dry matter yield of forage maize by 36.1 dt ha^-1 (19.9%) compared to 1987. Over the last 15 years, there has been a slowdown in upward yield trends. In addition, the NUE of grain and forage maize increased by 35.0% and 27.2%, respectively. From 1987 to 2023, grain yield gaps between variety trials and national on-farm yields reduced from 30.1 to 22.6% while the stagnation of on-farm forage maize yields resulted in an increased yield gap from 15.1 to 32.1%. This diverging trend can be attributed to a complex set of reasons, such as climate change, management practices and economic constraints. Looking at quality traits in the variety trials, starch content and digestibility of forage maize did not change, but starch yield (14.0%) and NUE of starch yield (18.3%) increased, while N yield of forage maize decreased by - 4.7%, though not significant. Our study shows that breeding progress of grain maize was successfully transferred into increasing on-farm yields, while a considerable yield gap remains for forage maize, what calls for additional research.