Maximizing forage yield and nutritive value in grasslands is becoming challenging, particularly in the face of climate change. The sum of temperatures (i.e. growing degree days) is used as a proxy for estimating forage yield and quality, but the validity of this index at large scale remains unclear. We conducted an experiment in 23 intensively managed Swiss permanent grasslands in 2017, 2018 and 2019. At each site, 12 plots of 1 × 5 m were established and harvested after either 2, 4, 6, or 8 weeks from the start of the growing season with 3 replicates for each harvest time. Forage yields, crude protein and fibre contents were assessed, which allowed estimating digestibility, net energy for lactation (NEL) and protein digestible in the intestine (PDIN). Despite variability in the data, we found a strong positive relationship to the sum of temperatures for forage yield, but a strong negative relationship for digestibility, crude protein, NEL and PDIN. Our data suggest that in Switzerland, mowing between 650 and 750􀂡C days provides the best trade-off between forage yield and nutritive value. Our findings show that the sum of temperatures could be integrated in decision-making tool for site-adapted management of permanent grasslands.