Man-managed pasturelands cover more than 30% of Earth’s surface and are an essential ecological and economic resource. The dynamics of forage quality (protein and fiber content) as feed for livestock is complex since different plant functional types can determine forage quality via their different phenology, which depends on environmental and biotic conditions. Forage quality monitoring requires frequent and accurate measurement over extensive areas, which is not feasible by traditional field sampling. We studied the effects of the grazing regime on forage quality in semiarid and Mediterranean grasslands by developing a comprehensive pasture quality index (PQI) and monitoring quality via remote sensing tools. We hypothesized that nutritional quality would decrease throughout the growing season as plants mature, and that the plant functional composition would be affected by prolonged heavy grazing, consequently affecting quality. We utilized a multi-scale approach of frequent multispectral satellite imagery and fine-resolution drone imagery, combined with machine learning regression versus field sampling measurements, in two long-term ecological research sites in Israel. Overall, our results show that the plant functional community composition was affected by the grazing regime, with a high occurrence of unpalatable thistles, mainly under intensive grazing. The overall PQI showed higher values at the mid-growth (winter) compared to peak growth (spring). The highest values were observed under intermediate grazing in the semiarid site. The results allowed accurate, comprehensive, large-scale, and continuous monitoring of forage quality changes in the heterogeneous Mediterranean and homogeneous semiarid grasslands, gaining insight into sustainable livestock management under the changing climate conditions.