In The Scythe Book, Tresamer talks about the tremendous diversity of plants that old-time pastures and hay fields used to have, and he also briefly mentions how farmers used to make special herbal hay with their scythes, for the health of their animals, and ultimately themselves. Here in this video below, a mountain farmer from Murau, Austria, explains the optimal stage at which to cut an herbal meadow, for the first cutting of hay, for maximum herbal strength and mineral content. She also talks about the healing properties of some of the herbs, and how much her cows enjoy and benefit from them. Modern fields have much less diversity, which is why she heads up high into the mountain to cut the old hayfields that mechanised farming hadn't adulterated.
As for timing the first cut, conventional wisdom now is that the best time to mow for hay, is before the grass has started to head out, because it has the most protein at this stage. But from what I've read in old German scythe literature, the traditional time to mow the first cutting was "when the meadow is in bloom", as Maria Berger (Women's World Scythe Champion in 2000) is explaining in the video. Usually around St. John's Day, which is close to the summer solstice. Also happens to coincide with the longest day length of the year, for the quickest drying time of this massive volume of hay. To everything there is a season.