There is no advantage to mixed grass in a hay field. The grasses all have similar yields and compositions, and besides, after a few years one predominates no matter how many you planted. The thick stems dry very slowly, often leaving one with the choice of baling it wet and getting moldy hay, or letting the hay get rained on. Red clover hay is always dusty.
Grasses do peak at different times, but they all become somewhat dormant in mid-summer.
Legumes are very helpful here, as they do well in heat, and have deep roots to get water in dry weather. Red clover suffers from toxic molds which cause slobbers and photosensitivity. Fortunately, it dies out after about two years. Alfalfa does not tolerate grazing well. Birdsfoot trefoil is hardy, tolerates grazing, makes good hay, etc etc.