An article of the different stages of grass growth: Developmental Phases of Grass
By the way, these pictures were taken of the LAWN, not the pasture. The grazing goal was to trim, not maximize production.
This grass technically falls in the 6-10 inches category, but the most nutritious part has already been grazed.
This grass is 10-12 inches and has already been grazed as well. The remaining grass is "stemmy" and over mature. Percentage of stems is something to watch for when buying hay as well.