Sure it wasn't seeded for pasture? It's definitely grass, but which is the question :)

Is this the same field as the grain crop you asked about? In which case it wasn't seeded. However, if it is a different field farmers do seed grass. That's how they get pasture as vera said. Usually a mixture including various rye grasses.

Could be Lolium perenne which is really shiny on the back of the leaves/ blades. It is a kind of high calory grass, so yes probably pasture.

Could be that the rest of the mix takes longer to germinate.