Definition: A low, green plant that grows naturally over a lot of the earth's surface; plants eaten by grazing animals.
Q: Can it also be used to mean 'hay' and 'straw?' Or, does it mean only grass and grassy?
A: Specifically for 'hay', one signs #HAY. Sometimes one can sign GRASS (not necessarily the green grass but grass in general) when talking about a cow eating grass/hay.