Jamieson, Fausset and Brown on Jeremiah 50:11

11. (Is 47:6).

grown fat--and so, skip wantonly.

at grass--fat and frisky. But there is a disagreement of gender in Hebrew reading thus. The Keri is better: "a heifer threshing"; the strongest were used for threshing, and as the law did not allow their mouth to be muzzled in threshing (De 25:4), they waxed wanton with eating.

bellow as bulls--rather, "neigh as steeds," literally, "strong ones," a poetical expression for steeds (see on Jr 8:16) [Maurer].