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 (Deu 25:4), they waxed wanton with eating.
bellow as bulls - rather, “neigh as steeds,” literally, “strong ones,” a poetical expression for steeds (see on Jer 8:16) [Maurer].