Contradiction with Genesis 1:29
Genesis 1:29 states that all plants and trees with seeds are given to humans for food, which contradicts Job 40:20 indicating that grass is for beasts.
Genesis 1:29: And God said, Behold, I have given you every herb bearing seed, which [is] upon the face of all the earth, and every tree, in the which [is] the fruit of a tree yielding seed; to you it shall be for meat. [bearing...: Heb. seeding seed] [yielding...: Heb. seeding seed]
Contradiction with Isaiah 11:7
Isaiah 11:7 describes a future time where lions eat straw like oxen, conflicting with Job 40:20 where the beasts feed on grass in the context of their current nature.
Isaiah 11:7: And the cow and the bear shall feed; their young ones shall lie down together: and the lion shall eat straw like the ox.
Contradiction with Romans 14:20
Romans 14:20 declares all things are clean for eating, which contradicts the designation of grass specifically for beasts in Job 40:20.
Romans 14:20: For meat destroy not the work of God. All things indeed [are] pure; but [it is] evil for that man who eateth with offence.
Contradiction with Genesis 9:3
Genesis 9:3 grants every moving thing that lives as food for humans, which challenges Job 40:20's implication that certain provisions are exclusive to beasts.
Genesis 9:3: Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things.
Contradiction with 1 Timothy 4:4
1 Timothy 4:4 states every creature of God is good for eating, potentially conflicting with the notion in Job 40:20 that only specific things like grass are for beasts.
1 Timothy 4:4: For every creature of God [is] good, and nothing to be refused, if it be received with thanksgiving: