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All attackers are chosen at once. You can't attack with Devoted Crop-Mate, return a creature card to the battlefield, and then attack with that creature.

All cards in the Amonkhet set that let you exert a creature let you do so as you declare it as an attacking creature, as do some of the cards in the Hour of Devastation set. You can't do so later in combat, and creatures put onto the battlefield attacking can't be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.

If a creature has a targeted triggered ability that triggers when you exert it, you can exert it even if there isn't a legal target for that triggered ability.

If an exerted creature is already untapped during your next untap step (most likely because it had vigilance or an effect untapped it), exert's effect preventing it from untapping expires without having done anything.

If the creature returned to the battlefield has any abilities that trigger when creatures attack or when you exert creatures, those abilities won't trigger.

If the mana cost of a card in your graveyard includes X, X is considered to be 0.

If you gain control of another player's creature until end of turn and exert it, it will untap during that player's untap step.

The mana value of a card in your graveyard is determined solely by the mana symbols printed in its upper right corner. The mana value is the total amount of mana in that cost, regardless of color. For example, a card with mana cost {1}{U}{U} has mana value 3.

You can't exert a creature unless an effect allows you to do so. Similar effects that "tap and freeze" a creature (such as that of Decision Paralysis) don't exert that creature.