Devoted Crop-Mate — ruling 1

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.

Devoted Crop-Mate — ruling 2

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

Devoted Crop-Mate — ruling 3

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.

Devoted Crop-Mate — ruling 4

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.

Devoted Crop-Mate — ruling 5

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.

Devoted Crop-Mate — ruling 6

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.

Devoted Crop-Mate — ruling 7

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.

Devoted Crop-Mate — ruling 8

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.

Devoted Crop-Mate : règles, fonctionnement et interactions (MTG)

Devoted Crop-Mate is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

You may exert this creature as it attacks. When you do, return target creature card with mana value 2 or less from your graveyard to the battlefield. (An exerted creature won't untap during your next untap step.)

How it works : Devoted Crop-Mate

Original English text — Wizards of the Coast / Scryfall

#1April 18, 2017· Wizards of the Coast
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 has mana value 3.
No other card is named in this ruling.
#2April 18, 2017· Wizards of the Coast
If the mana cost of a card in your graveyard includes X, X is considered to be 0.
No other card is named in this ruling.
#3April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#4April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#5April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#6April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#7April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#8April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#9April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.