A whole host of champions can shut Alistar out of last hitting*, and one of his problems is the time it can take to clear a minion wave as him without mana expensive Pulverize spamming.
*(Pretty much every ranged dps champion, for starters.)
An early Sunfire Cape is honestly the best way to improve Alistar's farm, if its that important to you.
Generally, you should be at least 10X, more concerned that the enemy champions die than about who gets the last hit.
Don't save your Headbutt for a last hit when there is need for it in a team fight.