Can a calf come into heat if it is still nursing it's mother?

Depends on the age of the calf or "calf" in question! Majority of the time calves should be weaned and separated from each other before they have a chance to start going into puberty and go into heat. Weaning age is or should be between 6 to 10 months of age. Some herds that are highly fertile are likely to have heifer calves going into heat before they are weaned, which could be as early as 3 to 4 months of age, which may be kind of scary for a producer because of the potential for that calf to have problems--due to small pelvic size--giving birth to a calf!