I emailed Scrapeline, before this thred really got started, so to be fair:
Calcium carbonate is just a standard, economical form of calcium. Like John said, there are lots of different calcium sources (chelated and inorganic). Most people, whether you are feeding deer, cattle, dogs, cats, horses or pigs will use calcium carbonate as the primary calcium source. Tums, the heartburn medicine, is calcium carbonate. most feeds will also have dicalcium phosphate or monocalcium phosphate. these are just two different products that have both calcium and phosphorus in them. when we balance feeds, we target a specific amount of phosphorus that we want in the feed, say 0.7%. We do this by adding a phosphorus source (monocal or dical). then we add calcum carbonate to the feed to bring the calcium amount to a desired level, say 1.4%. Now you have a 1.4:0.7 calcium to phosphorus level or said another way 2:1 ratio.
You can use different forms of calcium (chelated or not chelated) in your deer feed, but figuring out how much the deer is going to use (absorb and not waste) from the specific calcium source you are using is the challenge. Some of the calcium in plants is only 30% absorbed by the animal, calcium carbonate can be 70% absorbable. Calcium citrate (chelated) is more absorbable than calcium carbonate. So on and so on... Now to say this about this "milk calcium" product. I (like many others) do not agree with certain giving drugs to increase antler growth, but milk calcium is not going to shorten the lifespan of a deer. it is a source of calcium (elemental form) and phosphorous (phosphate form). Since the calcium is elemental, it is very absorbable (90-95%). Companies have been using milk calcium to put into yogurts for calcium deficient people. It is expensive because it is human grade.
Keep in mind that the body has a way of regulating the absorption of calcium. The amount of calcium (calcium carbonate) to add to your feed will vary depending on the current calcium and phosphorous levels. It also depends on the dry matter intake of the feed by each deer. once the calcium levels in the blood hit a certain concentration determined by the kidneys, the animal starts to absorb less calcium and starts to excrete more in the feces. too much calcium can interfere with absorption of other nutrients like phosphorus and some fatty acids. By giving a deer "too much of a good thing" like ZZ said, is always a concern no matter what you are giving them. too much calcium, phosphorus, or an imbalance of the two can cause bladder stones to form, block urine excretion and severely hurt or kill the deer. A good healthy, balanced feed is always the solution to sucessful deer farming. try to target a 2:1 ca to p ratio and concentrate on using common ingredients like ca carb. chelated calciums are sometimes hard to come by and very expensive. sometimes we hear things that really confuse us or make us think that we could be doing something better. Feeds and feeding 101...keep feeds simple and effective. Good luck to all.