Make your own eggshell powder (ESP) and bone meal powder
I used ESP and bone meal powder for my cat Pigpen who was allergic to chicken, duck, turkey, and game hen but thankfully not to their shells or their membranes. Over the 16+ years I have been running my list/forum, I have met far too many cats who are allergic to poultry. If your cat is allergic to poultry and won't accept whole prey, using ESP (substitute emu, quail, or ostrich for chicken eggshells) or bone meal powder still beats commercial food.
Eggshell powder (ESP)
Use eggshells without any coating (such as shellac) i.e. as natural as possible. I use local egg farms where the hens are free-range and pasture-fed. The egg yolks which I add to the cats' raw mix, are a beautiful bright orange color unlike the anemic yellow color one sees in grocery store egg yolks.
Baking is not necessary but is convenient as well as desirable for the following reasons:
The drier eggshells are, the easier they are to grind up.
As a rawfeeder I don't, but for those people who worry about salmonella, the heat from baking helps with this issue. Note: if you buy eggs from anything but a local farm you are familiar with, I would go ahead and bake the shells, just in case.
If you see a lot of egg whites sticking to the shells, rinse them out.
I do not remove any remaining membrane because it has therapeutic value e.g. for arthritis.
Bake in the oven e.g. while pre-heating oven or after it's turned off after baking a pizza or something like that or at 300 degrees for 10 minutes. Obviously time will vary based on your oven, so adjust accordingly.
Place eggshells in an open dish e.g. pie plate in a garden window or any south-facing (so it gets plenty of sun) window. Leave for a week or more until dry. You will know this when they are brittle and crumbly to the touch.
When they look dry enough to me I run through a coffee grinder for a few seconds. Depending on your grinder, you will need to stop and start a few times. Aim for the consistency of baking soda. Wait before opening or you'll end up with a cloud of powder.
How to use: Depending on how coarse or fine one's ESP grind is, 1 teaspoon of ESP contains anywhere from 1800 to 2200mg of elemental calcium (in carbonate form).
1 tsp of eggshell powder (ESP) contains somewhere between 4,500mg and 5,000mg total calcium (in carbonate form) depending on whether one goes by the US Egg Board or research by a vet at the Univ. of Florida (article used to be online, alas I did not make note of the reference).
A study published in Poultry Science (2000) referenced below, found that:
"ESP seems to have a beneficial composition with about 39% of elemental Ca, relevant amounts of Sr, and low levels of Al, Pb, Cd and Hg."
A colleague (fellow professor but unlike me, she's in Ag Science) and I ground up 3 large egg shells each in 3 different consistencies - medium, fine, and extra-fine, then weighed each of the resulting grinds in her lab. Our results ranged from 4,400mg to a shade under 5,080mg. In other words, not hugely different once one takes 39% of that number to get elemental Calcium, and enough to satisfy my curiosity. Deferring to the researchers in this field, I feel comfortable going with a range of 1,800-2,200mg of elemental Ca per teaspoon of eggshell powder.
Given the fact that teaspoon is a volume measure, and how finely ESP is ground determines just how much one can fit in a teaspoon, as long as one is not leaving the ground too coarse, somewhere in the 2,000mg elemental Calcium per teaspoon is a good ballpark.
I don't think it's safe to go too low with calcium, and since eggshell powder has been shown to not raise serum calcium levels (Schaafsma et al, 2002), if you have to err, go a bit over rather than under in amount.
Bone meal powder
One can use pretty much the same method as for eggshell powder.
After making stock or broth, remove all the meat from the bone.
Dry the bones in a toaster oven* at 375 to 400 degrees Fahrenheit,for at least one hour. Obviously times will vary depending on one's oven as well as on the animal/bird bone and type of bone e.g. thigh bones/drumsticks will take a lot longer than wings.
After bones are dried and brittle, pulverize/crumble to a fine powder in a Vita Mix or coffee grinder (obviously only for small bones and amounts:) or a Magic Bullet-type appliance.
*I don't put these out in the sun like I do with eggshells because I don't want to take a chance a kitty/puppy will run off with the bones but if you have a way to safeguard against this, by all means place bones in a garden window. Give it at least a week.
How to use: Note - this is based on my own powder made from venison & bison bones. I used a small food scale and weighed 1 teaspoon of the powder. It came to 4grams or 4,000mg.
Unfortunately, even this does not really tell us how much Calcium, Phosphorous, etc. is in a teaspoon because judging by commercial brands, e.g. NOW 1 rounded teaspoon i.e. 3,000 mg contains Calcium 1000 mg, Phosphorus 500 mg, Magnesium 25 mg
But Solgar lists 1 rounded teaspoon as. 5,000 mg and contains Calcium 1000 mg, Phosphorus 600 mg. Neither mentions source but the most common one is beef.
Bottom line - unless one is using powdered bone meal for just a few meals every 2-3 months, I wouldn't take a chance on assuming home-made powder is equivalent to a commercial brand. So how do I use it? I weigh the bones before drying and grinding, and set aside what I would like to add in as a % ratio of the overall mix. Then, I dry and grind those pre-weighed bones, and add the powder from those bones to the meat.
"Mineral, Amino Acid, and Hormonal Composition of Chicken EggshellPowder and the Evaluation of its Use in Human Nutrition", A. Schaafsma, I. Pakan, G.J.H. Hofstede, F.A.J. Muskiet, E. Van Der Veer, and P.J.F. De Vries, 2000 Poultry Science 79:1833–1838
Schaafsma A, van Doormaal JJ, Muskiet FA, Hofstede GJ, Pakan I, van der Veer E., "Positive effects of a chicken eggshell powder-enriched vitamin-mineral supplement on femoral neck bone mineral density in healthy late post-menopausal Dutch women", Br J Nutr. 2002 Mar;87(3):267-75.