Limestone, calcium carbonate
This is the most common source of calcium used in livestock feeding. It is almost pure calcium carbonate. Calcitic limestone contains 36-38% calcium and can safely be fed free choice (ad lib) mixed with salt. Dolomitic limestone contains at least 5 % magnesium carbonate and should not be used for poultry, but it is as good as calcitic limestone for other animals. Portland and natural cement can be used instead of limestone.