Dietary Calcium and Magnesium Deficiency in Dry Cows

Dr. Bill Weiss, Dairy Nutrition Extension Specialist, The Ohio State University (top of page) pdf file

Subclinical and clinical milk fever (hypocalcemia) are common and expensive problems on many farms.  Cows with hypocalcemia often require veterinary treatment, have reduced milk yields, and are a much greater risk for other health disorders, such as mastitis, metritis, and displaced abomasum.  Proper nutrition of dry cows can greatly reduce the occurrence of hypocalcemia.  The two primary approaches to controlling hypocalcemia are: 1) feed diets that are marginally deficient in available calcium and just adequate in phosphorus and 2) feed anionic diets [i.e., diets with a negative or dietary cation-anion difference (DCAD)], usually with excessive dietary concentrations of calcium.  A recent study reported in the Journal of Dairy Science (Volume 94, pages 1365 to 1373) provides an additional concern when excessive calcium is fed to dry cows.  In this experiment, cows were fed diets with approximately 0.5, 0.9, or 1.3% calcium (dry matter basis) during the last 3 weeks of the dry period.  The diet with 0.5% calcium slightly exceeded the National Research Council (NRC) calcium requirements.  Calcium carbonate and calcium phosphate were used to adjust dietary calcium concentrations.  All diets were approximately +20 mEq/100 g and all diets contained 0.3% phosphorus, 1.6% potassium, and 0.18% magnesium (dietary potassium, phosphorus, and magnesium exceeded NRC requirements).  Hypocalcemia was not severe for any treatment, but plasma calcium concentrations were lowest (7.8 mg/dL) for the highest calcium diet and greatest (8.6 mg/dL) for cows fed the lowest calcium diets.  The greatest effects of dietary calcium were on magnesium absorption.  Cows fed the highest calcium diet had the lowest plasma concentrations of magnesium the first 7 days of lactation (averaged approximately 1.6 mg/dL, which is indicative of subclinical magnesium deficiency).  The low plasma magnesium in cows fed high calcium diets during the dry period was caused by significantly reduced absorption of magnesium.  The apparent digestibility of magnesium by cows fed the low calcium diet was 17% (this absorption coefficient is very typical for dairy cows).  The apparent absorption of magnesium by cows fed diets with the intermediate or the highest concentrations of calcium averaged 4 and 0%, respectively.  The bottom line from this research is that there is no benefit from feeding high calcium diets to dry cows when anionic diets are not fed, but if high calcium is fed, then dietary magnesium needs to be increased substantially.  Cows consumed approximately 16 g/day of magnesium, and based on the reported quantities of magnesium excreted in the urine, cows fed the higher calcium diets would need to consume 22 to 40 g of magnesium to maintain magnesium status (approximately 0.25 to 0.45% magnesium).  The results from this study may or may not be applicable to cows fed anionic diets.  There is little risk of feeding diets with 0.45% magnesium to dry cows, suggesting that when high calcium diets are fed, regardless of whether a negative DCAD diet is fed, substantially increasing dietary concentrations of magnesium is the prudent course of action.