A study of magnesium metabolism, depletion and repletion in young calves

The objectives of this experiment were: (1) to produce hypomagnesemia in young male calves; (2) to study the effects of magnesium (Mg) deficiency and excess dietary potassium (K) on mineral metabolism in hypomagnesemic and normal calves; and (3) to study the effects of MgCl₂*6H₂O enemas on plasma, cerebrospinal fluid (CSF) and red blood cell (RBC) mineral levels. In this study, calves were used as models for adult, lactating cows. Preliminary studies were initiated to establish procedures and determine the effectiveness of a semi-purified, low Mg, dry diet. Calves did poorly on the initial diet used in 2 preliminary trials in-volving 2 calves each. The composition of the diet was altered and a calf was maintained on this diet alone for 7 weeks. After the calf became severely hypomagnesemic, plasma and CSF Mg level were repleted using an enema of MgCl₂*6H₂O solution. Procedures for administering the enema and taking samples of blood and CSF were refined and finalized with this calf. Two trials were run involving 8 calves each which were fed 4 different diets. The diets were: Mg deficient basal diet, basal diet plus K, basal diet plus Mg and basal diet plus Mg and K. For the balance study, only samples collected from 9 calves were used for statistical analysis because of problems with diarrhea and anorexia. Samples of feces were collected using canvas collection bags and aliquots of urine were collected daily for a 5 day balance trial. All 16 calves were used in the repletion study which was initiated immediately after the balance study for both trials. Each calf was given an enema of MgCl₂*6H₂O solution while under general anesthesia. The enema was held in for 5 minutes and samples of blood and CSF were taken at 5, 15, 30, 60, 120 and 180 minutes after administration of the enema. For calves fed the Mg supplemented diets, the intake, fecal excretion and balance of Mg were greater (P< .05) than calves fed deficient diets. Calf urinary excretion of Mg was not different among treatments. Supplemental K appeared to have no effect on Mg excretion or balance. Dietary K appeared to have no effect on plasma Mg level. The MgCl₂*6H₂O enemas caused a rapid increase in plasma Mg level. A significant increase in CSF Mg level was also observed in calves on the Mg deficient diets which had subnormal levels of CSF Mg. The MgCl₂*6H₂O enema appeared to have no effect on plasma calcium (Ca), and K, RBC minerals and packed cell volume (PCV). One calf in trial 1 had 4 episodes of tetany and was treated successfully 3 times with a MgCl₂*6H₂O enema. CSF samples taken during tetany had subnormal levels of Mg. Young dairy calves appeared to be good models for Mg studies in adult cows.