Magnesium chloride in dry cow silage to prevent hypocalcaemia
Milk fever, or parturient paresis, is the second most common disease in Swedish dairy cows. The disorder is associated with the onset of lactation when some cows are unable to meet the metabolic demands of calcium to support milk production and therefore develop a state of hypocalcaemia. Clinical hypocalcaemia (milk fever) may lead to coma and death in severe cases but subclinical hypocalcaemia has also been shown to have negative effects on e.g. feed intake and production and to increase the susceptibility of the cow to develop secondary diseases. The nutritional strategy applied precalving is of great importance in preventing milk fever and e.g. manipulation of the dietary cation-anion difference (DCAD) or the supply of an adequate dietary level of magnesium may affect the degree of hypocalcaemia at calving. The objective of the present study was to investigate if the degree of hypocalcaemia at parturition could be affected by feeding silage preserved with an additive containing magnesium chloride (MgCl2) to lower DCAD and to increase the amount of dietary magnesium in the prepartum ration. The purpose was also to investigate if MgCl2 would affect feed intake and if supplying magnesium above feeding recommendations would affect plasma calcium level at calving. Twelve heifers and 24 cows (all pregnant) of the Swedish Red Breed were divided into six blocks based on age and calving date. Animals within block were randomly allocated to one of three experimental diets; a control diet (C) consisting of late harvested silage, a magnesium chloride diet (MCL) with 8 g MgCl2/kg DM silage added at ensiling and a magnesium oxide diet (MO) with 22.7 g/day of magnesium oxide manually mixed in the silage at feeding. Animals began receiving the diets ~21 days before expected parturition and collection of blood and urine samples was started and continued until 7 days after parturition. Urine pH, Ca and Mg and blood Ca, Mg, parathyroid hormone (PTH) and C-terminal peptide (CTx, marker of bone resorption) were analyzed. Fresh crop and silages were analyzed for DM, energy, crude protein and minerals and DCAD was calculated using ([Na+] + [K+]) ? ([Cl-] + [S-]). The results showed that the degree of hypocalcaemia at calving was not affected by a lower DCAD or an increased dietary content of magnesium. However, MgCl2 caused a significant increase in urinary excretion of calcium in the week before calving, which may imply that the feeding of MgCl2 lead to an increased activation of the mechanisms regulating calcium homeostasis and/or an increased responsiveness of these mechanisms to the secretion of PTH close to parturition. Feed intake was not affected by treatment and the plasma level of CTx was only affected by age and calving. The extra amount of magnesium supplied in the MO and MCL diets caused a higher urinary excretion of magnesium prepartum compared to control. Therefore it was concluded that the amount of magnesium provided in the control diet was sufficient to meet magnesium requirements in prepartum dairy cows.