Magnesium is an essential mineral that is important for several physiologic and biochemical processes in the body. Deficiency of magnesium is mainly associated with grass tetany, but can also appear in cows during the indoor period. Since long it is known that potassium has a negative effect on the absorption of magnesium. Due to the fact that potassium fertilisation leads to a higher potassium content in forage, it is of interest to study interactions of potassium on the magnesium balance in dairy cows. There are few studies about magnesium feeding to dairy cows in Sweden and the purpose of this study was to investigate how much magnesium and from which feed-sources dairy cows in Sweden get their magnesium.
Data was collected from seven farms in the northern part of Uppland and from seven farms in Skaraborg in Sweden. Four of the investigated farms had an organic production system (KRAV). At the farm visits information concerning the feeding of the cows and their milk production was gathered. Furthermore urine samples were collected from 10 % of the cows at each farm. A surplus of absorbed magnesium is excreted in the urine, thus by determining the content of magnesium in urine a good indication of the magnesium status in the cows could be achieved. The content of magnesium and potassium in the diets was determined based on feed analysis data obtained from the farmers, analyses of collected feed samples and information by the feed manufacturers.
Calculated magnesium intake per cow was in average 50 g/day and varied between 30 and 70 g/day. The average urinary excretion of magnesium was 5.8 g/day and cow, varying from 2.2 to 8.2 g/day. Magnesium intake was compared with the magnesium excretion in urine and the variation between farms was explained partly by differences in magnesium intake and magnesium concentration in the diet, but also of the influence of potassium on the magnesium absorption.
The absorption, calculated as the proportion of the intake that was excreted in urine and milk, was in average 17 %, which did not differ from the literature value of 16 % for the coefficient of absorption for magnesium from natural feedstuffs. Recommended value of total diet K/Mg-ratio is 4-5 and the average in this study was 6.8. A low K/Mg-ratio in the diet gave higher urinary excretion of magnesium, which indicates a that high potassium level mat negatively interact with magnesium uptake The K/Mg-ratio in the forage for the four organic farms had strong influence of magnesium absorption, a low K/Mg-ratio gave a higher coefficient of absorption.
In average...
