Cation-anion balance in organic silage in relation to prevention of milk fever

Summary

Studies of organic farms in Norway have documented that cows in organic herds are less prone to milk fever compared to the overall average incidence of milk fever in Norway. Milk fever occurs most frequently in high-yielding cows, fat cows and older cows. On average, cows on organic farms are lower yielding, but also older. Dry cow diets relatively high in the anion chloride (Cl–) and low in the cations potassium (K+) and sodium (Na+) can prevent milk fever. In organic farming no inorganic fertilisers are used. This may result in low content of K in the plants and with that a low content of K in the fodder. Often there also is greater diversity of plants (clover, herbs, “weeds”) in an organic meadow than in a conventional meadow. Plants vary in their uptake of different elements. One can therefore expect a different composition of minerals in organically produced fodder compared to fodder from conventionally managed fields.
Therefore, the mineral content in fodder from eight organic farms and eight conventional control farms was examined to see if low frequency of milk fever is connected to the cation-anion balance (CAB) in the fodder. Seven cows were selected from each farm. Fodder given to the cows in the dry period was analysed for Na, K and Cl, and the botanical composition of the roughage were determined. The cows’ urinary pH was measured during the dry period, using pH-papers. The fodder was also analysed for other minerals (Ca, Mg, P, Mn, Fe, Cu, Zn and Mo) to get a broader picture of the ration. The cows’ body condition scores in the dry period were measured.
The mean CAB in both the organic and conventional fodder was 366 mEq kg-1 DM, and the cows’ urinary pH was around 8.5. Based on results from this project we cannot see any connection between the lower frequency of milk fever in organic milk production and CAB in organic fodder. Not unexpectedly, the organic fodder contained more Ca and Mg. There was no difference in content of Fe, Cl, Na, K, P, Mn, Zn and Cu. Several fodder samples had more then 2% K of DM. The Ca/P ratio was high, together with a relatively low content of P. Both high K content and high Ca/P ratio together with low P content are connected to increasing risk for milk fever. The results from the mineral analysis of the fodder samples indicate an unfavourable mineral composition regarding milk fever in both conventionally and organically produced fodder.