Magnesium is involved in both milk fever (low blood calcium) and grass staggers (low blood magnesium) problems.  The first usually occurs in older, fatter, high producing cows within a day or two of calving; the latter can affect any cow at any time of the year.  Both can be clinical (you can actually see the symptoms – slow, wobbly cows; nervous, aggressive cows; downer cows or dead cows) or subclinical (in which case there is nothing to see, but the cows will be affected in a number of ways, and have reduced production).


If you are getting a high number of clinical cases (>3%), then you almost certainly have a higher number of subclinicals – the problem will be costing you production and money. It is worth getting it right, and the best way to ensure this is by blood testing.  Depending on the circumstances it may be appropriate to test springers and/or animals in early lactation.


The key part of dealing with these problems is supplementing with mag – it directly prevents staggers, and because of a biochemical tie-up between calcium and magnesium, helps prevent milk fever. The amount of supplementation required depends on the amount of mag in the feed (spring grass is especially low), the amount of feed actually offered and eaten, and the cow’s production levels.


In most situations, 15gm of magnesium per cow per day from 3 weeks before calving should be adequate.  Magnesium chloride (“Mag-C”) and magnesium sulphate (“Mag-S”) can be added to the water, but each is only about 10% magnesium, i.e. 150gm of these products are required per cow per day, to give 15gm of mag. However Mag-C or Mag-S above about 60gm will decrease the palatability of the water (if you wouldn’t drink the trough water, why would you expect the cattle to?), and if water intake is depressed, all sorts of problems can follow. For this reason it is best to give only part of the full requirement via the water.


Magnesium oxide (“Causmag”) for dusting or spreading on silage is 55% magnesium. So if this is the only source, cows must eat 28gm.  The amount offered must allow for wastage, so most would work on an allowance of up to 100gm/cow/day.


Magnesium levels in the body fluctuate quite markedly, because the daily turnover is high compared to the body stores.  Thus, it is best to supplement a smaller amount several times a day, than the entire daily dose in a single hit.


Other than palatability, feeding too much mag can cause as many problems as feeding too little. High mag levels can cause scouring, depress metabolism, and paradoxically, lead to more metabolic problems.  It is definitely not a case of “the more the better”.  If you are getting milk fevers, it’s not just a case of “upping the mag” because that can, in fact, make things worse.


One more thing to consider is the concept of “DCAD”. In lay terms, this means making the blood more acidic, to aid calcium metabolism. Practically, this is done by - feeding sulphates and chlorides (Mag-C and Mag-S); avoiding potassium (don’t graze effluent paddocks with springers or colostrums); avoiding nitrogen and potassic fertilisers in spring; and not feeding lime flour pre-calving (although it may be of benefit to colostrum cows and milkers).  The confusing part is that the oxide part of Mag Oxide makes the blood more alkaline (leaving you stuck between a rock and a hard place), but almost always the net benefit is in favour of feeding Mag Oxide (for the beneficial effect of the mag part).