March 2010 edition
Minimizing the risk of milk fever
Nathan Shannon, Dairy Extension, Department of Primary Industries, Wodonga
We know that milk fever is bad news on our farms around calving time. Minimising the incidence of hypocalcemia or milk fever also reduces the incidence of other associated metabolic disorders such as retained fetal membranes, abomasal displacement, and metritis.
Milk fever is caused by the cow’s inability to mobilise enough calcium into her blood to meet her needs. At calving, a cow’s requirement for calcium increases enormously because calcium is removed in large quantities in both colostrum and milk (one to three grams of calcium per kilo).
The key to managing milk fever is to manipulate the mineral nutrition of the sprinters. The current theory on reducing the incidence of milk fever and associated disorders involves the dietary cation-anion difference (DCAD). The DCAD value is calculated from the milliequivalents (mEq)/kg of potassium plus sodium, minus chloride and plus sulphur of the springer’s diet.
Pre-calving diets with a low DCAD make the cow’s blood more acidic. The cow responds by releasing more carbonate from her bones to buffer this acid and maintain normal blood pH. As a result, the cow is much better able to cope with the huge demands for calcium after calving, avoiding milk fever and other metabolic disorders associated with low blood calcium levels. An ideal pre-calving diet has a DCAD between minus 100 and zero mEq/kg. This is achieved by reducing the amount of potassium and sodium in the diet (generally these minerals are abundant in pasture, legume hays and silage) and increasing the chloride and sulphur in the diet through inclusion of anionic salts or commercial anionic feed supplements.
Given the excellent spring conditions last spring, many farmers will be tempted to feed their springing cow’s high amounts of silage they conserved last year. This can be dangerous, particularly if the silage was boosted in late winter with a fertilizer blend containing potassium. This combined with some ‘green pick’ which may have grown following early March rains could cause a dramatic increase in the incidence of milk fever this autumn.
So what can we do? For the two to three weeks before calving, cows should be fed a diet with a low and preferably negative DCAD. To do this, green feed, silage and legume hays should be removed from the diet and replaced with cereal hay or straw (low DCAD) as the dominate fiber source. Cows should also be fed anionic salts to ensure the DCAD is in the negative zone. These can come in a number of forms which can be fed with grain (low DCAD) or supplied in the water source.
Feeding small amounts of grain (less than two kilos) prior to calving allows starch digesting rumen microbes to increase in number which will be beneficial when the cows enter the dairy after calving and grain may be a large part of their ration. This grain pre-calving will also help to ensure cows are receiving their daily energy requirements, particularly if straw is been used as the fiber source and not mining body condition.
At calving however, DCAD levels should switch from negative to positive because milk production depletes cation levels, and blood needs additional buffers supplied by positive DCAD to combat the high acid load of peak milk production.
For more information on pre-calving diets contact a reputable ruminate nutritionalist or call Nathan Shannon at the Wodonga DPI.
