9 November 2018
Milk fever is a metabolic disease around the time of calving caused by a low blood calcium concentration.
This happens because the cow has a sudden increase in calcium requirements after calving and cannot respond quickly enough to mobilize calcium from body stores.
As calcium is needed for muscle function, milk fever is typically seen as muscle weakness, such as a ‘downer cow’. However, for every case of clinical milk fever, there can be more than eight cases of cows with below normal blood calcium, predisposing cows to retaining cleanings, lower intakes, displaced abomasum, reduced immune function, reduced fertility and lower milk yield.
As a case of milk fever is costly due to treatment costs and reduced productivity, it is important to prevent milk fever by nutritional strategies such as making sure cows are in optimum body condition, managing the calcium and magnesium concentration of dry cow diets and using anionic salts.
1. Body Condition Score (BCS)
Having dry cows in the correct BCS is important for the control of milk fever.
Target BCS for dairy cattle
- Drying off: 2.75 – 3.0
- Calving: 3.0 – 3.25
Cows in higher or lower BCS have a have a greater risk of developing milk fever compared to those in optimum BCS.
2. Dietary calcium during the dry period
Feeding a dry cow diet low in calcium ensures that the hormones are primed for the high calcium demands after calving therefore, lowers the risk of milk fever. The dietary calcium content should be less than 30g per cow per day for calcium restriction to work successfully.
Cows at risk of milk fever can be given a highly available source of oral calcium using Reviva, immediately after calving.
3. Dietary magnesium during the dry period
Magnesium has an important role in activating hormones that help with calcium absorption from the intestine and bone metabolism.
- The target dry cow dietary magnesium content should be 0.4 %
- An easy to remember target – check the dry cow diet has 20 to 25 g of supplemented magnesium during the two weeks before calving
4. Anionic salts during the dry period
Anionic salts such as chlorine (Cl) and sulfur (S) have a negative charge when dissolved in solution and lower the cow’s blood pH, resulting in a metabolic acidosis. This triggers more active bone mobilization and dietary absorption of calcium, helping to reduce the risk of milk fever at calving.
The difference in the concentration of dietary anions (negative charge) and cations (positive charge) in the diet is referred to as the ‘dietary cation and anion difference’ (DCAD). The more anions in a diet, the closer to zero (or even negative) the diet becomes, which will reduce the risk of milk fever.
The anionic salts that can be used are: -
- Calcium sulfate, magnesium sulfate and ammonium sulfate. The sulfates are more palatable than chlorides
- Calcium chloride, magnesium chloride and ammonium chloride. These have a stronger effect on DCAD per gram than sulfates
As anionic salts are unpalatable, they may depress intakes however, keeping the feeding level below 100g per cow per day generally avoids this.
Note, as potassium (K) is a cation, when the forage K is greater than 2.5%, changing the forage fed to dry cows may help reduce milk fever.
Dr. Mark Little MVB CertDHH PhD MRCVS