Subclinical hypocalcaemia or the silent form of milk fever

What is subclinical hypocalcaemia?

Subclinical hypocalcaemia is the silent, non-symptomatic form of milk fever. It is characterised by a fall in calcaemia (calcium levels in the blood) below 80 mg/l. The difficulty lies in identifying this disease, which has no visible signs.

The number of cows affected is huge. Up to 72% of cows in 4th lactation are affected. However, there are differences depending on the number of lactations. As a recent study (Mullimann, 2017) shows, primiparous cows are generally less impacted, with 16% of cows appearing to be affected by subclinical hypocalcaemia. But this prevalence increases rapidly with the number of lactations. 52% of cows in 2nd lactation are affected and almost 72% of cows in 4th lactation. This simply means that from the second calf onwards, more than half of French dairy cows are affected!

If we look a little further afield, Germany and the USA don’t seem to be outdone. According to studies, the prevalence of this disorder is between 23% and 55% of cows in Germany and between 41% and 50% in the USA.

Table 1: increase in the frequency of early lactation disorders linked to hypocalcaemia

The consequences of hypocalcaemia for cows

Hypocalcaemia in itself does not cause any clinical signs, but it does have some negative consequences: reduced appetite, stress, lower production and even reproductive problems. In the study published by Nydam and Cornell, success rate at 1st artificial insemination is 63% for the control group and falls to 31% for cows with hypocalcaemia (<80 mg/l).
Hypocalcaemia is now well known to be a factor in the onset of early lactation problems.

It increases the risk of mastitis by a factor of 8 and the risk of ketosis by a factor of 5.5. Researchers have also shown that hypocalcaemia has an impact on metritis, retained placenta, displaced abomasum and difficult calving (see Table 1).
This is linked to the negative impact of hypocalcaemia on muscle tone and immunity. Some vets consider that subclinical hypocalcaemia is 4 times more costly than overt milk fever (GTV 2017).

For breeders wishing to reduce the risk of hypocalcaemia, here are a few tips. Firstly, pay attention to the body condition of the cows: around 3 at dry up, no recovery during the dry period.
The breeder must also work on a ration for dry cows with a negative anion-cation balance or at least as close to zero as possible. In practice, a negative anion-cation balance is difficult to achieve.

Be careful with the forages used: some, such as grass silage and other clover-rich hay, produce a high anion-cation balance. For these rations, consider using specific minerals or nutrients with a very negative anion-cation balance (below -3000 mEq)
To protect at-risk cows, breeders can also provide an oral dose of calcium from the first signs of calving: liquid calcium, boluses, etc. In this case, take care to distribute 50 g of calcium per application, in a highly assimilable form (formate, chloride, acetate, pidolate). Here is the profile of the ideal candidate: > 3 lactations, high producer, history of milk fever or difficult last calving.

Phosphorus is also sometimes added to these solutions, as hypophosphataemia often goes hand in hand with hypocalcaemia. Calcium can also be combined with vitamin D3, which facilitates calcium absorption.

Table 2: prevent subclinical hypocalcaemia (Mulligan et al., 2006)