TOO SOON OR TOO LATE?
Calcium is the most abundant mineral in our body. It is essential for growth, milk production, muscle contraction and bone & teeth health. The biggest challenge about calcium in ruminant nutrition is around calving: we shouldn’t supply calcium too early to avoid hypocalcaemia and we shouldn’t supply calcium too late to avoid… hypocalcaemia as well.
A mineral cannot be produced by an animal. It is necessarily come from diet. The quantity of mineral in the blood is extremely stable and represents a small part of the total body mineral composition. Most of the mineral are stored in bones and could be mobilized when the needs are increasing (lactation, growth). For calcium, 99% of total calcium are stored in bones and teeth against only 1% in extracellular serum. We talk about calcaemia for the calcium concentration in blood. Calcium is mainly absorbed in the small intestine by an active transport molecule named Calcium Binding Protein. This protein can complex ion Ca2+ and transport it from the top of the enterocytes to its bottom. Then, it is exported to the blood and it can be stored in the bones or be used for milk production. The absorption rate of calcium can be modulated by hormones or vitamin D according to the needs.
A normal calcaemia for a cow is at 85-100mg/L. During the lactation, the balance is easily done by the cow as she can easily get calcium from the diet or mobilize from her bones (cf Figure 1).
Absorption rate is at 52%, so 90gr of Calcium intake means that 43gr will finish in the faeces. The remaining calcium intake (47gr) is used for milk production, maintenance and the foetus. During drying off, this calcium intake is still high compare to the needs and the cow organism adapt her hormone production to reduce absorption. The day of calving or the day before, cow starts producing milk again but the hormonal system is not able to increase absorption capacity of the cow as fast as her needs. Moreover, the cow doesn’t produce just milk (1.2g of calcium/L) but colostrum (2.3g of calcium/L).
We see the problem coming: lower intake of calcium (low absorption rate) and higher needs (colostrum richer than milk about calcium) in an extremely short period of time (24h) without an efficient mobilization from bones calcium reserve. It is the post calving hypocalcaemia also called milk fever.
Milk fever in dairy cow is a post-partum pathology related to a blood calcium deficit. Clinical milk fever (Calcaemia < 50mg/L) are less and less common as soon as feeding during close up is well managed. Nevertheless, 30 to 50% of dairy cow still face a subclinical hypocalcaemia (Calcaemia < 75mg/l) in early lactation. Direct consequences of milk fever are loss of appetite, difficulties to stand up and poor milk production. This hypocalcaemia is one of the worst ways to start a lactation and it will trigger numerous other post-partum disorders: (cf Table 1).
Table 1: Incidence rate of post-partum disease in case of hypocalcaemia
It is important to note that frequency of post-partum hypocalcaemia is strongly related to number of lactations of the animal: 52% of 2nd lactation cows are concerned and 72% for 4th lactation cows. Clinical hypocalcaemia must concerned less than 5% of the herd. Subclinical hypocalcaemia is more difficult to detect and can concern up to 50% of the herd. The first method to decrease milk fever incidence is to manage the Dietary Cation Anion Difference (DCAD) in dry cow feed. DCAD represent the sum, in milliequivalent per kg, of anions and cations in the blood (K+ + Na+ + Cl- + S-). When the DCAD is negative (-50 to -100 mEq/kg), it means there is more anions than cations in the diet and it stimulates bone mobilisation before calving. To do so, it is recommended to use anionic salts like magnesium chloride, magnesium sulphate, calcium chloride or calcium sulphate. The second method is to reduce calcium supplementation before calving (<20g/day). If the calcium is provided too soon, the cow will not stimulate her calcium mobilization mechanism. Basically, it means for farmer that they should reduce calcium carbonate quantity or replace fresh grass by straw. To get a negative DCAD, sodium bicarbonate must be removed from close up diet as it has very high DCAD value. After calving, many cows can face hypocalcemia if we supply calcium too late. It is important to give her readily absorbable calcium. This calcium must be given in the feed or by drenching as soon as possible. Some sources, calcium pidolate for example, are well known to improve efficacy of calcium transportation to the blood by providing precursor of the Calcium Binding Protein. It is one of the most efficient ways to provide calcium to the animal. If you want to know more about calcium source in ruminant, do not hesitate to contact me.