The ultimate goal is to prepare the animal for rapidly increasing metabolic needs at a time when intake is not maximized. The required needs are not only protein and energy inputs but also minerals such as calcium and magnesium.
Calcium
Calcium is involved in many mechanical functions of the body, such as contractions of the smooth and skeletal muscles necessary for calving. Calcium also plays an important role in various metabolic functions, like nerve impulse transmission, permeability of cell membranes, secretion of enzymes and proteins, coagulation and vasoconstriction mechanisms, and immunity cell activation.
We know the demand for calcium is very high in the days around calving, and therefore diets must contain adequate levels to avoid any problems. The contractions of the uterus, necessary for the expulsion of the fetus, exert an enormous pressure on the metabolism of the animal, and the cow’s needs must be met. To offset this, we must first and foremost understand the different mechanisms surrounding this regulation, absorption and use of minerals by the animal. Since everything is a question of balance, the dairy cow’s body has several processes that allow it to absorb or release calcium based on its needs.
One of the parameters that manage this self-regulation of calcium is the parathyroid hormone (PTH), a hormone secreted naturally by the parathyroid glands when calcium demand is growing (Figure 1).
PTH has a couple of roles:
1. The stimulation of osteoclast activity during the decrease in serum calcium. These cells are responsible for bone resorption in order to release calcium. In contrast, osteoblasts allow bone formation by mineralizing osteoids in bone tissues.
2. To indirectly control the absorption of calcium in the gastrointestinal system via the synthesis of the active form of vitamin D (calcitriol). When PTH is active, a source of quality calcium is important to efficiently maximize availability for the cow. The hormone calcitriol also works with the PTH to decrease the loss of calcium in the urine.
In turn, calcitonin is synthesized by thyroid cells. It is a hypocalcemic hormone and therefore plays an antagonistic role to PTH. It inhibits the reabsorption of calcium in the kidneys, inhibits bone resorption, while also promoting bone formation.
Magnesium
Magnesium is also essential for the functioning of several enzymes; it aids in the formation of adenosine triphosphate as well as in the transfer of nerve impulses. Magnesium is important to the integrity of bones and teeth. Like other minerals, magnesium has absorption mechanisms available to the animal, and several factors can affect this process. The main factor is the concentration of magnesium in the ruminal fluid.
Therefore, the solubility of the magnesium source used in the diet as well as the dietary intake will play an important role. Unlike calcium, soluble magnesium is largely absorbed in the rumen epithelium; this process is not directly regulated by a hormone.
However, aldosterone, secreted by the adrenal glands, has an indirect action on the regulation of magnesium by regulating the excretion of magnesium via urine. The main role of aldosterone is to regulate the concentration of sodium and potassium in saliva and in the rumen.
As mentioned above, since the absorption of magnesium is independent of the action of a hormone, it is essential that intake is sufficient. Otherwise, the animal does not have the opportunity to draw from bone reserves to maintain its homoeostasis.
Like calcium, magnesium is an essential mineral for a successful transition and is one of the most important factors in the prevention of milk fever. Magnesium actively participates in the homoeostasis of calcium by influencing the absorption of calcium, and this role is even more important when an anion strategy is used. The use of anionic salts in the close-up period activates the release of PTH.
Magnesium deficiency makes the body and target tissues less receptive to this hypercalcemic hormone. Magnesium is also involved in the synthesis of the active form of vitamin D (calcitriol). Therefore, a lack of magnesium drastically increases the risk of milk fever.
In conclusion, not all available mineral sources have the same bioavailability. In addition, some elements may interfere with the absorption of other essential elements. For example, diets rich in potassium may limit the absorption of magnesium. These aspects should therefore be considered when formulating rations and in choosing a nutritional strategy to be applied on the farm.
The success of a lactation period is certainly due to many factors. The transition period is critical for the health of the cow, as well as for the following lactation period, and therefore it is crucial to emphasize this part of the production cycle. 
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Gabrielle L. Dumas
- Technical Representative
- DCL Animal Health and Nutrition
- Email Gabrielle L. Dumas
