THE IMPORTANCE OF CALCIUM AND MAGNESIUM IN THE SUCCESS OF THE TRANSITIONAL PERIOD
The transition period is probably the most critical period for successful lactation. As a result, many factors play a crucial role in the success of this period: p. ex. comfort and ventilation, the overall health of the animals, the quality of the food served and the feeding strategy used for the calving preparation period. The
Success factors are multiple, but the ultimate goal is to prepare the animal for rapidly increasing needs at a time when consumption is not at its peak. The needs implied here include protein and energy inputs, but also different minerals such as calcium and magnesium.
Calcium is involved in certain mechanical functions such as contractions of the smooth and skeletal muscles necessary for delivery. Calcium also plays an important role in various metabolic functions such as nerve impulse transmission, permeability of cell membranes, secretion of enzymes and proteins, coagulation and vasoconstriction mechanisms, and the importance of calcium and magnesium in the success of the transition period in the activation of immune cells. We know that the demand for calcium is very high in the days surrounding calving and therefore the rations must contain enough to avoid any problems. The contractions of the uterus necessary for the expulsion of the fetus exert enormous pressure on the metabolism of the animal, which must be able to meet the demand. To do this, we must first and foremost understand the different mechanisms surrounding the regulation, absorption and use of minerals by the animal. Since everything is a question of balance, the body of the dairy cow has several processes that allow it to absorb or release calcium depending on its needs. One of the parameters that governs this self-regulation of calcium is parathyroid hormone (PTH) – a hormone secreted naturally by the parathyroid glands when the calcium demand is increasing (see Figure 1).
The PTH has multiple roles:
1. One of them is the stimulation of the activity of osteoclasts during a decrease of the calcemia. In turn, these cells are responsible for bone resorption to release calcium. In contrast, osteoblasts constitute bone tissue by mineralizing osteoids and thus, they allow the formation of bones.
2. Another role of PTH is 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 paramount to meeting this demand and making this process more efficient by maximizing availability for thecow. The hormone calcitriol works in conjunction with the PTH also allowing to decrease the losses of calcium in the urine when necessary. In turn, calcitonin is synthesized by thyroid cells. It is a hypocalcemic hormone and therefore plays an antagonistic role to that of PTH. This hormone inhibits rather the reabsorption of calcium in the kidneys, it inhibits bone resorption and promotes bone formation.
Magnesium is also essential for the functioning of several enzymes, it intervenes inter alia in the formation of the adenosine triphosphate (ATP) as well as in the transfer of nervous influxes. Magnesium is also important to the integrity of bones and teeth. Like other minerals, magnesium has absorption mechanisms of its own to be made 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 ration as well as the dietary intake will play an important role. Unlike calcium, magnesium is largely absorbed in the rumen epithelium once soluble and 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 the urine. The main role of aldosterone is more in regulating 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 the food intake is sufficient. Otherwise, the animal does not have the opportunity to draw on its bone reserves to maintain its homeostasis. Like magnesium, calcium is an essential mineral for the success of the transition period and is one of the most important factors for the prevention of milk fever. Magnesium actively participates in calcium homeostasis by influencing the absorption of calcium and this role is all the more important when the anion strategy is employed. The use of anionic salts in preparation for calving 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). As a result, a lack of magnesium drastically increases the risk of milk fever.
To conclude, not all available mineral sources have the same bioavailability. In addition, some elements may interfere with the absorption of other essential elements. For example, rations rich in potassium may limit the absorption of magnesium. These aspects should therefore be taken into consideration when formulating rations and in choosing the nutritional strategy to be applied on the farm. The success of a successful lactation is certainly multifactorial. However, the transition period is decisive for the health of the cow as well as for the production of the coming months. Emphasizing this period of the production cycle is therefore a wise choice.
Gabrielle L. Dumas, agr., DCL nutrition + animal health