The pre-calving period is one of the most critical times for a dairy cow and ultimately for the productivity and profitability of the dairy herd. Milk produced during lactation is the end product that generates income, but if the dairy cow does not enter lactation in optimum health and condition then milk yield will be compromised along with fertility and ultimately profitability. Many problems arising around calving and early lactation are caused by low blood calcium levels, or milk fever. Many farmers believe that milk fever is only a problem when the cow actually collapses, but sub-clinical milk fever can cause many problems including low feed intakes, difficult calvings, retained placentas, metritis, displaced abomasums, ketosis, etc. It is important to understand how these problems are linked in order to feed dairy cows correctly and avoid calcium related problems, which can be very expensive if you calculate the potential loss of milk yield and the fertility problems, not to mention veterinary costs. Understanding the process also lets us understand how a properly formulated dry cow mineral supplement, Vital Dry can be used to not only supply essential minerals to mother and foetus but also manipulate dry cow physiology to reduce the incidence of milk fever.
Milk fever, or hypocalcaemia, is cause by low blood calcium levels around the time of calving. There is only a small amount of calcium circulating within the blood, and this is insufficient to meet the calcium requirements of the cow in early lactation. At parturition there is a substantial loss of calcium in the birthing fluids, and in addition, colostrum contains twice as much calcium as milk. Therefore, at calving there is a sudden large increase in the calcium requirement, which combines with low blood calcium levels often results in milk fever, or sub-clinical milk fever. Cows that have once had milk fever are more susceptible to this condition at subsequent calvings. Under normal calcium homeostasis (the natural balancing ability of the animal’s body) falling blood calcium levels trigger the release of a hormone which helps stimulate the absorption of calcium from bone and from the gut, and also from the resorption from the urine. However, if calcium levels within the dry cow diet are too high, this calcium metabolisation mechanism can become ‘lazy’ and absorption of calcium from the diet stops, so even if there is calcium in the diet the cow becomes unable to absorb it. This can be made worse by low magnesium intakes in the dry period. Milk fever is a self- perpetuating syndrome as food intakes are depressed by low calcium, therefore further reducing calcium absorption from the gut.
The role cations and anions play in the cause of milk fever is now better understood and recognised, switching the emphasis of blame from calcium. It has now been proven many times that the big milk fever risk factor is the excess of cations, which tend to come from forages high in potassium or sodium; predominantly green forages like grass silage. These alkaline-based cations get absorbed and attempt to influence the blood chemistry of the cow, which she responds to by switching off mobilisation and absorption of calcium sources at a time when her need for calcium is at its greatest. With the input of an on-farm nutritionist it is possible to manipulate the dietary cation and anion balance (DCAB) and include higher levels of calcium in the diet pre- calving. However on smallholder units where such nutritional input is not available we continue to advocate the use of low-calcium, high- magnesium dry cow supplement such as Vital Dry.
When cows have clinical milk fever, their muscle activity is much reduced, which leads to the inability of the animal to stand up or to defecate due to insufficient muscle power. If the animal is not treated in sufficient time, it will roll onto its side, preventing normal rumen gases from escaping. Eventually, the animal will die due to the pressure on the heart from the bloated gases. Even if a cow does not suffer clinical milk fever, subclinical milk fever can have a large impact on cow performance. If we consider the role of calcium in muscle contraction it is easy to understand how many of the problems commonly occurring around calving begin. Calcium is required for muscle contraction, and one of the largest muscles in the body is the gastrointestinal tract. If the contractions within the gastrointestinal tract slow down, so does the passage of food, hence food intakes fall. As a result, nutrient intakes are lower, resulting in energy deficiency problems (such as ketosis, fatty liver syndrome). If the energy balance is affected, milk yield drops and this can be very difficult to compensate for later in lactation. Longer term, fertility can be seriously impaired.
Another major muscle in the body is the uterus. If contractions are less strong due to calcium deficiency this can result in difficult calvings, or retained placentas. As a result, metritis becomes a high risk and the impact on fertility can be severe. For the last 8 weeks of gestation, calcium intakes for dry cows should be reduced by feeding a supplement such as Vital Dry, not feeding DCP (dicalcium phosphate) and reducing fresh green forages high in potassium. Such management changes allow the homeostatic mechanisms of calcium metabolism to be triggered more strongly and help prepare the cow for parturition and the sudden demand for calcium at calving.
A lower level of phosphorous is essential for dry cows but because we want to lower calcium levels a phosphorous source supplying zero calcium must be used. This is the reason for not feeding DCP but using the formulated dry cow mineral Vital Dry to supply the mineral needs at this time. Phosphorous sources that do not contain calcium are more costly than calcium phosphates but the benefits of their use during the dry period far outweigh the costs with regards to cow health. Magnesium phosphate is an excellent source of available phosphorous and it also supplies good levels of magnesium. People believe that by feeding a high quality lactating cow mineral to their dry cows they are providing the best for their animals, but in reality they are creating more of a challenge for their cows because these are generally high in calcium and not high enough in magnesium. A specific dry cow mineral containing magnesium phosphate such as Vital Dry should be used as this supplies trace minerals, good levels of phosphorous whilst reducing calcium intakes and increasing magnesium intakes.
In addition to feeding Vital Dry in the last few weeks of gestation, the following points can help to optimise cow health and subsequent productivity: