Hypocalcemia impacts on cow health future milk production and reproductive performance
Nutan Chauhan, Bilal Chamadia
Animal Nutrition Division
ICAR-National dairy research institute, Karnal, Haryana
Calcium, including gastrointestinal motility and skeletal muscle strength, is important for skeleton tissue and smooth muscle and nerve function. The lowest blood calcium concentration normally occurs within 12 to 24 hours of calving and typically returns to normal within 2 to 3 days after calving in healthy cows. Incidence of hypocalcemia increases with higher milk production and successive lactation. Clinical hypocalcemia is rarely produced by first-calf heifers because they produce less colostrum and milk and can mobilize bone calcium more quickly in their growing skeleton.
Blood calcium concentrations are closely controlled by regulating the absorption of dietary calcium and the release or absorption of calcium from the bone. Two hormones, parathyroid hormone (PTH) and 1,25-dihydroxy vitamin D3, control these processes. As the calcium content in the blood decreases, PTH is secreted and works at the kidney to decrease the urinary excretion of calcium. This move allows for only minor changes in blood calcium concentrations. When higher levels of calcium are needed, as with lactation initiation and maintenance, PTH works on the bone and calcium is reabsorbed and released into the blood. Sufficient magnesium and slightly less alkaline blood pH (known as metabolic acidosis) are required to secrete and effectively bind PTH to its receptor, highlighting the need to include adequate amounts of magnesium in pre-fresh diets and to balance these diets to provide a negative cation-to-anion difference (DCAD) in order to avoid hypocalcemia.
Subclinical Hypocalcemia
Subclinically hypocalcemic cows with blood calcium at or below 8.0 mg/dl (2.0 mmol/l) but not exhibiting clinical signs are considered subclinically hypocalcemic. Prior to calving, the first step to handling fresh cows is to have a healthy ration. After freshening, a well formulated ration may help sustain blood calcium levels that influence the future milk production and post-calving health of a cow. It is significant to have body condition leading up to calving. The BCS should ideally be 3.25 to 3.5.
Sub-clinical hypocalcemia (SCH) in dairy cows is an economically important disease and significantly increases a cow’s susceptibility to mastitis, retained fetal membranes, displaced abomasum, dystocia and ketosis, which can reduce a cow’s productive life Due to the increased need for nutrients to sustain milk synthesis, considerable adaptations occur in the dairy cow with the initiation of lactation and continuing milk development. Thus, metabolic adaptations must take place to support the increased need for calcium. If they do not occur early enough or of appropriate severity, the blood calcium concentration drops below the critical threshold and can result in clinical and subclinical hypocalcemia, or milk fever.
Hypocalcemia decreases immune cells’ ability to respond to stimuli, thereby leading to infections such as mastitis. Hypocalcemia decreases smooth muscle contraction, leading to decreased rumen and abomasal motility, resulting in abomasum displacement and decreased feed intake. Its effects on muscle contraction prevent efficient teat closure, which contributes to mastitis. Cows with SCH don’t display traditional signs of hypocalcemia, such as lethargy, cold ears or the inability to stand. Instead, they are likely to consume less feed and produce less milk. Because every dairy farm is different, addressing hypocalcemia through prevention is not a one size-fits-all venture.
Prevention of Hypocalcemia
- Low calcium diets pre-fresh: Bind the diet calcium for at least 10 days before calving. When successful, the cow’s body is tricked into thinking it is calcium deficient, and it begins to produce hormones to increase diet calcium absorption and bone calcium resorption mechanisms prior to calving. Diets must have less than 20 g of available calcium to be successful. Readily available source of magnesium should be supplied because hypomagnesemic cows will struggle with calcium homeostasis as well.
- Low potassium forages in the diet of close-up (pre-fresh) group:Focus on procuring or growing low-potassium grasses for dry cow diets by avoiding manure application to fields used to grow dry cow forage. Also, harvest grasses when more mature than perhaps you have in the past, as mature forages contain less potassium. Use warm season grasses, such as corn silage, when possible for a portion of the dry cow diet. However, the use of low potassium forages in pre-fresh dairy cow diets will minimize the risk of clinical hypocalcemia, but not the occurrence of subclinical hypocalcemia. If low potassium forages are fed without additional dietary modifications of chlorine and sulfur, changes in the dietary cation-anion difference (DCAD) may not be significant enough to induce metabolic acidosis and avoid a subclinical decrease in blood calcium concentration.
- Feeding anionic salts for 21 days pre-fresh: Add anions to the diet to acidify the cow to restore tissue sensitivity to the calcium regulating parathyroid hormone. Many nutritionists use ration balancing programs that calculate the diet cation-anion difference. 21 days pre-fresh has been shown to avoid clinical (five-fold reduction) and subclinical hypocalcemia by eating a negative DCAD diet. Diets should be formulated using the most palatable anionic mineral supplements to lead to a dietary DCAD of -10 to -15 mEq/100g dietary dry matter. Many commercially available supplements based on anionic minerals or proteins are available for use in the formulation of these diets. To avoid hypocalcemia, close-up diets should be formulated with around 1.0 per cent calcium and 0.35 per cent magnesium. Phosphorus concentration of close-up diets should be 0.25% to 0.3% because excess phosphorus (0.4% total diet) increases the risk for hypocalcemia. As an indication of whether DCAD management is successful, urine pH should be used. When urine pH of the herd averages 6 to 6.6 the week before calving, the diet is correct. Excessive anions are fed (coming from both feed and water sources) if the average urine pH is between 5.0 and 5.5, and the diet needs to be reformulated to avoid a decrease in dry matter intake. Feeding anionic salts to virgin heifers increases feed costs especially with unclear benefits.
- Oral sources of calcium:Calcium supplemented orally (not part of the diet) immediately after calving gives positive response for preventing a drop in concentration of blood calcium. After administration, several oral supplements are absorbed within 30 minutes and blood calcium levels are raised for 4 to 6 hours. Oral supplementation of calcium often is in the form of calcium chloride and calcium sulfate in gel or paste forms. Both of these sources of calcium are considered acidogenic, which, similar to the DCAD ration, would help fresh cows maintain sufficient blood calcium levels. The calcium chloride will be rapidly absorbed, while the calcium sulfate will provide sustained release of calcium during the post-calving period. The calcium chloride can result in respiratory problems if aspirated, and as such, care must be taken when administering it. Solid bolus coated with fat containing calcium chloride and calcium sulfate is effective at increasing the concentration of blood calcium for that two doses (one at calving, a second 12 hours post-calving) should be given after calving. Coated boluses will help minimize the chances of the commodity being aspirated by cows. Watch out for products containing calcium carbonate [limestone] for supplementation. This is the cheapest source of calcium and is absorbed too slowly to really make a difference in calcium levels in the blood. Some boluses will also contain other ingredients such as vitamin D, but fresh cows really just need rapidly absorbed calcium from a quality bolus that’s easy to swallow.
Implications of Hypocalcemia on Performance
Hypocalcemia affects the health of fresh cows, the development of potential milk and reproductive success. Studies have also shown that immune function in milk cows with low calcium levels in the blood is impaired. Cows with lower blood calcium concentrations within the first day after calving are more likely to have a displaced abomasum, ketosis (and fatty liver), retained placenta and resulting metritis and mastitis. Some studies have shown a decrease in feed intake and rumination and corresponding higher non-esterified fatty acid (NEFA) concentrations after calving.
Bottom Line
Prevention of hypocalcemia around calving is an important component when designing transition cow programs for optimum post-calving health, reproduction efficiency, and milk production. For colostrum and milk synthesis, muscle and nerve function, and immunity, adequate calcium is necessary. It is easy to identify clinical cases of hypocalcemia and for milk managers to recognize that improvements in feeding and management are needed to avoid potential cases. Subclinical hypocalcemia, on the other hand, is not easy to diagnose and can be a risk factor for herds with a high metabolic disorder occurrence rate. Subclinical hypocalcemia potentially occurs in over 50% of dairy cows, does not present with recognizable symptoms, and can only be diagnosed when blood samples are collected within the first 1 to 2 days post-calving and blood calcium concentration is determined to be below 8 mg/dl. Prevention is the key, as with all metabolic disorders, and the use of anionic salts and other management strategies can help to avoid this metabolic disorder.
