Lactation, breed and milk yield

Lactation number is a critical factor in the etiology of HC. Reinhardt et al. (Reference Reinhardt, Lippolis, McCluskey, Goff and Horst2011) described that plasma calcium concentrations decreased much less during the first 48 h postpartum in primiparous cows compared to multiparous cows (2.15 v. 1.85 mmol/l, respectively). These data suggest that the mechanisms for calcium homeostasis become less effective as the cow undergoes subsequent lactations and may contribute to greater or prolonged HC in older animals. In agreement with this observation, Fleischer et al. (2001) and Caixeta et al. (2017) described a positive correlation between HC and lactation number. In primiparous cows, remodeling processes in the bones are very active, and therefore calcium resorption from the bones is rapid and efficient compared to multiparous cows (Iwama et al., 2004). Aging also results in a decline in the active transport of calcium in the intestine and impaired production of calcitriol (Woodrow et al., 2006), affecting the capacity of the animal to rapidly modulate imbalances in blood calcium concentrations.

In addition to lactation number, breed also contributes to HC susceptibility in dairy cows. Henderson (1938) reported significant differences in HC incidence between cow breeds (15.3% in Brown Swiss, 13.3% in Shorthorn, 8.6% in Guernsey and 6.0% in Ayrshire). Lean et al. (2006) demonstrated that HC incidence is 2.25 times higher in Jersey cows than in Holstein cows. Similar results were reported by Chiwome et al. (Reference Chiwome, Kandiwa, Mushonga, Sajeni and Habarugira2017) on HC incidence in Jersey and Holsteins cows (14.78% and 4.82%, respectively). Despite several studies having described HC incidence among breeds, the mechanism involved in these differences remains unclear. Goff et al. (Reference Goff, Reinhardt, Beitz and Horst1995) determined that Jersey cows have fewer intestinal calcitriol receptors than Holstein cows (95 v. 142 fmol/mg protein, respectively). The relative amount of intestinal calcitriol receptors in Jersey cows compared to Holsteins cows may affect the amount of calcium that can be absorbed in the intestine, which could be a potential explanation for the differences in HC susceptibility between breeds. It has also been suggested that differences in HC incidence between Jerseys and Holsteins may be related to the amount of milk produced by kilogram of BW (Blake et al., 1986; Chiwome et al., Reference Chiwome, Kandiwa, Mushonga, Sajeni and Habarugira2017), which is higher in Jerseys (average BW = 454 kg; average milk production = 31.7 kg/day) compared to Holsteins (average BW = 680 kg; average milk production = 35 kg/day) (Capper and Cady, 2012). However, not only the relation between milk yield and BW, but milk yield alone appears to be positively correlated with HC incidence in dairy cows. Jawor et al. (2012) demonstrated that cows suffering HC during the first 48 h postpartum are those that produced approximately 6.4 kg of milk more per day compared to cows without HC. Similarly, Gild et al. (2015) described that cows with subclinical HC produced 3.17, 2.71 and 1.90 kg more milk compared to normocalcemic cows on the first, second and third days of lactation, respectively. It is assumed that cows with greater capacity for milk production may also be prone to greater net loss of calcium into the milk shortly after parturition (Jawor et al., 2012), exceeding the already saturated adaptive capacity of the animal.

Mineral balance

Adequate mineral balance is crucial for the normal function of the metabolic and physiological processes in animal systems. Deficiencies or imbalances in the mineral content can lead to abnormal nerve function, muscle contraction, immune response, the blood coagulation cascade and other physiological processes. Minerals present in organisms in large amounts are calcium, phosphorus, magnesium, sodium, potassium, chloride and sulfur. In the specific case of calcium, several studies have detected a correlation between low circulating magnesium concentrations and low circulating calcium concentrations in humans (Classen et al., 1986; Paunier, 1992), mice (Zimmermann et al., 2000) and dairy cows (Kronqvist et al., Reference Kronqvist, Emanuelson, Sporndly and Holtenius2011 and Reference Kronqvist, Emanuelson, Traven, Sporndly and Holtenius2012). Low magnesium concentrations have been shown to reduce the secretion of PTH (Suh et al., 1973), as well as decrease tissue responsiveness to PTH (Reddy et al., 1973; Rude, 1998). Phosphorous status also affect cow’s ability to regulate calcium concentrations in blood around parturition. Indeed, Cohrs et al. (2018) showed how reduced dietary phosphorus intake in late pregnancy increased the calcium resorption from bones, which in turn increase circulating calcium concentrations around parturition.

Based on these facts, it can be assumed that blood mineral status (i.e., magnesium and phosphorous) directly affects the efficiency of the mechanisms involved in calcium homeostasis during the periparturient period in cows, which may result in increased susceptibility to HC.

Traditional management practices for the prevention of hypocalcemia

The use of diets formulated with a negative dietary cation-anion difference (DCAD) prior to parturition has long been established to increase calcium concentrations around parturition (Ender et al., Reference Ender, Dishington, Helge-Bostad and Martinsons1962; Dishington, Reference Dishington1975; Block, 1988). Shohl and Sato (1923) were the first to hypothesize that mineral interrelationships were related to acid–base status. Then, Shohl (1939) proposed that the body excretes dietary cations and anions excess in order to maintain normal acid–base balance. Based on these findings, Dishington (Reference Dishington1975) established that the net acid intake in dairy cows, based on dietary difference between cations and anions, was mostly influenced by sodium, potassium, chloride and sulfur. Therefore, DCAD diets for dairy cows are calculated based on the following formula: DCAD = (sodium + potassium) – (chloride + sulfur) mEq/kg of DM (Charbonneau et al., 2006). Negative DCAD diets aim to reduce the charge ion difference in blood, which decreases blood and urine pH, resulting in strong ion metabolic acidosis. This metabolic acidosis is compensated in part by the bone accepting hydrogen ions (H⁺) in exchange for calcium (Lemann et al., 2003). Thus, the metabolic acidosis created by feeding negative DCAD diets inhibits renal calcium reabsorption prepartum, increasing urinary calcium excretion. This fact triggers the homeostatic mechanisms that increase calcium influx (Gaynor et al., 1989; Phillippo et al., 1994; DeGaris and Lean, Reference DeGaris and Lean2008) and target tissue responsiveness to parathyroid hormone (Horst et al., 1997). In addition, feeding negative DCAD diets (i.e., −59 mEq/kg DM) results in increased calcitriol concentrations in blood compared to cows being fed positive DCAD diets (68 v. 40 pmol/l on day −3 relative to parturition, respectively) (Phillippo et al., 1994). However, decreased palatability of anionic salts is thought to be a limitation of this strategy, which can dramatically affect the already reduced feed intake in dairy cows around parturition (Moore et al., 2000). More recently, the combination of diets with reduced basal cations in combination with more palatable anion sources have been used to increase circulating calcium concentrations around parturition without negative effects on feed intake (DeGroot et al., Reference DeGroot, Block and French2010; Weich et al., 2013; Weiss et al., Reference Weiss, Azem, Steinberg and Reinhardt2015). However, the relation between decreased feed intake and DCAD composition is still controversial. According to a recent publication by Zimpel et al. (2018), reduced feed intake in cows fed negative DCAD diets prepartum (−114 and −113 mEq/kg DM) seems to be a direct consequence of the metabolic acidosis status of the animal and not a consequence of the addition of acidogenic products or salts containing chloride in the DCAD diet. The number of days that cows need to be fed with DCAD diets is another factor that may influence the capacity of these diets to regulate calcium homeostasis at parturition. Lopera et al. (2018) fed two different negative DCAD diets (−70 and −180 mEq/kg DM) to dairy cows for either 42 or 21 days prepartum. They showed that cows fed for 21 days prepartum showed a similar metabolic acidosis at calving as those fed for 42 days postpartum. Therefore, it seems that feeding cows with a negative DCAD for 21 days prepartum is enough to cause a metabolic acidosis at calving and prevent or reduce HC incidence. Metabolic acidosis level prepartum can be easily monitored by measuring either urinary calcium concentrations or urinary pH (Martin-Tereso and Martens, 2014). As described by Leno et al. (Reference Leno, Ryan, Stokol, Kirk, Zanzalari, Chapman and Overton2017) cows fed a negative DCAD diet prepartum (−74 mEq/kg DM) increased urinary calcium excretion (8.2 v. 0.7 g/day, in negative DCAD diet and control cows, respectively) and maintained urine pH between 5.5 and 6.0 compared to the control cows (urine pH 7.89). Additionally, circulating calcium concentrations (2.27 v. 2.16 mmol/l), feed intake (3.07% v. 2.88% of BW) and milk yield (43.9 v. 40.8 kg/day) increased postpartum in cows fed a negative DCAD diet compared to control cows. Similar results have been recently shown by Diehl et al. (2018) using a negative DCAD diet (−222 mEq/kg DM) during the last 21 days prepartum.

As an alternative to negative DCAD, feeding low calcium diets prepartum have also been used to induce homeostatic adaptation of calcium metabolism in dairy cows. Calcium requirements for dry cows are minimal (22 g/day for a 500 kg cow; NRC, 2001), being necessary only for maintenance and fetal skeletal development. Feeding <20 g of calcium per day during the dry period decreases calcium concentrations in blood, which activates calcium homeostatic mechanisms involving calcium mobilization from bones, intestinal calcium absorption and renal calcium resorption (Kichura et al., 1982). This prepartum feeding strategy results in increased calcium mobilization from bones into the bloodstream and contributes to compensate calcium requested for milk production (Thilsing-Hansen et al., Reference Thilsing-Hansen, Jorgensen, Enemark and Larsen2002a). Additionally, cows fed low calcium diets before parturition increase intestinal efficiency for calcium absorption at parturition. In order to activate calcium homeostatic mechanisms before parturition, low calcium diets should provide <20 g of calcium per day. Therefore, high calcium forages such as alfalfa (12 g of calcium/kg DM) need to be exchanged by low calcium forages such as corn silage (1.7 g of calcium/kg DM) and barley silage (3.4 g of calcium/kg DM) in dry cow diets in order to reduce calcium intake. However, this is difficult to achieve in some places of Europe and North America, due to the predominant forages available for dairy cows. Therefore, an alternative strategy was explored in instances where calcium content of forages is relatively high, which is the inclusion of rumen-protected rice bran in the diet (Martin-Tereso et al., Reference Martin-Tereso, Martens, Deiner, van Laar, den Hartog and Verstegen2016). The low calcium content of this product (5.5 g/kg DM) and the calcium-binding capacity of the phytic acid present in rice bran (61.5 g of phytic acid/kg DM) reduce the dietary calcium available for intestinal absorption, activating the homeostatic calcium mechanisms. Indeed, Martin-Tereso et al. (Reference Martin-Tereso, Martens, Deiner, van Laar, den Hartog and Verstegen2016) showed that cows receiving 140 g of rumen-protected rice bran/kg DM prepartum exhibited higher calcium concentrations compared to controls at parturition (1.98 v. 1.87 mmol/l, respectively), 6 h postpartum (2.05 v. 1.85 mmol/l, respectively) and 12 h postpartum (2.15 v. 1.92 mmol/l, respectively). As described above, these extraordinary physiological conditions for efficient calcium absorption can be used in cows after parturition by feeding them a diet without zeolite and with high calcium content. Large amounts of calcium absorbed in the intestine will contribute to the required amount of calcium for milk production and will maintain circulating calcium concentration constant during the first days after parturition. An additional strategy was developed by Thilsing-Hansen et al. (Reference Thilsing-Hansen, Jorgensen and Ostergaard2002b) to maintain low calcium concentrations in dairy cow diets prepartum. These authors aimed to increase the efficiency the intestinal calcium absorption by feeding dietary zeolite (sodium aluminum silicate) during the last 2 weeks prepartum. Similar to rice bran, zeolite acts by binding dietary calcium, reducing the amount of calcium available for intestinal absorption. Therefore, the functional calcium deficiency prepartum caused by the dietary zeolite stimulates the intestine to increase calcium absorption efficiency at parturition. The use of zeolite appears to be effective in the prevention of HC as cows supplemented with zeolite prepartum had higher calcium concentrations compared to the controls around parturition (2.35 v. 2.05 mmol/l, respectively) (Thilsing-Hansen et al., Reference Thilsing-Hansen, Jorgensen and Ostergaard2002b). Similar results were shown by Khachlouf et al. (Reference Khachlouf, Hamed, Gdoura and Gargouri2019) in cows supplemented with 200 g of sodium aluminum silicate per day during the last 40 days prepartum. In this study, supplemented cows showed higher circulating calcium concentrations at calving than the control cows (~2.34 v. 2.23 mmol/l, respectively).

Another strategy for the prevention of HC consists on the administration of exogenous calcitriol or its precursors in order to increase calcitriol concentrations around parturition, resulting in increased calcium concentrations in blood. Cholecalciferol is one of the precursors of calcitriol. Cholecalciferol is converted to calcidiol, the substrate for 1-α-hydroxylase and renal synthesis of calcitriol. Consequently, the role of cholecalciferol in the prevention of HC has been thoroughly investigated. Hibbs et al. (1947) demonstrated that supplementation with 1 to 5 million IU of cholecalciferol per day during 2 to 4 weeks before parturition was not sufficient to prevent HC in Jersey cows. In subsequent studies, Hibbs and Pouden (Reference Hibbs and Pounden1955) demonstrated that daily supplementation with 30 million IU of cholecalciferol between 8 and 3 days prevented HC in Jersey cows with previous HC problems. However, several studies have shown that the amount of dietary cholecalciferol required for increasing calcium concentrations is close to the toxic dose (Swan, 1951; Capen et al., 1966; Julien et al., 1977), causing marked anorexia, loss of BW, dyspnea, tachycardia, recumbency and severe cardiovascular calcifications (Thilsing-Hansen et al., Reference Thilsing-Hansen, Jorgensen, Enemark and Larsen2002a). According to nutrient requirements for dairy cattle published by the NRC (2001), approximately 20 000 IU of cholecalciferol per day are necessary to support calcium and phosphorus homeostasis during lactation. In another study, a single subcutaneous injection of calcitriol (300 µg) within the first 6 h postpartum successfully increased calcitriol concentrations from 58.5 to 1271 pmol/l 24 h, resulting in increased total calcium concentrations from 2.52 to 3.02 mmol/l (Vieira-Neto et al., Reference Vieira-Neto, Lima, Lopes, Lopera, Zimpel, Sinedino, Jeong, Galvao, Thatcher, Nelson and Santos2017). Additionally, the combination of DCAD diets together with dietary supplementation of either cholecalciferol or calcidiol prepartum in dairy cows has been explored (Martinez et al., Reference Martinez, Rodney, Block, Hernandez, Nelson, Lean and Santos2018). Feeding prepartum cows a diet containing a negative DCAD diet (−130 mEq/kg DM) regardless of the source of vitamin D resulted in increased calcium concentrations around parturition (2.18 mmol/l) compared to cows fed a positive DCAD (+130 mEq/kg DM) diet (1.96 mmol/l). This suggests that the source of vitamin D supplementation does not matter prepartum as long as it is in combination with a negative DCAD diet.

Novel treatments for the prevention of hypocalcemia

In the last decade, the knowledge surrounding the endocrine regulation of calcium during lactation has increased. Based on this new knowledge, factors such as 5-HT have been manipulated to develop novel and potentially more efficient strategies to prevent HC in parturient dairy cows.

In addition to the ability of the mammary gland to synthesize 5-HT, the mammary gland is able to respond to 5-HT as the bovine mammary epithelium expresses multiple 5-HT receptors such as 5-HT-1B, 5-HT-2A, 5-HT-2B, 5-HT-4 and 5-HT-7 (Hernandez et al., Reference Hernandez, Limesand, Collier, Horseman and Collier2009). At the level of the mammary gland, 5-HT is thought to increase PTHrP production (Hernandez et al., Reference Hernandez, Gregerson and Horseman2012); however, it is not known how 5-HT induces the production of PTHrP in the mammary gland. Based on these findings in rodents, research has been performed to evaluate the role of 5-HT on calcium homeostasis in dairy cows (Weaver et al., Reference Weaver, Prichard, Endres, Newhouse, Peters, Crump, Akins, Crenshaw, Bruckmaier and Hernandez2016; Hernández-Castellano et al., Reference Hernández-Castellano, Hernandez, Sauerwein and Bruckmaier2017a and Reference Hernández-Castellano, Weaver, Hernandez and Bruckmaier2017c; Weaver et al., Reference Weaver, Prichard, Maerz, Prichard, Endres, Hernandez-Castellano, Akins, Bruckmaier and Hernandez2017). In these studies, 5-hydroxy-l-tryptophan (5-HTP), a hydroxylated derivative of the amino acid tryptophan, has been intravenously infused to promote the synthesis of 5-HT by the peripheral tissues in dairy cows prior to parturition. Furthermore, 5-HTP has been used as an oral supplement to increase 5-HT concentrations in dairy calves (Hernández-Castellano et al., Reference Hernández-Castellano, Özçelik, Hernandez and Bruckmaier2017b). In Holstein cows, increased 5-HT concentrations in the prepartum period resulted in a less dramatic decrease in calcium concentrations compared to control cows on day 1 (1.93 v. 1.62 mmol/l, respectively) and day 2 postpartum (2.07 v. 1.83 mmol/l, respectively) (Hernández-Castellano et al., Reference Hernández-Castellano, Weaver, Hernandez and Bruckmaier2017c). Similar results were observed in Holstein cows in Weaver et al. (Reference Weaver, Prichard, Endres, Newhouse, Peters, Crump, Akins, Crenshaw, Bruckmaier and Hernandez2016). However, these authors observed that the administration of 5-HTP prepartum resulted in decreased calcium concentrations in Jersey cows before parturition (2.60, 2.25, 2.15 and 2.10 mmol/l from day 4 to 1 prior to parturition, respectively). Jersey cows treated with 5-HTP prepartum increased circulating calcium concentrations earlier postpartum compared to controls (2.25 v. 2.00 mmol/l on day 1 postpartum, respectively). Therefore, it appears that Holstein and Jersey cows differentially respond to 5-HTP administration prepartum. It is possible that in Jersey cows, decreased circulating calcium concentrations prepartum may trigger an earlier recovery of circulating calcium concentrations at parturition. The intravenous infusion of 5-HTP prepartum also resulted in an increase of PYD concentrations in blood on day 1 postpartum (Hernández-Castellano et al., Reference Hernández-Castellano, Hernandez, Sauerwein and Bruckmaier2017a). These results suggest that cows with increased 5-HT concentrations prepartum potentially mobilized calcium from bone tissue prior to control cows. Furthermore, the administration of 5-HTP did not result in a change of either PTH (Hernández-Castellano et al., Reference Hernández-Castellano, Hernandez, Sauerwein and Bruckmaier2017a) or calcitriol concentrations (54 and 48 pmol/l in 5-HTP and control groups, respectively; Hernández-Castellano and Bruckmaier, unpublished data). Therefore, the current data suggest that 5-HT possibly increases calcium resorption from the bones through a different mechanism than PTH, potentially via PTHrP secretion by the mammary gland. The administration of 5-HTP prepartum also resulted in increased 5-HT concentrations in colostrum (37 v. 24 nmol/l in 5-HTP and control cows, respectively) (Hernández-Castellano et al., Reference Hernández-Castellano, Weaver, Hernandez and Bruckmaier2017c). As 5-HTP infusions ceased at parturition, no differences in milk 5-HT concentrations were detected between groups on day 7 postpartum (12 nmol/l). Additionally, Slater et al. (Reference Slater, Endres, Weaver, Cheng, Lauber, Endres, Olstad, DeBruin, Crump, Block and Hernandez2018) combined the use of negative DCAD (−55 v. +14 mEq/kg DM) with the administration of 5-HTP (1 mg 5-HTP/kg BW) prior to parturition, resulting in further increases of both ionized and total calcium concentrations compared to negative DCAD or 5-HTP treatment alone.