hypocalcemia in dairy cattle

HypocalcemiaIn Diary Cattle

By

Stefanie A NewhouseSenior SeminarDr. Tera Montgomery

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Table of Contents

TABLE OF CONTENTS........................................................................................................................................... 2

ABSTRACT........................................................................................................................................................... 3

INTRODUCTION.................................................................................................................................................. 4

MEDICAL IMPACT OF HYPOCALCEMIA...............................................................................................................................4ECONOMIC IMPACT OF HYPOCALCEMIA.............................................................................................................................5

PHYSIOLOGICAL EFFECTS OF HYPOCALCEMIA...................................................................................................... 6

NEUTROPHIL FUNCTION.................................................................................................................................................7BONE RESORPTION MARKERS.........................................................................................................................................8CALCIUM SIGNALING IN IMMUNE CELLS............................................................................................................................9

CAUSES OF HYPOCALCEMIA................................................................................................................................ 9

DCAD THEORY..........................................................................................................................................................10HYPOPARATHYROIDISM................................................................................................................................................11

PREVENTION OF HYPOCALCEMIA...................................................................................................................... 12

EFFECTS ON BEHAVIOR................................................................................................................................................12DIETARY POTASSIUM (K)..............................................................................................................................................13

TREATMENT OF HYPOCALCEMIA....................................................................................................................... 14

SECONDARY HEALTH EFFECTS.......................................................................................................................................15

CONCLUSION.................................................................................................................................................... 15

FUTURE RESEARCH......................................................................................................................................................15

REFERENCES...................................................................................................................................................... 17

3HYPOCALCEMIA IN DAIRY CATTLE

AbstractSubclinical and clinical hypocalcemia during the transition period in dairy cattle has detrimental effects

on animal health, welfare, and production. An array of causes for decreased calcium levels in high-

production dairy cattle have been discovered over the last few decades. These causes can include the

level of calcium in the pre-partum diet, Dietary Cation-Anion Difference (DCAD) diet level being fed, and

tissue sensitivity to parathyroid hormone (PTH) levels. Another important area of interest for

researchers has been the physiological effects of hypocalcemia on the immune cells, neutrophil

function, and bone resorption markers. Along with dietary changes, behavior has been studied as a

possible way to prevent hypocalcemia. Behavioral traits have been studied, but few correlations have

been made between a cow’s behavior and the development of hypocalcemia, especially subclinical

hypocalcemia. Several treatment options for chronic clinical hypocalcemia have been implemented to

on-farm use, including oral calcium supplements and intravenous infusions of calcium. Overall, there is a

greater understanding of hypocalcemia in dairy cattle. However, there is still future research that needs

to be conducted in order to prevent and detect more hypocalcemia cases in dairy cattle before it has a

negative effect on the production for the farmer.

Key words: dairy cow, hypocalcemia, neutrophil function, bone resorption markers, immune cells,

behavior, parathyroidism

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IntroductionMilk production in dairy cattle has continued to increase steadily over the past several decades. Since

calcium is the main mineral component of milk, the cow’s need for readily available calcium at the

beginning of her lactation has also continued to increase. However, if the demand by the mammary

gland for calcium is not met, the cow becomes hypocalcemic. Hypocalcemia is a disorder that develops

when a cow is unable to maintain adequate blood-calcium concentrations. Each year, approximately 5-

10% of dairy cattle develop clinical hypocalcemia, and an additional 25-50% of dairy cattle are affected

by subclinical hypocalcemia.

Any mammal can develop hypocalcemia during the transition period. However, dairy cattle have been

bred to produce such a large amount of milk that they are particularly susceptible to becoming

hypocalcemic. When the cow’s body has such a large need for calcium, an increase in calcium

mobilization from the bone occurs. The mobilization of calcium from bone happens because dietary

calcium is not sufficient to support the demands of lactation. Hypocalcemia has been linked to

significant negative effects on milk yield and reproduction. Additionally, hypocalcemia can lead to a

variety of secondary health problems or disorders such as metritis, retained placenta, mastitis, or a

displaced abomasum. This array of issues, stemming from hypocalcemia, can cost dairy producers over

250 million dollars each year in the United States alone.

Medical Impact of Hypocalcemia

Hypocalcemia is defined as the deficiency of calcium in the bloodstream. This disorder can be clinical in

which the animal is physically displaying signs such as muscle weakness and a decrease in body

temperature, or subclinical where the animal is not showing any readily observable symptoms. Some of

the initial signs of clinical hypocalcemia can be excitability, hypersensitivity, and restlessness. Once the

calcium concentrations get so low that tetany begins to set in, the cow may experience tachycardia or


mild hyperthermia. As the cow’s health continues to decline, muscles will weaken to the point at which

the cow will be sternally then laterally recumbent. The cow will appear to be trembling at first, but then

she will become a “downer” cow once she has reached the stage of recombency. Some of the more

severe cases will exhibit symptoms such as constipation, mild to severe bloating, weak pulses, poor

pupillary light response, flaccid paralysis, coma or even death (Bovine Postparturient Paresis).

Most cows that suffer from hypocalcemia will be diagnosed based on their symptoms they exhibit. Since

the illness is so rapid in nature, a laboratory test is not a feasible diagnostic tool if the animal is showing

clinical symptoms already. However, if a cow has subclinical hypocalcemia, diagnosis is much more

difficult. Subclinical hypocalcemic cows are diagnosed after blood is drawn and a laboratory test reveals

low serum calcium levels (Bovine Postparturient Paresis).

Economic Impact of Hypocalcemia

It is easy to understand the negative effect clinical hypocalcemic cows have on the profitability of the

farm. First of all, a clinical hypocalcemic cow would most likely need to be treated by a veterinarian

using an intravenous infusion of calcium. Along with the cost of the veterinary treatment, studies have

shown clinical hypocalcemia causes a decrease in milk production, as well as a higher susceptibility to

other secondary health conditions, which include: metritis, displaced abomasum, retained placenta, and

mastitis. Additionally, with many of the secondary health problems, more expenses from the producer

will be involved for the treatment of that animal. Clearly, a clinical hypocalcemic cow can be very costly

for a producer.

Similar to clinical hypocalcemic cows, studies have shown that subclinical hypocalcemic cows have a

higher instance of secondary health problems, decreased milk production and a decreased chance of

being bred back. Since subclinical hypocalcemia affects approximately half of the cows on dairy farms in

the United States, most of them go undiagnosed and untreated. Thus, the issues that arise and decrease

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production result in the loss of over 250 million dollars each year in the United States due to

hypocalcemia.

Hypocalcemia has a huge economic impact on the dairy industry in the United States and around the

world. In recent decades, the study of hypocalcemia has continued to grow in order to try and

understand the disorder better to reduce the amount of money lost to this disorder. A lot of knowledge

relating to the causes, prevention, and treatment of hypocalcemia have been discovered and continue

to be discovered by scientists.

Physiological Effects of HypocalcemiaWhen a dairy cow experiences hypocalcemic conditions, they need to compensate for the decreased

blood calcium because calcium is a very important part of regulating a whole host of bodily functions,

including smooth muscle contraction, intracellular activity, and etc. Calcium absorption by the intestine

will typically increase. However, the primary mode for the body to restore homeostatic calcium levels is

through calcium resorption from the bone.

The primary hormone that is responsible for the mobilization of calcium from the bones at the onset of

lactation in mammals is parathyroid

hormone related-protein (PTHrP), which

is synthesized within the mammary gland

and is only detectable in the circulatory

system during lactation and times of

metastatic bone cancers.

PTHrP is similar to parathyroid hormone

(PTH) and it has been extensively studied


in the human and rodent models. However, little research has been done relating to its function in dairy

cattle. Serotonin, 5-hydroxytryptamine (5-HT) regulates the induction of PTHrP. The molecular

mechanisms governing this action are still unknown. Manipulation of 5-HT near the end of a pregnancy

period may be a critical factor in preventing the onset of hypocalcemia during early lactation. 5-HT is a

homeostatic regulator of the mammary gland. 5-HT is produced in a two-step pathway from the amino

acid L-tryptophan using the rate-limiting enzyme tryptophan hydroxylase 1 (TPH1). After L-tryptophan is

converted into 5-hydroxy-L-tryptophan, it undergoes decarboxylation to form 5-HT.

Preventing hypocalcemia could greatly benefit the dairy industry. Cows that remain healthy through the

transition period are more likely to produce an adequate amount of milk, as well as another calf, and

both of these factors are critical in maintaining the economic viability of the dairy farms and the

sustainability of the human food production system.

Neutrophil Function

Martinez conducted a study that was published in the Journal of Dairy Science in 2013 about the effects

of the neutrophil function in hypocalcemic dairy cattle. The study, conducted at the University of Florida

Dairy Unit in Gainsville, Florida, directly compared the neutrophil function of normcalcemic and induced

subclinical hypocalcemia in ten non-pregnant, non-lactating Holstein cows (Martinez).

Among the most important data collected, they discovered that the neutrophil function was suppressed

in cows with subclinical hypocalcemia. Cows that had induced subclinical hypocalcemia had a reduced

percentage of neutrophils with phagocytic activity. Additionally, subclinical hypocalcemic cows had a

reduction in the percentage of neutrophils with oxidative burst activity. Finally, phagocytosis and

oxidative burst declined at 72 hours after the beginning of the infusion in subclinical hypocalcemic cows,

where as those values both increased in normcalcemic cows (Martinez).

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The main outcome of this study revealed the drop in calcium in the blood that occurs during

hypocalcemia negatively effects the neutrophils and makes the cow less likely to fight against

pathogenic bacteria. The depression of bacterial killing is caused from the neutrophil suppression of

phagocytosis. This point also supports the fact that hypocalcemic cows have an increased instance of

secondary health issues. Extra caution should be taken for these cows in order to prevent secondary

bacterial infections.

Bone Resorption Markers

A study conducted by Liesegang, published in the Journal of Dairy Science in 1998, explored the bone

resorption markers in hypocalcemic dairy cattle. The study, conducted at the University of Zurich in

Switzerland, investigated whether hydroxyproline, deoxypyridinoline, or the carboxyterminal

telopeptide of type I collagen could be used as markers to provide evidence of bone resorption during

hypocalcemia in dairy cows (Liesegang).

Among the most important data that was collected, cows showing symptoms of hypocalcemia had

increased urinary hydroxyproline concentrations from parturition to day 14. Deoxypyridinoline

concentrations in the urine were increased after parturition until day 9 and carboxyterminal

telopeptides of type I collagen peaked at day 5. However, these values were the same between cows

with hypocalcemic symptoms and cows without (Liesegang).

The main conclusion researchers made was that assays for urinary deoxypyridinoline and serum

carboxyterminal telopeptide of type I collagen determinations are useful tools to follow the course of

degradation of bone collagen in dairy cattle during hypocalcemia. Further research could develop on-

farm tests to determine levels of these bone resorption markers in order to determine the severity of a

hypocalcemic cow.


Calcium Signaling in Immune Cells

A study published in the Journal of Dairy Science in 2005 by Kimura outlined the influences of

hypocalcemia on immune cells in dairy cows. The study, conducted at the National Animal Disease

Center in Ames, Iowa, tested if the increased demand for calcium in periparturient cows adversely

affects intracellular calcium stores of immune cells (Kimura).

Among the important data they collected, researchers were able to conclude that intracellular calcium

stores decreased in peripheral blood mononuclear cells (PBMC) before parturition and development of

hypocalcemia. This is the cause of a blunted intracellular calcium release response to an immune

activation signal. The study suggests that systemic calcium stress precedes measurable hypocalcemia,

especially in cows that will develop clinical hypocalcemia. PBMC intracellular calcium stores are a more

sensitive measure of calcium stresses in transition cows (Kimura).

The conclusion researchers developed with the data from this study shows a depletion of intracellular

calcium stores starts several days prior to calving in hypocalcemic cows. Further research should be

done in this area to develop a simple test for on-farm use that measures PBMC. PBMC is responsible for

the flux of calcium that would ordinarily activate the cells and prevent the cow from experiencing

immunosuppression (Kimura).

Causes of HypocalcemiaThe physiological effects of hypocalcemia are very complicated and it is difficult to study just one part of

the calcium homeostatic regulatory system. Since the entire process is so complex, the causes of

hypocalcemia are hard to pinpoint. Historically, high levels of dietary calcium have been believed to be

the major cause of hypocalcemia in dairy cattle. Scientists have begun exploring other factors that could

contribute to a cow’s risk of developing hypocalcemia. A major area of nutrition related to the cause of

hypocalcemia is the Dietary Cation-Anion Difference (DCAD) Theory. Hypoparathyroidism, another cause

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of hypocalcemia, has recently been linked to hypocalcemia because it affects the body’s tissue

sensitivity to parathyroid hormone. Overall, understanding some of the major causes of this disorder is

an important part of understanding hypocalcemia in dairy cattle.

DCAD Theory

The Dietary Cation-Anion Difference Theory (DCAD) has become a topic in dairy science that is being

studied more and more nowadays. DCAD is basically an equation of the cations (potassium and sodium)

and the anions (chlorine and sulfur) in the diet of a dairy cow in a relationship such as

(Na+K )−(Cl+S). DCAD is responsible for influencing the animal’s acid-base homeostasis, calcium

status around calving, and mineral element utilization. Previous studies have shown decreasing the

DCAD improves calcium homeostasis at the onset of lactation. Additionally, the risk of developing

hypocalcemia is greatest in cows in a state of metabolic alkalosis, which is induced from feeding a high-

DCAD diet.

A study conducted in 2005 by Lean was published in the Journal of Dairy Science, revisited previous

work related to the DCAD Theory. The study, conducted at the Bovine Research Australasia in Australia,

aimed to examine which form of the DCAD equation provided the best estimate of hypocalcemia risk

and to clarify roles of calcium, magnesium, and phosphorus concentrations of two pre-partum diets in

the pathogenesis of hypocalcemia (Lean).

Among the most important data the researchers found, hypocalcemia risk was highest with pre-partum

dietary concentrations of 1.35% calcium. Additionally, increasing pre-partum dietary magnesium

concentrations had the largest effect on decreasing incidence in hypocalcemia. Finally, increasing dietary

phosphorus concentrations pre-partum increased the risk of hypocalcemia (Lean).

The main suggestion researchers made to producers was the data strongly supports the need to

evaluate macro mineral nutrition apart from DCAD of the diet. It is critical for producers to assess and


control the levels of dietary macro minerals included in this study of the pre-partum diet in order to

prevent the instances of hypocalcemia in the herd. The diet taking DCAD levels into account in this study

is so far the best model currently available for predicting hypocalcemia incidence in dairy cattle (Lean).

Hypoparathyroidism

A study published in the Journal of Dairy Science in 2013 conducted by Goff aimed to look at the effect

of high-DCAD diets fed to pre-partum cows to see if it reduces tissue sensitivity to the parathyroid

hormone (PTH), inducing a pseudohypoparathyroid state that diminishes calcium homeostatic

responses. The study, conducted at the United States Department of Agriculture – Agricultural Research

Service in Ames, Iowa, directly compared cows fed low-DCAD and high-DCAD diets to induce a

compensated metabolic alkalosis and acidosis state, respectively, followed by synthetic PTH injections (J.

P. Goff).

Among the most important data they collected, they noticed cows fed the high-DCAD diet had plasma

calcium concentrations that increased at a much lower rate. Additionally, the cows that were fed the

high-DCAD diet produced significantly less 1,25-dihydroxyvitamin D in response to the PTH injections

than the cows that were fed the low-DCAD diet. Finally, the cows that were fed the high-DCAD diet had

numerically lower serum

concentrations of the bone resorption

marker carboxyterminal telopeptide of

type 1 collagen than the cows fed the

low-DCAD diet. However, the numbers

were not statistically different (J. P.

Goff).

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Researchers in this study made one main suggestion to dairy producers to try and decrease instances of

hypocalcemia in their cattle. It is important that cows that are nearing parturition, especially

multiparous cows, are consuming low-DCAD diets. Low-DCAD diets can increase the intestinal

absorption of calcium, as well as enhance the release of calcium from bones. According to Goff,

“because cows fed low-DCAD diets do excrete more calcium in their urine, it has been suggested that

renal reabsorption of this potential pool of calcium (5-7 g of Ca/d) could provide the calcium needed to

prevent the development of hypocalcemia and milk fever.”

Prevention of HypocalcemiaPreventing clinical and subclinical hypocalcemia cases in a dairy herd is the key to reducing the

production losses from the effected cows. By preventing the hypocalcemic issues from occurring, the

cow’s chances of having secondary health issues are eliminated as well. Several ways hypocalcemia is

being prevented is through the study of a cow’s behavior around the transition period and through the

manipulation of the potassium levels in the cow’s diet.

Effects on Behavior

A 2011 study, published in the Journal of Dairy Science by Jawor, aimed to describe the associations of

subclinical hypocalcemia with milk yield, feeding, drinking and resting behavior during the period around

calving. This information could help producers predict when cows are being affected by subclinical

hypocalcemia based on their behaviors. The study, conducted at the University of British Columbia’s

Dairy Education and Research Centre, directly compared these behaviors in Holstein dairy cows which

had subclinical hypocalcemia with control cows (Jawor).

Among the important data they collected, cows with subclinical hypocalcemia produced an average of

5.7 kg/d more milk during weeks 2, 3, and 4 of lactation compared to control cows. Also, hypocalcemic

cows that were in their third lactation sustained greater milk yields throughout 280 DIM. Additionally,


dry matter intake was, on average, 1.7 kg/d greater during the weeks -2 and -1 and fewer visits to the

water and feed bins occurred in the first few weeks after calving among cows with subclinical

hypocalcemia. Finally, the cows with subclinical hypocalcemia on average stood for 2.6 h longer during

the 24-hour period before calving and 2.7 h less standing during d +1 (Jawor).

The researchers made two main suggestions to dairy producers relating to hypocalcemic cows’ behavior.

First, dairy cattle with subclinical hypocalcemia during the 24-h period postpartum did not exhibit any

major changes in production or behavior that would typically be associated with poor health. For this

reason, cows need to be closely monitored during this time period in case symptoms do appear.

Secondly, the results related to the amount of trips to the feed and water bins post-partum displayed

the importance of ensuring cows have adequate access to feeding and drinking areas during the period

following calving. This is necessary in order to allow the cows to meet their nutrient requirements.

Therefore, it is important to manage the pens of fresh cows closely to ensure a low stocking density.

Dietary Potassium (K)

A study conducted by R. L. Horst in 1997, published in the Journal of Dairy Science, looked at strategies

for preventing hypocalcemia in dairy cattle. The study, conducted at the National Animal Disease Center

in Ames, Iowa, studied ways to minimize the effects of dietary potassium on hypocalcemia incidences

(Horst).

Among the most important data collected, Horst found direct evidence that cows fed a diet low in

potassium and sodium have less milk fever than those on diets high in K or Na. Second, over fertilization

of alfalfa with K results in plants with high concentrations of K that are detrimental to the health of the

periparturient dairy cow. Finally, since many dairy producers purchase a portion of their feedstuffs, soil

K concentrations continue to build over time as most of the K brought onto the farm in purchased feed

remains on the farm (Horst).

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Researchers made one main suggestion with this study relating to dietary K. It is important to feed low

levels of dietary K in dairy cattle rations because K plays a significant role in predisposing cows to

hypocalcemia. The researchers also showed the importance of the acid-base balance of the diet on

calcium metabolism. Finally, Horst suggested, “future research should focus on effective methods for

managing dietary K, resulting in decreased reliance on anionic salts.”

Treatment of HypocalcemiaThe amount of research being done related to the treatment of clinical and subclinical hypocalcemia has

been slowly decreasing over the years because of the fact that causes and prevention for hypocalcemia

are becoming the focus. The specific type of treatment for hypocalcemia depends on the severity of the

disorder. A standing cow would be treated differently than a cow which is unable to stand up on her

own (Oetzel).

Cows that are experiencing subclinical hypocalcemia or are displaying clinical signs of hypocalcemia and

are still able to stand on their own are in the very mild stages of the disorder. At this point, cows can be

treated with oral calcium supplementation. Within a half hour after the supplementation, the cow is

able to absorb an effective amount of calcium into her bloodstream. The calcium concentrations in the

cow’s blood will remain elevated from the supplementation for 4-6 hours afterward (Oetzel).

In more severe hypocalcemia cases, when the cow is unable to stand, a faster treatment is required. The

most rapid treatment for treating severely hypocalcemic cows is with intravenous calcium infusions.

With this treatment, calcium levels in the blood are restored almost immediately. The downfall to this

type of treatment is that blood calcium concentrations are raised to extremely and potentially

dangerous levels. Therefore, if a cow is experiencing severe hypocalcemia, a veterinarian should be

involved in the treatment.


Secondary Health Effects

A wide array of issues can stem from subclinical and clinical hypocalcemia in dairy cattle. First of all,

studies have shown metabolic disorders impair immune function which predisposes the cow to uterine

infections and mastitis following parturition and at the beginning of their lactation. Additional

researcher has linked the instances of ketosis with subclinical hypocalcemia. Finally, retained placenta

and endometritis has higher susceptibility rates in cows that have had hypocalcemia at the beginning of

their lactation. If a cow develops hypocalcemia, a producer should take extra care to insure the risk of

any of the secondary issues is reduced. A veterinarian can help determine the best treatment and

aftercare for a hypocalcemic cow (J. P. Goff).

ConclusionSubclinical and clinical hypocalcemia are major health problems in a majority of dairy cattle and often go

untreated. Cows with subclinical hypocalcemia typically do not show signs or symptoms until the

problem is severe. A cow’s behavior should be closely monitored before and after parturition, and

proper management strategies should be enforced on dairy farms. Taking these simple steps as a

producer could end up saving them thousands of dollars every year from the early detection of

hypocalcemia in the cows. By focusing the efforts on the prevention of hypocalcemia, through focusing

on major factors contributing to hypocalcemia, such as pseudohypoparathyoidism, the instances can be

greatly decreased. In all, this would be beneficial to dairy cows and producers worldwide.

Future Research

The study of clinical and subclinical hypocalcemia has been increasing over the last few decades. This is

due to the large economic impact that the disorder has on dairy industry. However, studies are moving

away from the treatment of hypocalcemia and shifting toward focusing on the causes and prevention of

the disorder. A lot of research should continued to be done focusing on the development of on-farm

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detections for hypocalcemia and preventions to aid the producer and minimize production losses in the

years to come. Overall, hypocalcemia is a very important disorder to study in the dairy industry to help

make the industry more productive as a whole.


ReferencesBovine Postparturient Paresis. 2015. VetCentric. online. 1 February 2015. <http://www.vetmedclinic.com/?p=279>.

Goff, J. P. "Diet-induced pseudohypoparathyroidism: A hypocamcemia and milk fever risk factor." Journal of Dairy Science 97 (2014): 1520-1528. In print. 1 February 2015. <http://dx.doi.org/10.3168/jds.2013-7467>.

Goff, Jesse P. Impact of Milk Fever and Hypocalcemia on Reproductive Performance of the Dairy Cow. 26 January 2011. Iowa State University. online. 1 February 2015. <http://www.extension.org/pages/26036/impact-of-milk-fever-and-hypocalcemia-on-reproductive-performance-of-the-dairy-cow#.VQ8L7454pqI>.

Horst, R. L. "Strategies for Preventing Milk Fever in Diary Cattle." Journal of Dairy Science 80 (1997): 1269-1280. In print. 1 February 2015.

Jawor, P. E. "Associations of subclinical hypocalcemia at calving with milk yield, and feeding, drinking, and standing behaviors around partrition in Holstein cows." Journal of Dairy Science 95.3 (2011): 1240-1248. in print. 1 February 2015. <http://dx.doi.org/10.3169/jds.2011-4586>.

Kimura, K. "Parturition and Hypocalcemia Blunts Calcium Signals in Immune Cells of Diary Cattle." Journal of Dairy Science 89.7 (2006): 2588-2598. in print. 1 February 2015. <http://www.sciencedirect.com/science/article/pii/S0022030206723359>.

Lean, I. J. "Hypocalcemia in Diary Cows: Meta-analysis and Dietary Cation Anion Differnce Theory Revisited." Journal of Dairy Science 89.2 (2006): 669-684. in print. 1 February 2015. <http://www.sciencedirect.com/science/article/pii/S0022030206721300>.

Liesegang, A. "Comparison of BoneResorption Markers During Hypocalcemia in Dairy Cattle." Journal of Dairy Science 81.10 (1998): 2614-2622. in print. 1 February 2015. <http://www.sciencedirect.com/science/article/pii/S0022030298758199>.

Martinez, N. "Effects of induced subclinical hypocalcemia on physiological responses and neutrophil function in dary cows." Journal of Dairy Science 97 (2014): 874-887. In print. 1 February 2015. <http://dx.doi.org/10.3168/jds.2013-7408>.

Oetzel, Garrett R. "Minimizing Hypocalcemia During Early Lactation." Tri-State Dairy Nutrition Conference (2013): 23-34. in print. 1 February 2015. <http://tristatedairy.osu.edu/Proceedings%202013/Garrett%20Oetzel.pdf>.

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