Contents

Introduction
Digestion, Absorption and Water Movement
Electrolyte and Water Control Mechanisms
Water Loss and Diarrhea
Oral Rehydration Therapy
Components of Electrolyte Solutions
Summary

Introduction

The importance of administering electrolytes to scouring calves is well recognized. Most calf facilities include electrolytes as a standard item in their arsenal of medications and treatments for calf scours. It is interesting that even with the widespread use of electrolytes, the method of administration, amount given, timing, frequency, expectations and results are quite varied. That's not surprising considering the misunderstanding, confusion and disagreement surrounding the basic principles of electrolyte and water balance in the body.

The discussions that follow begin with the routine processes of water movement into and out of the digestive tract. This basic treatment of digestion and absorption sets the stage for discussions of water loss, rehydration therapy and electrolyte formulation presented in sections 3 through 5. Section 2, on the other hand, provides a more detailed look at how the body regulates electrolytes, water balance and works to maintain an electroneutral environment. It's definitely worth taking a look at. The principles presented in Section 2 provide a solid foundation for assessing actual situations and developing successful, cost effective treatment and prevention protocols. The information presented in this text is not unique to baby calves, and accurately describes electrolyte and water movement in other animal species.

1. Digestion, Absorption and Water Movement

Overview. This section describes how water moves into and out of the small intestine relative to ingestion, digestion and absorption of nutrients. An overview of intestinal mucosa structure and function is provided. The role of sodium in water movement and the relationships among sodium, amino acids and glucose absorption are also discussed. These are key concepts necessary for understanding electrolyte function, water loss, rehydration therapy and electrolyte formulation.

In healthy animals, large amounts of water are regularly secreted into the small intestine to help digest and absorb nutrients. Most of this water is recovered as the nutrients are digested and absorbed. Nearly twice the total volume of water in an animal's body cycles into and out of its digestive tract each day.

The mucosa, or lining of the small intestine is made up of villi and crypts. Figure 1. Villi project into the open space, or lumen, of the small intestine and are mainly involved in nutrient absorption. Each villus is well supplied with blood and lymph vessels that rapidly move absorbed nutrients away from the digestive tract and into the body. Crypt cells, on the other hand, are primarily concerned with secretion of substances, including water, into the intestinal lumen.

Water movement into the small intestine. As food enters the small intestine, water readily "leaks" between the mucosa cells of the upper small intestine into the lumen. During digestion large food particles are broken down to small absorbable nutrients, increasing the concentration of particles inside the intestine. This concentration, referred to as osmotic pressure, is much greater inside the intestine than it is in the cells and fluids of the body surrounding the digestive tract. Since water flows toward areas of high osmotic pressure, water moves from the body into the intestinal lumen.

Water can also be moved into the intestine through specific action of crypt cells. By pumping chloride ions Cl- into the crypt space of the lumen, crypt cells actively draw water into the intestine. These Cl- ions attract sodium ions (Na+) into the crypt space, increasing the local osmotic pressure. As the osmotic pressure increases, water is pulled into the intestine. Some bacteria, such as E. coli, produce enterotoxins that trigger this pumping mechanism causing hyper-secretion of water. Cholera, which has resulted in the deaths of millions of humans is perhaps the most infamous of these organisms that lock this pump system in the "ON" position.

Water resorption from the small intestine.Water is resorbed from the digestive tract as a result of nutrient absorption, with sodium playing an important role in this process. As a rule: water follows sodium. For example, amino acids and carbohydrates are co-transported with Na+ out of the lumen and into the mucosa cells of the small intestine. Once inside the cell, Na+ is rapidly pumped into the extracellular fluid surrounding the cell, away from the intestinal lumen. As a result of these nutrient movements, a series of osmotic gradients are created which move water from the lumen into the cell, and then from the cell into the extracellular fluid. The ability to concentrate Na+ in the extracellular fluid surrounding mucosa cells, drawing water from the digestive tract, increases as food particles move through the small intestine. By the time food reaches the large intestine, about 80% of the water has been resorbed.

2. Electrolyte and Water Control Mechanisms

Overview. Section 2 describes how cellular, kidney and lung mechanisms regulate the body's electrolytes and water. The kidneys and lungs regulate the chemical composition of blood, providing primary control over electrolytes and water in body fluids. A discussion of acid-base balance describes how the concerted efforts of the kidneys and lungs help maintain an electroneutral environment.

This elementary exploration of the physical chemistry of biological solutions is the most technical in this publication. Although an understanding of this material is not required before reading the remaining sections, these discussions should provide a better understanding of electrolyte and water balance, dehydration, and electrolyte formulation. To skip to the next section, click on Water Loss and Diarrhea.

Cellular mechanisms work in conjunction with the kidneys and lungs to regulate and maintain normal water and electrolyte balance within the body. The kidneys have the largest responsibility for maintaining blood chemistry, and in concert with the lungs are responsible for regulating the acid-base balance within the blood. These processes involve both electrolytes and water.

Figure 2 shows the typical chemical composition of body fluids. Extracellular fluid makes up about 40% of the body's water and includes the fluid in blood and the interstitial space between cells. The remaining 60% is intracellular fluid, residing inside of cells. Extracellular fluid provides all of the nutrients, oxygen, waste removal, pH and temperature control for the cell. Cells simply react to conditions in the extracellular fluid.

Cells. Sodium (Na+) is the primary extracellular ion, and is concentrated outside the cell. Potassium (K+), the main intracellular ion is concentrated inside the cell. Since cell membranes are permeable to Na+, it diffuses into the cell where its concentration is lower. To maintain the low intracellular Na+ concentration, the cell quickly pumps Na+ into the extracellular fluid. Sodium pumps located in the cell membrane pump Na+ out and K+ into the cell, maintaining normal osmotic gradients.

Kidneys. The kidneys filter the blood to remove harmful metabolic acids and wastes and reabsorb those substances the body needs. They help control plasma ion concentration and maintain pH by differential removal of strong ions such as Na+, K+ and Cl¯ from plasma into urine. By removing more Na+ than Cl¯ for example, the kidneys lower the plasma Strong Ion Difference [SID]. SID is simply the difference between the concentrations of positive and negative strong ions. To offset the increase of negative charges in plasma created by removal of Na+, the hydrogen ion concentration [H+] increases. Increasing [H+] lowers pH, making plasma more acidic. On the other hand, removing more Cl¯ than Na+ raises plasma [SID], which lowers [H+] and raises pH.

The kidneys also control blood volume by regulating the amount of water in extracellular fluid. Two hormones, aldosterone and antidiuretic hormone (ADH) help the kidneys control the fluid volume of blood. By altering Na+movement and membrane permeability to water, these hormones affect the amounts of Na+ and water remaining within the kidney tubules and consequently the amounts excreted in urine. For example, when blood volume decreases as a result of water loss, aldosterone and ADH production increase. Elevated aldosterone increases Na+pump activity in the kidney tubules. As a result, more Na+ is removed from the tubules and concentrated within the kidney, reducing Na+ excretion. The increased Na+ concentration in kidney tissue creates an osmotic gradient that pulls water out of the tubules. The elevated ADH level works in conjunction with aldosterone by increasing tubule permeability to water allowing water to follow Na+ out of the tubules, reducing urinary water loss.

Lungs. The lungs play an important role in regulating plasma CO2 and pH. Since CO2 is a gas, the term partial pressure, PCO2, is used to describe its concentration in liquids. Changes in respiration rate relate to changes in the partial pressure of CO2. For example, CO2 is produced during exercise and is removed from muscle tissue by the blood, increasing PCO2. To quickly rid the blood of excess CO2, respiration rate increases. In this process, CO2 from the muscle combines with water to form carbonic acid in the blood, which then dissociates to H+ and bicarbonate ions (HCO3-). Bicarbonate is the primary storage and transportation form of CO2 in plasma. In the lungs, this process is reversed with CO2 and water being exhaled.

As the rate of CO2 production increases, PCO2, [HCO3-] and respiration rate increase. If CO2 production outstrips the lungs' ability to convert HCO3- to water and CO2, plasma [HCO3-] will continue to rise. As a result, plasma [H+] increases to offset the additional negative charges, leading to a decrease in plasma pH.

Acidosis = ↑PCO2 , ↑HCO3- , ↓pH

Alkalosis = ↓PCO2 , ↓HCO3- , ↑pH

Acid - Base Balance. By regulating blood chemistry, the kidneys and lungs maintain an electroneutral environment. The processes involved in maintaining electroneutrality are referred to as acid-base balance. The power of the lungs to excrete large quantities of carbon dioxide enables them to compensate rapidly, while the smaller capacity of the kidneys corresponds to a relatively slower rate of compensation through metabolic means. Both must work in concert to maintain an acid-base balance.

Metabolic Regulation. Strong ions are fully dissociated from each other in plasma and form charged water complexes. For example, Na+and Cl- do not associate with each other to form NaCl in plasma. Instead, Na+ associates with the O- component of water while Cl- associates with the H+ component. The orientation of water molecules to a strong ion counterbalances and isolates the ion's charge, and exposes either the positive or negative portion of the water molecules. Figure 4.

Charged water complexes of Na+ have an overall positive charge that attracts OH-, while negative charges associated with Cl- water complexes attract H+. The attractive force between water molecules is such that hydrogen ions of one water molecule are strongly drawn to the oxygen of another. As a result, hydrogen ions readily "jump" to adjacent water molecules causing water to dissociate, forming hydronium ions [H3O+] and hydroxyl ions (OH-). In acidic solutions, [H3O+] > [OH-], while [OH-] > [H3O+] in basic solutions. Although the term [H3O+] better describes the hydrogen ion status of biological solutions, the term [H+] is more commonly used and is the terminology used in this text.

By selective removal of Na+ or Cl-, the kidneys adjust the relative proportion of H+ to OH- in plasma. As Na+ is removed, the OH- required to offset positive charged water complexes decreases. Consequently, the amount of OH-in the solution decreases. As OH- decreases, the relative amount of H+ increases, bringing about a reduction in pH. On the other hand, removal of negative charged water complexes reduces the amount of H+ needed in the solution to offset negative charges. As the relative amount of OH- increases, the solution becomes more basic and pH rises. In a nutshell, that's how the kidneys help maintain electroneutrality. Keep in mind that in biological solutions, H+ and OH- exist as part of charged water complexes, not as independent entities.

Respiratory Regulation. Bicarbonate (HCO3-) is the primary buffering agent in plasma. Since bicarbonate is the plasma transport form of CO2, its regulation falls under the jurisdiction of the lungs, not the kidneys. As previously discussed, the lungs quickly adjust the partial pressure of carbon dioxide in plasma (PCO2) by either increasing or decreasing respiration rate. As a result, plasma bicarbonate either decreases or increases.

Respiratory & Metabolic Regulation. Although they have their own regulatory processes, the lungs and the kidneys work together to maintain plasma electroneutrality. As an example, consider a baby calf that is undergoing the common summertime problem of heat stress. In an attempt to get rid of extra heat, the calf's respiration rate increases. Although rapid breathing rids the body of some excess heat, it also causes a loss of CO2, which lowers plasma PCO2 and HCO3-. Reducing HCO3- also reduces the [H+] required to neutralize the negative charges associated with bicarbonate. As plasma [H+] decreases, the relative [OH-] increases, causing plasma to become more alkaline. Plasma pH rises. To compensate, the kidneys remove Na+ ions. As Na+ is removed, the [OH-] required to offset the positive charge associated with sodium is reduced. This reduction in plasma [OH-] increases the relative [H+], bringing plasma pH down to normal.

Through their combined actions the lungs and kidney may have averted a couple of potentially life-threatening situations. Nevertheless, the calf has lost body water during the heat stress and plasma [SID] was decreased below normal to maintain electroneutrality. Oral electrolyte therapy is an obvious remedy for both the water and electrolyte loss. If administered early enough, the electrolyte treatment could have averted or at least lessened the heat stress and resulting physiological changes in the calf.

pH. The pH of intracellular fluid is about 7.0 and about 7.4 for extracellular fluid. At normal body temperature, the pH of a solution is 6.8. Therefore, body fluids are actually maintained at a slightly alkaline pH. The pH range of physiological solutions is small, with a pH change of 1.0 being fatal.

Ion movements in body fluids cause changes in pH, making it a dependant variable. As demonstrated above, significant changes can occur in plasma chemistry that result in virtually no change in pH. Furthermore, equal changes in [H+] and [OH-] in physiological solutions do not bring about equal changes in pH. To summarize, a change in pH indicates a problem. It does not, however, indicate what is causing the problem or what needs to be corrected. pH is not a very sensitive measure for evaluating the acid-base status or changes in status of body fluids.

3. Water Loss & Diarrhea

Overview. Pathogens, feed characteristics and management influence digestive function and can result in water loss through the digestive tract. There are four types of digestive water loss. Each is defined in this section. A diarrheic animal may actually suffer from more than one type of water loss at the same time. The process is dehydration and the clinical signs associated with progressive water loss in calves are also discussed.

Types of Water Loss: Increased Permeability. Microbes cause inflammation and damage to the intestinal mucosa resulting in increased water movement into the intestine. This type of water loss is commonly caused by viruses (rotavirus, coronavirus) and protozoa (coccidia, cryptosporidia).

Hypersecretion. This type of water loss is similar to increased permeability in that large amounts of water move into the intestine, but there is no tissue damage. Bacterial enterotoxins stimulate cellular pumps in the crypt cells of the intestinal mucosa to secrete large amounts of ions into the intestinal lumen. These ions draw water into the small intestine. Hypersecretion in calves is most commonly caused by E. coli.

Malabsorption. Epithelial damage of the small intestine reduces nutrient absorption. Viruses and protozoa damage the villi in the small intestine leading to villous atrophy, and can damage the mucosa of the large intestine as well. Normal amounts of water may be secreted into the digestive tract, but tissue damage results in poor nutrient and water absorption. Malabsorption results in increased nutrients reaching the large intestine. These additional nutrients can cause bacterial overgrowth and excessive production of volatile fatty acids (VFAs). As a result, osmotic changes occur that may worsen fluid loss.

Maldigestion. Changes in feed management may lead to maldigestion. A sudden change in feed, use of poor quality ingredients, the presence of feed allergens or other detrimental factors and digestive disorders can lead to maldigestion. Maldigestion usually results in malabsorption.

Dehydration. During diarrhea, large amounts of water and electrolytes are lost from the body. Water moves from the extracellular fluid of the body into the intestinal lumen. This reduces the amount of water in the extracellular fluid. As a result, the concentration of ions and other solute particles in extracellular fluid increases. This means an increase in the osmotic pressure of the extracellular fluid. Since water moves toward areas of high osmotic pressure, water flows out of the cells and into the extracellular fluid. This movement of water from the cells lowers the osmotic pressure of the extracellular fluid and increases its volume. Since the cells lose water in the process, they become dehydrated. In other words, plasma volume is maintained at the expense of cellular fluids.

As dehydration progresses, tissues tend to shrink, skin becomes dry and wrinkled, and eyes become shrunken and soft. Fever develops as dehydration worsens. If water loss continues and plasma volume falls, the kidneys reduce urine output in order to conserve water. As urine output decreases, waste products accumulate in the blood.

Reduced kidney function causes changes in plasma ion concentrations and a reduction in plasma pH. As pH is reduced, acidosis occurs. Both dehydration and acidosis interfere with the animal's ability to maintain its body temperature and lead to hypothermia. The animal's attitude and posture are related to the severity of these factors. Table 1.

Acidosis is more severe in older calves and may contribute more to depression and weakness than in younger calves. As shown in Table 1, the plasma ion deficit of older calves is more severe than younger calves showing the same clinical signs. E coli infections are most common in calves under a week of age and tend to cause hypersecretion diarrhea with rapid and severe water loss. The speed with which dehydration occurs during these infections may not provide enough time for the lungs to compensate for the rapid onset of acidosis. As a result, dehydration rather than acidosis may be more related to attitude and posture in younger calves. Respiratory compensation is consistent with prolonged metabolic acidosis, and could be the reason older calves present a more severe acidosis in each category (Naylor, 1987)

As water loss reaches about 8 - 10% of body weight, blood viscosity increases causing a decrease in cardiac output and a rise in pulse rate. As water loss continues, acidosis progresses, lowering plasma pH to the point that cell membranes start to depolarize. Potassium begins to leave the cells and increases in the extracellular fluids. The reduced membrane potential interferes with muscular contractions, causing the heart to beat irregularly. Blood pressure decreases resulting in circulatory failure and reduced blood flow to the lungs. The pulse weakens and the calf goes into an irreversible shock and becomes comatose. Death results from heart failure.

4. Oral Rehydration Therapy

Overview. The amount and timing of electrolyte replacement therapy is critical for rapid recovery from dehydration. Section 4 describes the relationship between the degree of water loss and the amount of electrolyte solution required to offset the loss. An overview of the effect of different pathogens on water loss and rehydration therapy, regulation of voluntary intake and effects of tube-feeding are presented. The importance of regular milk replacer feedings on the hydration status of the animal and in maintaining nutrient intake is also explored.

The timing of electrolyte replacement therapy is critical. A common mistake is waiting too long before administering electrolyte solutions to affected calves. Table 2 shows that calves can lose as much as 6% of their body weight before showing visible signs of dehydration. Giving fluids too little, too late allows progressive fluid loss. As a result, the calf's condition continues to deteriorate. Most calves that die of scours usually die from loss of water and electrolytes, not from direct action of pathogenic organisms. The focus of any treatment plan should be on replacing lost fluids and restoring acid base balance.

Good candidates for oral rehydration therapy are those calves that can stand and suckle. Weak calves with a poor suckle reflex may need to be tube-fed. Calves that have lost the suckle reflex, are recumbent and unable to rise, are poor candidates for oral rehydration therapy. Subcutaneous and/or intravenous infusions are indicated in more advanced stages of dehydration and acidosis. The calf's ability to recover declines as the severity of dehydration and acidosis increase.

The amount of supplemental fluids a calf needs each day depends on its level and rate of dehydration. Table 2 shows the minimum amount of electrolyte solution required daily by a 100 lb. calf. The amount of fluid indicated at each level is the amount of electrolyte solution that needs to be fed in addition to regular milk replacer feedings. Substituting an electrolyte solution for a regular feeding of milk replacer does nothing to correct fluid loss.

Electrolyte solutions should be fed between milk replacer feedings at least two hours after the milk replacer. This routine provides a more even distribution of liquid consumption throughout the day. A properly formulated electrolyte solution is designed to maximize the absorption and utilization of both water and the various electrolyte ingredients specified on the label. Many electrolytes contain sodium bicarbonate that may alter the pH in the digestive tract and adversely affect the efficiency of nutrient utilization if fed in conjunction with milk/milk replacer. It's best to feed electrolytes and milk replacer separately for maximum utilization of both.

Efficacy of Treatment. enterotoxigenic E. coli causes a hypersecretion type of water loss. In this situation, only about 60% of the electrolyte solution is absorbed, so the frequency of administration needs to be increased. In this case, 40% of the electrolyte solution will pass through the calf's digestive tract, adding to the calf's fecal water loss. This makes the diarrhea appear to be worsening with electrolyte therapy even though the treatment is effective.

Rotavirus, coronavirus and cryptosporidia invade and damage the intestinal villi causing an increased permeability type of water loss. These organisms tend to affect calves over a week old causing a somewhat slower rate of water loss and a more prolonged infection than with enterotoxigenic E. coli. Electrolyte therapy reduces the metabolic acidosis associated with these infections. As a result, the suckling reflex increases, helping the animal to recover without other treatments.

Voluntary Intake. Mammals regulate their sodium intake. As sodium loss increases, calves preferentially consume sodium-containing fluids. This is one reason why sodium is such an important component of an effective electrolyte solution. The solution should be made available at the onset of infection since calves that become acidotic cannot effectively regulate sodium intake. Free choice electrolyte solutions have been shown to decrease mortality in baby pigs from 20% to 7% under naturally occurring diarrhea conditions.

Tube feeding. It may be necessary to tube-feed calves with a weak suckle reflex. In this situation, there is little or no stimulation for closure of the esophageal groove causing the electrolyte solution to enter the rumen rather than the abomasum. Some solution overflows into the abomasum and is absorbed as efficiently as nursed solutions, but rumen bacteria may be washed away.

Although it is critical to rehydrate scouring calves, the possibility of negative effects of tube feeding on rumen microflora does exist. These effects depend on the calf's age and degree of rumen development. Most electrolyte solutions, for example, contain glucose. In the developing rumen glucose is fermented to volatile fatty acids and lactate causing a decrease in rumen pH. This lower pH can destroy certain rumen bacteria, slowing the calf's return to normal feed digestion and absorption.

Milk/Milk Replacer Feeding.Dehydration is reported as the primary reason that scouring calves die. Second place goes to starvation. When normal digestive and absorptive functions of the intestine are impaired, calves cannot absorb adequate nutrients from the diet. Since young calves have precious little in the form of stored nutrients to sustain them, digestive and absorptive problems can progressively lead to rapid weight loss, weakness and death. This situation is made worse when milk replacer is withheld during the treatment process.

Withholding milk replacer does reduce nutrients available for gut pathogens, but also reduces nutrients for the calf. This reduction in nutrients not only compromises the normal gut flora, it also reduces nutrients available for immune function and contributes to intestinal villi atrophy. Figure 3 clearly demonstrates the effect of a sudden reduction in nutrient flow to the small intestine. The villi on the left are healthy intestinal villi of a pig at weaning. The picture on the right shows villi two days later before the pig has adjusted to the new diet.

University of Missouri

The digestive tract requires more energy to keep it going than any other organ in the body. If the inflow of nutrients is greatly reduced, the digestive tract begins to shut down, conserving energy by reducing functions. Villus atrophy reduces nutrient absorption and compromises the protective barrier function they provide against pathogens. There is strong evidence that withholding nutrients also prolongs the duration of diarrhea and slows recovery.

5. Components of Electrolyte Solutions

Overview. Electrolyte solutions should be formulated for their ability to enhance water absorption, water retention and to reduce acidosis. Since diarrhea can have a serious effect on bacterial populations in the digestive tract, inclusion of specific direct-fed microbials favors conditions for growth of beneficial bacteria and reestablishment of a normal intestinal environment.

Water is the most important nutrient for sustaining life. Water corrects dehydration and acts as a solvent for electrolytes.

Sodium (Na+) is the major extracellular ion in the body. Water follows the movement of Na+, making it also the major electrolyte component. As Na+ is absorbed from the digestive tract into the blood and into cells, it causes water to move along with it. Sodium is absorbed by cells through simple diffusion, is also co-transported into cells along with glucose and amino acids (glycine). Sodium can also play a role in voluntary uptake of electrolyte solutions. Since mammals regulate sodium intake, calves will preferentially drink sodium-containing solutions as the calf's sodium loss increases. Sodium bicarbonate is often included in electrolyte formulations. In liquid, sodium bicarbonate dissociates to its basic components, Na+ and bicarbonate. As indicated above Na+ enhances water absorption. However, administration of bicarbonate in oral electrolyte solutions generally provides no effect on water or electrolyte absorption. Bicarbonate. is also an antacid and can have negative effects on nutrient digestion and absorption. Although administration of bicarbonate to calves with severe metabolic problems due to advanced dehydration (recumbent, will not suck) can temporarily reduce acidemia, bicarbonate does not appear to have a beneficial role in oral electrolyte solutions. Click here for more information on Sodium Bicarbonate In Oral Electrolyte Solutions

Glucose is a sugar, or carbohydrate, that facilitates sodium absorption. Glucose is co-transported with Na+ from the digestive tract. This enhances Na+ absorption and water uptake from the small intestine.

Glucose also provides a minor energy source for the calf. However, an electrolyte solution should not be looked at as a replacement for energy and other nutrients provided by milk replacer.

High glucose electrolyte solutions are sometimes presented and used as a replacement for milk or milk replacer during diarrhea. Six quarts of such a solution provides about 75% of the daily energy needed by a baby calf for maintenance, while providing none of the protein required by the calf. Glucose, which is absorbed more quickly than lactose (milk sugar), causes a rapid increase in plasma glucose. Insulin is released into the calf's bloodstream to lower the elevated plasma glucose level. This insulin response is excessive in young calves. Within three hours after administration of the high glucose electrolyte solution, plasma glucose is lower than the pretreatment level.

Glycine is an amino acid that is co-transported with Na+ and works along with glucose to facilitate sodium and water absorption. Glycine is the most easily synthesized amino acid and is most often included in electrolyte solutions.

Potassium (K+) is the major intracellular ion. Potassium helps maintain the integrity of the cell membrane and is involved in neural function and muscular contraction. Advanced dehydration leads to acidosis and severe electrolyte imbalance, causing a loss of cell membrane potential and cell death. A high supplemental level of K+ can be lethal.

Chloride (Cl-) is the major anion, or negatively charged extracellular ion that can be lost during diarrhea. As the kidneys filter blood, they work to maintain electroneutrality by regulating the level of Cl- (as well as Na+) in the body.

Ascorbic Acid (vitamin C) cannot be synthesized by calves until they are about 3 weeks old, and is therefore considered an essential nutrient for calves under three weeks of age. Ascorbic acid is an antioxidant and is found in high concentrations in steroid secreting cells. The concentration of ascorbic acid in plasma is lower in stressed calves than non-stressed calves. Oral supplementation of ascorbic acid elevates the ascorbic acid level in plasma of preruminant calves.

Direct-Fed Microbials (DFMs) are specific, genetically superior species of bacteria that support conditions in the intestinal tract that are favorable to the growth of beneficial microorganisms and are unfavorable for pathogens. DFMs help prevent intestinal colonization by pathogens through production of antimicrobial compounds such as lactic acid, hydrogen peroxide, modified bile acids, and bacteriocins, which are effective bactericidal/bacteriostatic compounds. DFMs compete with pathogens for attachment sites for growth, compete for nutrients, neutralize toxins and stimulate the host immune system. Lactobacillusbacteria rapidly colonizes the newborn intestinal tract and is the predominate microorganism in the small intestine. Lactobacilli survive the digestive process and attach to the epithelial lining. They grow best at a pH of 5.5 and are very effective against E. coli.

Bifidobacteria are primary colonizers of the large intestine, growing best at a pH between 6.5 and 7.0. Bifidobacteria are antagonistic toward E. coli and Clostridium.

Fructo-oligosaccharides (FOS). FOS are naturally occurring plant sugars. When fed to animals, they travel intact to the large intestine where they provide a source of nutrients for beneficial bacteria such as Bifidobacteria . FOS have been shown to increase volatile fatty acid production in the large intestine and improve calcium and magnesium absorption. FOS cannot be digested by the animal or by pathogenic bacteria, and are an excellent complement to direct-fed microbials containing bifidobacteria.

Glutamine/Glutamate are amino acids that have been shown to improve villi height and overall intestinal morphology during periods of stress and following injury. Both glutamine and glutamate provide a local fuel source for enterocytes, the absorptive cells in intestinal villi.

6. Summary

Maintaining a calf's electrolyte and water balance is key to optimizing digestive function and minimizing the impact of intestinal pathogens. The presence of pathogens in the digestive tract can lead to changes in the normal processes of nutrient digestion, absorption and water movement. Diarrhea, or scours, occurs when normal movement of water into and out of the digestive tract is disrupted, resulting in water loss and dehydration.

The kidneys and lungs work in concert with cellular mechanisms to control electrolytes and water in body fluids. Loss of fluids through diarrhea is accompanied by loss of body salts. The kidneys and lungs attempt to control the chemical composition of blood and maintain an electroneutral environment. As dehydration progresses, they begin to lose control. This fluid and electrolyte loss produces a change in body chemistry that can lead to severe depression in the calf and eventual death.

Rehydration therapy with an effective electrolyte solution can help alleviate effects of dehydration and help restore a normal electrolyte balance. Timing and amount are critical. To be effective, an electrolyte solution must be properly formulated with the primary purpose of enhancing water absorption. The secondary purpose is to provide a source of major extracellular and intracellular ions to help replenish key electrolytes lost during dehydration. And finally, an electrolyte solution should provide ingredients that help rebuild and maintain a healthy intestinal environment.

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