Nutritional Significance of Dietary Cation Anion Difference (DCAD) in Transition Cows

Dietary Cation-Anion Difference(DCAD) balancing is a complex concept but has simple and profound effect on performance and health of dairy animals. It exerts strong and linear effect on metabolic disease (Hypocalcaemia or milk fever). It plays vital role in transition period. DCAD deals with mineral homeostasis and acid base balance. Milk fever (periparturient paresis) occurs in dairy cattle after calving because of low blood calcium levels as result of calcium moving into milk. There are about 23 grams of calcium in 10 litres of colostrum, and when this is added to the normal amount of calcium needed for maintenance, the needs of the cow can be more than 10 times the supply of calcium in her bloodstream. When the demand for calcium is greater than the supply in the blood this can cause the problems of milk fever, unless the cow can rapidly mobilize stored calcium in her body (e.g. in bones) to offset the situation. A nutritional approach to managing milk fever involves monitoring specific elements in the diet.
Dietary Management during Prepartum period
For purpose of optimal nutritional management of dairy cows which are fed prepared feeds, the dry period is divided into two distinct categories-far-off dry and close- up or transition groups respectively. Minerals nutrition of close-up group is highly significant. Minerals should be provided to close-up cows in known quantities either as part of grain mixture or total mixed ration (TMR).
Calcium and Phosphorus nutrition 
Diets high in calcium during the prepartum can result in high incidence of milk fever and diets low in calcium will reduce the incidence of milk fever in dairy cows. Feeding more than 100gm of calcium daily during dry period is associated with increased incidence of milk fever. A cow weighing 500 kg requires only 33gm daily Ca to meets its fetal and maintenance demands in last two months of gestation. Diets with low calcium 20gm/day fed during last two weeks before calving is highly effective. Low dietary calcium activates calcium homeostatic mechanisms before calving which helps cow to absorb intestinal calcium and to mobilize calcium from bone. At least 14 days low calcium diets are required for minimizing the risk of milk fever. Supplementing dry cow ration with sodium aluminium silicate and soyabean oil during last two weeks of pregnancy is effective in preventing milk fever. They both act as dietary calcium binder. Increased level of phosphorus, >80gm per head /day, can also increase the incidence of milk fever. Increased serum phosphorus has an inhibitory effect on renal enzymes, when lowered will reduce intestinal calcium absorption.
Cation-anion difference (DCAD)
The calculation for DCAD requires converting the various anions and cations in a diet into milliequivalents (mEq). This is done because of the different chemistry of each element and a system that accounts for the impact of each one in the balance calculation is necessary. DCAD is reported in mEq/kg of diet. The DCAB equation involves subtracting the mEq of anions from the mEq of cations and the result can be positive or negative.An important aspect of evaluating a diet for DCAD is that the mineral content of the diet has been accurately determined. A mineral analysis by wet chemistry, not by near-infrared (NIR) analysis is important. Another consideration is to adjust for the cations and anions present in drinking water. Not all minerals in water are necessarily nutritionally available to the cow but water high in cations or anions could affect the DCAB.
DCAD only measures the levels of four macro-minerals in the diet:
Cations (+): Sodium (Na) & Potassium(K)
Anions (-): Chloride (Cl) & Sulphur (S).
Measured in mEq/kg of dry matter
Sodium + Potassium) – (Chloride + Sulphur) = DCAD
The equation does not include other dietary cations and anions like Ca2, Mg2, and PO4 which have a minor role. This is a more accurate and practical method of controlling milk fever.
Milliequivalents (mEq):
Milliequivalents are calculated by multiplying the content of each element in the diet by a conversion factor.

Positive DCAD
More Cations (+) than anions (-) in feed leads to the production of more blood buffers and less hydrogen ion (metabolic alkalosis).This should be strictly avoided in transition period.
Negative DCAD
More anions (-) than cations (+) in feed leads to the reduction of blood buffers and allows for accumulation of hydrogen ion (metabolic acidosis). Diets high in cations, especially sodium and potassium, tends to induce milk fever compared with those high in anions viz chloride and sulphur. Most studies indicate that a DCAD of -50 to -100 mEq/kg DM is optimal for prevention of milk fever. Supplementation of diets in the last 3 weeks prepartum with anionic salts at a rate sufficient to decrease DCAD to -15mEq/100gm of dietary DM and urine pH to 6.0 prevented most cases of parturient hypocalcaemia. Monitoring urine pH can be a useful aid to find the effective intermediate inclusion rate and it is suggested that a urine pH of about 6.5 is ideal. Commercial anionic products fed to non-lactating dairy cows in a total mixed ration, after 4 days reduced urine pH below the desired threshold level of 6.5.
Effect of DCAD on Dry matter intake, performance in pre-and postpartum dairy cows
Cows had numerically lowered DMI in transition period with reduced DCAD. It is advisable to select more palatable anion sources or a reduced dose of anion sources if DMI is Ammonium chloride, Magnesium sulphate, Magnesium chloride. Ammonium chloride is more effective than most other salts as an acidifier and useful practically in control of milk fever. About 11% dry matter intake (DMI) and 9% milk yield were increased in early lactating cows fed +200 vs. −100 DCAD diet. Various research says that milk fat and total solid percentages and 4% FCM yield increased for cows fed positive DCAD in early lactation period. 21% incidence of milk fever is reduced when ammonium chloride is used.
Effect of DCAD on Metabolic disorders in pre and postpartum dairy cows
The prevalence of hypocalcemia is as high as 70% for multiparous cows, although only 8% exhibited clinical hypocalcemia that lowers the 16% yearly milk yield. Feeding low DCAD during the 3 to 4 week before calving had beneficial effects on systemic acid-base status, calcium metabolism, Ketosis, fatty liver, mastitis, udder edema and also postpartum productive performance. However, feeding negative DCAD to periparturient dairy cows proved a useful nutritional practice. Hypocalcemia is a predisposing factor for dystocia, prolapsed uterus, retained placenta and early metritis. So feeding negative DCAD and positive DCAD pre and postpartum is seen highly significant to control metabolic disorders.
Role of fodder in DCAD balancing
Green fodder plays vital in DCAD either it is positive or negative. During close up phase for achieving negative DCAD green fodder like sorghum, jowar, berseem, alfalfa and dry fodder wheat straw should be eliminated from diet because they are rich source of sodium and potassium, Instead, one should use corn silage, maize green and concentrates. Practical experience shows that feeding green fodder like sorghum, jowar and leguminous fodder has detrimental effect on health status of cow which are seen in terms of udder oedema, Retention of Placenta (ROP) teat bleeding soon after calving.
Role of Magnesium in DCAD balancing
In transition dietary magnesium should be balanced to 0.4% DM It is seen that complications like milk fever and Downer’s Cow are more prevalent in postpartum. Magnesuim chloride is commonly used in transition to cure such complications.
Management of DCAD by application of gluconeogenic precursors and anionic salts. 
Commonly it is seen that there is reduced DMI in transition phase which leads to negative energy in transition cows, if this negative energy is not cured milk fever, ketosis, Downer’s cows, phosphorus deficiency like metabolic disorder prevails which is lethal for milk production. In that case, a gluconeogenic precursor is highly effective, It not only combats negative energy with help of anionic salts like ammonium chloride, calcium chloride, magnesium chloride but also counteracts with disorders like milk fever, ketosis and other metabolic problems which means that simultaneous use of anionic salts help in management of DCAD.
Supplementation of –ve DCAD during transition leads to increase blood calcium, which prevents milk fever, reduces udder odema, retained placenta and displaced abomasum and ketosis in postpartum cows.1. If a diet is calculated to be +200 mEq/kg dry matter or more, a switch to lower potassium forages in the dry cow diet should be considered first. In some cases, the introduction of anionic products, particularly the older anionic salts, can reduce feed intake because they may be unpalatable. Reduced feed intake before calving can create bigger problems than milk fever, such as displaced abomasum and ketosis. Simultaneous use of anionic salts like calcium chloride or ammonium chloride along with some gluconeogenic precursor, not only combats negative energy but helps to manage DCAD efficiently.2. The DCAD in the transition diet should be between negative-50 to -100mEq/kg dry matter to effectively control milk fever and low blood calcium.3. Avoid usage of green fodder like jowar, sorghum, berseem, alfa-alfa and wheat straw to attain negative DCAD, because they are rich in sodium and potassium.  Use corn silage, corn green to attain negative DCAD.4. Monitor cow urine when using anionic products. Urine pH is a reasonable indicator of metabolic pH status and reflects the effectiveness of anionic products. Urine pH should be 6.0 to 6.5 for Holsteins and 5.5 to 6.0 for Jerseys.5. A gradual introduction to anionic products and gluconeogenic precursor incorporating them into a total mixed ration (TMR) can reduce palatability problems.
To know more about gluconeogenic precursor, please contact  manish.pathak@kemin.comReference are available on request.