Calculating the Dietary Cation-Anion Difference (DCAD) should be done when rations are formulated for both close-up dry cows and lactating cows.  Even though it sounds quite complicated, DCAD’s can be easily calculated by hand, or most computer ration formulation programs will calculate it automatically if the right information is entered from the feed analysis.  Cations are positively charged ions such as Sodium (Na+) and Potassium (K+), and Anions are negatively charged ions such as Chloride (Cl¯) and Sulfur (S¯²).  Even though there are other cations and anions present in rations, these are the main ones we are concerned with when calculating DCAD’s for dairy cow diets. 

The main formula that is most widely accepted among nutritionists is as follows:

DCAD (meq) = (Na + K) – (Cl + S)

The actual calculation involves converting the percent of these ions in the ration to milliequivalents of cations and anions, and is beyond the scope of this paper.  However, we will discuss the ranges of DCAD that are used in calculating both close-up dry cow rations and lactating cow rations, and the reasons these are important to the health and productivity of the dairy cow.

Close-up Dry Cow Rations

Most dairy owners are well aware of all the health complications that can occur around the time of calving, and the importance of good transition cow management and nutrition.  One of the most significant factors that occurs at calving, is that a high percentage of the calcium that is circulating in the bloodstream crosses the blood-milk barrier and ends up in the colostrum.  This massive movement of calcium from the bloodstream into the udder results in a significant drop in calcium levels in the bloodstream in cows as well as first calf heifers.  If this drop in blood calcium is severe enough, the cow will show clinical signs of milk fever (hypocalcemia or low blood calcium). 

Most dairy owners are under the assumption that if the fresh cows are not showing any signs of milk fever, the calcium levels in the bloodstream are normal.  However, a high percentage of the cows may be suffering from a metabolic disease called subclinical hypocalcemia.  In these cows, the blood calcium is below normal, but not low enough for the cow to go down with milk fever.  Subclinical hypocalcemia is like an iceberg, cows with clinical hypocalcemia (milk fever) would be represented by the tip of the iceberg above water, and those with subclinical hypocalcemia would represent the majority of the iceberg below the surface.  Calcium is necessary for all muscle contraction in the body.  This includes the entire gastro-intestinal tract and the uterus as well as the muscles of the legs involved in movement.  Therefore, the problems resulting from subclinical hypocalcemia, are directly related to decreased muscle function in the body.

The efficiency of muscle contraction in the reticulum, rumen, omasum, abomasum, and intestines is reduced, causing a slowing down of the passage rate of the feed consumed.  The decreased passage rate causes the feed to remain in the rumen for longer periods of time, giving the cow a feeling of fullness.  This results in a decreased appetite and less feed consumed during the early fresh period.  This decrease in feed consumption, results in a more severe negative energy balance early in lactation.  It is common to see excessive weight loss in early fresh cows with subclinical hypocalcemia.  This weight loss also has a significant effect on reproduction after the voluntary wait period has passed.  Cows experiencing this problem will not achieve the expected first test day milk weight and peak milk.

The abomasum normally accumulates gas during the digestive process.  However, normal contraction of the abomasum pushes this gas back into the rumen to be expelled through belching.  Cows with subclinical hypocalcemia tend to accumulate more gas in the abomasum since the ability for muscle contraction has been compromised.  These cows are much more prone to develop a displaced abomasum as a result.

The muscle tissue in the wall of the uterus is important in the process of expulsion of the placenta and normal involution or shrinking down of the uterus after calving.  Subclinical hypocalcemia may result in a higher incidence of retained placenta and post partum uterine infections.  These may cause severe health problems, especially if a septic metritis (uterine infection) results.  It is also common for these uterine problems to result in delayed breeding and poor conception.  Severe uterine infections often result in additional weight loss and a more severe negative energy balance.  Cows that go off feed because of a uterine infection will commonly develop a displaced abomasum because of lack of rumen fill.  Ketosis is also common as a result of going off feed from loss of appetite due to the uterine infection.

In addition to the importance of calcium in muscle contraction, it has more recently been shown that calcium plays a very important role in the animals’ immune system.  In fact, it is often referred to as “The second messenger of the immune system”.  Recent research has shown that animals experiencing subclinical hypocalcemia have severely compromised immune systems.  One of the White Blood Cells called the Neutrophil, loses about 50% of its ability to migrate to an area of infection, engulf bacteria, and destroy them.  A study completed in 2011 showed that animals suffering from subclinical hypocalcemia have a 66.6% increase in the incidence of uterine infections.  Subclinically hypocalcemic animals also have an increased incidence of ketosis, elevated NEFA (non-esterified fatty acids resulting from excessive fat mobilization) in the bloodstream, higher levels of blood cortisol (a hormone that suppresses the immune system), and an increase in retained placentas.

In order for a cow to expel the placenta, it has to successfully mount an effective immune response against the fetal placental tissue.  If they have lower blood calcium levels, their immune response is compromised, and a retained placenta occurs.  Cows that experience extra stress around the time of calving will also have higher levels of blood cortisol which also suppresses their immune system.  For this reason, cows that have twins or require assistance at calving, will also have an increased incidence of retained placenta.

As you can see, subclinical hypocalcemia can result in a myriad of health problems that greatly reduces the productivity of the early lactation dairy cow, and makes her more susceptible to infectious disease.  However, it is possible to formulate rations for close-up dry cows, by using the DCAD principle, which will significantly reduce the incidence of  subclinical hypocalcemia, resulting in a much healthier and more productive cow early in lactation.

Milk fever was initially thought to be caused by an excess of calcium during the close-up dry period since it was noticed that a high percentage of animals eating alfalfa hay or silage developed milk fever, and alfalfa was high in calcium.  As a result, rations were formulated without alfalfa hay and with low levels of calcium in order to prevent milk fever.  It was thought that by shorting the cow on calcium just prior to freshening, the process of mobilizing calcium from the bones would already be in place at the time of calving and the cow could draw on its calcium supplies in the bones. This approach was fairly successful in preventing clinical signs of milk fever.  However, since the amount of calcium supplied in the ration was intentionally low, the blood levels tended to drop into the area of subclinical hypocalcemia, resulting in the problems previously discussed.  These problems were not directly associated with subclinical hypocalcemia at that time, and were considered to be separate disease entities caused by other circumstances.

Now that subclinical hypocalcemia is well-defined as a metabolic disease entity, close-up dry cow rations are being formulated using negative DCAD’s to help decrease the incidence of subclinical hypocalcemia in fresh and early lactation cows.  There has been much research conducted to confirm the benefits of negative DCAD rations in preventing both clinical and subclinical hypocalcemia.  A negative DCAD ration (acidic diet) stimulates the process of calcium resorption from the bones.  It is also thought that it allows more calcium to be absorbed from the intestine.  Studies have shown that a well-formulated negative DCAD ration for close-up dry cows results in increased dry matter intake in early lactation, increased milk production, decreased disease incidence, decreased retained placentas and uterine infections, decreased displaced abomasums, decreased udder edema in first calf heifers, and improved reproductive performance.

When formulating a close-up dry cow ration, it is important to have the same type of feed ingredients in the ration that are contained in the fresh cow and/or lactating cow ration as long as they do not interfere with the DCAD ratio.  This allows the rumen microorganisms to adapt to these feeds prior to calving so a severe adjustment does not have to be made in the fresh cow before the ability to digest these feeds is developed.  Adapting to new feeds may take as long as two to three weeks.  This will obviously cause a more severe negative energy balance early in lactation.  The majority of lactating cow rations in the U.S. contain some alfalfa, with those in the Western U.S. containing a high percentage of alfalfa.  Since alfalfa is high in potassium, it can create severe problems with both clinical and subclinical hypocalcemia.  When the negative DCAD principle was first discovered, anionic salts such as Ammonium Chloride, Calcium Chloride, etc., were fed.  It was rapidly observed that these anionic salts were not very palatable and would often result in decreased dry matter intake prior to calving. It is obviously not good for the close-up dry cows to drop in dry matter intake. However, we now have commercial products available that are very palatable, and are high in the Chloride anion, such as Bio-Chlor™, Soy Chlor™, Animate™, etc., that allow us to formulate negative DCAD rations even when feeding forages high in potassium such as alfalfa hay.  These products now make it possible to utilize the same types of feeds being fed to the lactating cows in the diets for the close-up dry cows, making the transition from close-up ration to the lactating ration much smoother.

In some areas of the country, it is common to see close-up dry cow rations that are formulated at a slightly positive DCAD.  This type of ration is fairly successful at preventing milk fever (clinical hypocalcemia), but often predisposes the fresh cows to subclinical hypocalcemia.  Dairy herds on this type of ration often see excessive weight loss early in lactation, increased incidence of uterine infections, and poor reproductive performance.  Other complications sometimes seen are displaced abomasums, retained placentas, and ketosis.  Dairies that have switched to the recommended negative DCAD ration have seen a dramatic decrease in the incidence of subclinical hypocalcemia.

A current ongoing study is looking at the number of cows that have subclinical hypocalcemia.  So far, 344 blood samples from 22 different dairies have been analyzed at the veterinary diagnostic lab at the University of Michigan for blood serum calcium levels.  Samples were collected 12 hours after calving. Subclinical hypocalcemia is usually defined as the blood serum calcium levels being less than 8.5 mg/dl, while some define the cutoff at 8.0 mg/dl.  Ten of the 22 farms were supposedly feeding a negative DCAD ration.  However, within these 10 farms, the DCAD range was a -3 to a +13.  The results showed that 78% of the samples from these 10 herds were below 8.5 mg/dl and 52% were below 8.0 mg/dl.  This illustrates the importance of formulating the DCAD at a point that is negative enough to stimulate the absorption of calcium from the GI tract and to stimulate reabsorption of calcium from the bones.  A general recommendation would be to formulate at a negative DCAD of between -15 to -18.  However, it is still important to check the urine pH to make sure it is at the desired level of 6.0 to 6.5 for Holsteins.

How do you know if your ration is formulated correctly at the optimum negative DCAD?  Once the close-up cows have been on the ration for at least three days, the pH of the urine can be tested.  Urine pH is normally at 8 to 8.5.  The urine pH on cows fed a properly formulated negative DCAD ration should be around 6.0 to 6.5 for Holsteins and 5.5 to 6.0 for Jerseys.  Most cows can be stimulated to urinate when locked up, and urine pH’s are usually checked on a weekly basis to make sure the ration is formulated and mixed correctly.  If heifers and cows are mixed in the same pen and the younger animals have urine pH’s that are too high in comparison to the older cows, the pen is more than likely overcrowded.  If all urine pH’s are too high, either the ration is not formulated correctly, or there is a problem with the loading and mixing of the ration.  If the nutritionist has access to a good forage analysis of the forages being fed, a negative DCAD ration (about -15 to -18 meq per 100 gm of ration dry matter) can be formulated that will greatly decrease the incidence of clinical and subclinical hypocalcemia.  Urine pH’s can be monitored on a weekly basis to make sure that the close-up dry cows are receiving the benefits of a well–formulated ration.  The DCAD should be adjusted to reach the desired urine pH since it is the main indicator of a properly acidified diet.  Close-up dry cows should be on this ration for at least 3 weeks.

It is also important to supplement calcium in the diet when formulating a negative DCAD ration.  Since the animal is being stimulated to increase the absorption of calcium from the intestinal tract, additional calcium needs to be added to the diet.  The majority of nutritionists will usually add somewhere between 100 and 130 grams of calcium per head per day to the close-up dry cow diet.  Also, recent research has shown that the NRC recommended level of Vitamin D may be too low to maintain the desired level of Vitamin D in the bloodstream. The current level recommended by NRC is 21,000 units per cow per day.  This recent research is recommending that the vitamin D level be increased to 40,000 – 50,000 units per cow per day.  Vitamin D plays an important role in the mobilization and absorption of calcium in the body.  Magnesium is also important in this process and it is recommended that at least 0.45% magnesium be fed in the close-up dry cow ration.

Lactating Cow Rations

As soon as the cow calves, she is placed on a fresh cow or lactating cow ration which contains a higher level of non-fiber carbohydrate (NFC), which is mostly starches and sugars.  The fermentation of these nutrients in the rumen results in the production of larger amounts of volatile fatty acids (VFA’s). Production of VFA’s without the presence of substantial amounts of rumen buffers often results in rumen acidosis.  The goal of the close-up dry cow ration was to have a negative DCAD and acidify the animal in order to increase calcium absorption.  This is not deleterious to the close-up cow because of the higher forage to concentrate ratio in her diet.  However, we have just the opposite goal once the cow has freshened.  Now we want to formulate a diet that has a positive DCAD which tends to buffer the rumen and raise the rumen pH, thus decreasing the chances of developing rumen acidosis.

There have been numerous research studies published over the past several years that support the use of rations for lactating dairy cows that are formulated in the range of a +35 to +45 meq per 100 gm of ration dry matter.  These studies show that rations meeting these parameters result in higher dry matter intake early in lactation, higher milk yield and higher 4% fat corrected milk.  These animals were less likely to develop rumen acidosis and were more consistent on their feed intakes.

The DCAD of a lactating cow ration can be increased by adding rumen buffers to the ration such as sodium bicarbonate and potassium carbonate.  Most people do not realize that milk contains a relatively high level of potassium.  High producing cows excrete a substantial amount of potassium in their milk and this potassium needs to be replenished on a regular basis.  Besides buffering the rumen, potassium plays other major roles in the body’s metabolic processes that are essential to optimal health of the animal.  The major source of natural sodium bicarbonate in the cow is saliva.  When the cow is chewing her cud, she secretes large amounts of saliva containing sodium bicarbonate.  High producing dairy cows that are consuming high energy rations with lower levels of forage, chew their cud less than cows consuming high forage rations.  These cows need to be supplemented with substantial levels of sodium bicarbonate to compensate for that lost due to decreased salivation.

Potassium supplementation has also been shown to help cows better cope with heat stress.  The main way that cows dissipate body heat when experiencing heat stress is to pant.  Panting allows the cow to “blow off” hot moist air from the lungs, thus decreasing body temperature.  When cows are panting, they are not able to chew their cud, and as a result, the normal buffering of the rumen from sodium bicarbonate in the saliva is not accomplished.  Adding substantial amounts of sodium bicarbonate and potassium carbonate is especially important during periods of heat stress.  Providing sodium bicarbonate free choice is also beneficial, and large amounts are often consumed during the warm summer months.

Several different researchers have shown that increasing the DCAD of lactating cow diets increases feed efficiency as well as fat corrected milk.  When sodium bicarbonate was added to the diet there was an increase in dry matter intake that resulted in an increase in fat corrected milk (FCM).  When potassium carbonate was added, the increase in FCM was due to an increase in feed efficiency.  Other recent research has shown that adding potassium carbonate to the diet will improve butterfat test by decreasing the formation of fat metabolites in the rumen that suppress milk fat.  Feeding lactating cow diets that are formulated for a higher DCAD will result in improved dry matter intakes, increased levels of FCM, improved feed efficiency, and improved fat metabolism to prevent the formation of metabolites that suppress milk fat.

The benefits of utilizing negative DCAD rations in close-up dry cow diets and positive DCAD’s in lactating cow rations have been well documented in peer-reviewed published research.  The DCAD’s can be easily calculated either by hand or automatically by the majority of the computer ration formulation programs available on the market today.  A good forage analysis is important.  Forages should be tested for sodium, potassium, chloride and sulfur.  Chloride is often not tested for routinely and has to be requested for specifically.  Utilizing the information available on formulating rations with negative and positive DCAD’s is another tool that can be beneficial in maximizing the health and productivity of the dairy herd.