DCAD: Important for dry and lactating dairy cattle
When we say “electrolytes” on a dairy farm, one of the first things that come to mind is feeding electrolytes to scouring calves. Electrolytes play an important role in the cow’s diet as well. When we’re talking about electrolytes for adult animals, the term we commonly use is DCAD.
Electrolytes are classified as cations or anions:
- Cations are positively charged and include sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg).
- Anions carry a negative charge and are represented by sulfur (S), chloride (Cl) and phosphorus (P).
The dietary cation-anion difference (DCAD) of a ration is a measurement of strong electrolyte concentration. Although several equations can be used to calculate DCAD (expressed in milliequivalents (meq)), the most commonly used is:
DCAD = meq (Na + K) – (Cl + S)
In prefresh cows, the focus is on providing a diet that contains a negative DCAD, whereas, in lactating cows, the focus is on providing a diet that contains a positive DCAD.
Anions for prefresh cows
The main focus in prefresh cows – and providing a diet with a negative DCAD – is to aid in the prevention of hypocalcemia, or milk fever. Hypocalcemia is caused by low blood calcium levels in the early postpartum period. Hypocalcemia occurs when demands for calcium exceed what the cow is able to absorb from dietary sources and mobilize from storage in bone.
Recent information indicates that, during the transition period, approximately 25 percent of primiparous cows and 50 percent of multiparous cows suffer from subclinical hypocalcemia; between 5 and 10 percent of cows develop clinical hypocalcemia. Cows that experience hypocalcemia are more likely to develop other metabolic disorders, such as fever, ketosis and metritis. Reduced pregnancy rates (longer calving intervals) and productivity can result.
Normally, blood pH is alkaline (pH about 8.0) since feedstuffs typically contain more cations than anions. Early studies in the 1960s and 1970s showed that adding anions to prefresh diets lowered blood pH (and, as a result, urine pH) and caused blood calcium concentration to increase.
The changes in calcium metabolism that result from lowering of blood pH have been more recently investigated and are complex. Multiple organ systems (mammary gland, bone, kidneys) as well as several hormones (Vitamin D-1,25, parathyroid hormone, serotonin) are involved. The end result of a properly implemented low DCAD diet, however, is a reduction in hypocalcemia.
Implementation at the farm level is through incorporation of strong anion sources in the prefresh ration. Sources that contain chloride are the most potent, such as hydrochloric acid (HCl), ammonium chloride (NH4Cl) and calcium chloride (CaCl2). Other sources include sulfates(SO4), such as calcium sulfate (CaSO4) and magnesium sulfate (MgSO4).
Urine pH should be monitored for successful application of a negative DCAD program. The goal is to reduce urine pH to levels below what is physiological. Monitoring of blood calcium concentrations can be useful as well.
Cations for postfresh cows
In comparison to prefresh cows, lactating cows often benefit from a positive DCAD diet. Here we want to focus on strong cations, notably sodium and potassium.
Due to higher incorporation of rapidly fermentable carbohydrates in their diets, lactating cows tend to experience increased levels of acid buildup, both ruminally and in their blood. These rations also tend to support less rumination, which reduces the production of salivary bicarbonate, the major buffer of acids in both the rumen and blood. During periods of heat stress, panting and reduced rumination increase the loss of bicarbonate, which reduces blood pH and blood buffering capacity.
The cationic salts of sodium bicarbonate (NaHCO3), sodium sesquicarbonate (Na3H(CO3)2, and potassium carbonate (K2CO3) can be feed to help maintain rumen pH. The bicarbonate (HCO3) or carbonate (CO3) portion aids in maintaining blood pH.
Sodium bicarbonate, sodium sesquicarbonate and potassium carbonate have also been implicated in alleviating milk fat depression.
In addition, several reviews have been published showing lactation performance benefits of increasing dietary DCAD. Improvements in dry matter intake, milk production and milk component content have been observed. Feedstuffs or supplements that increase dietary DCAD include high-quality haylage, sodium bicarbonate, sodium sesquicarbonate and potassium carbonate. Which source of strong cation (sodium or potassium) is most beneficial is being argued in academic circles.
Contact your Vita Plus consultant to learn how DCAD balancing can be applied on your operation.
About the author: Dr. Eric Schwab grew up in a rural town in New Hampshire. He attended the University of New Hampshire – Durham, where he received bachelor’s degrees in dairy management and environmental and resource economics in 1998. While working in northeast Wisconsin, he met Dr. Randy Shaver and returned to academia at the University of Wisconsin – Madison to pursue his graduate degrees. In Shaver’s lab, Schwab’s master’s degree research focused on kernel processing and chop length in BMR corn silage. His Ph.D. dissertation focused on B vitamin nutrition and ruminal B vitamin synthesis in lactating dairy cows. In September 2005, Schwab joined Vita Plus on the dairy nutrition and technical services team. He lives in Deerfield, Wis. with his wife and their three sons.
Transition and reproduction