For the past three decades, dairy producers have known that a negative DCAD (dietary cation-anion difference) diet for in close-up cows benefits cow health and early lactation milk production. Chloride and sulfate in the negative DCAD diet, at the proper levels, induce a small shift in the acid-base balance, resulting in lower blood pH, mild metabolic acidosis and improved calcium flux, or mobilization of calcium from the cow’s bones. In this way, the negative DCAD diet reduces the risk of sub-clinical or clinical hypocalcemia.
Even herds that have a very low incidence of milk fever will likely make money by implementing negative DCAD. Economic benefits come in many forms, including reduced incidence of a number of disorders and diseases, improved reproduction, improved early- and total-lactation milk production and decreased culling rates.
In some dairies though, a trend toward more extreme DCAD diets could bring excess costs while also compromising the reliability of urine pH as a monitoring tool for metabolic acidosis. In a November article titled “Moderation: Best for DCAD Diets Too,” now available on BovineVetOnline.com, we outlined how veterinarians can help dairy clients refine their DCAD programs.
José Santos, DVM, PhD, at the University of Florida, conducts applied and basic research in dairy cattle nutrition, reproductive biology and management, including the use of negative DCAD diets. Here, Dr. Santos addresses questions regarding DCAD levels and evaluation of DCAD programs.
Q. How can dairies determine the “ideal” DCAD levels for their herds?
JS: The target DCAD value for dairies feeding acidogenic diets prepartum should be anywhere between -50 to -150 mEq/kg of dietary dry matter [calculated using the following formula: DCAD = (mEq K + mEq Na) – (mEq Cl + mEq S)]. We really do not know what the ideal DCAD is to optimize postpartum performance and health, likely in that range; however, we do know that values in that range (-50 to -150 mEq/kg) will minimize the risk of milk fever, subclinical hypocalcemia, retained placenta, metritis, and improve productive performance in cows.
Q. Are there any risks or downsides to moving toward more moderate DCAD programs?
JS: In general, as the DCAD becomes more negative (smaller value), the risks of milk fever (clinical hypocalcemia) and subclinical hypocalcemia decline. With the current knowledge, there are very little data to show any detrimental effect of reducing the DCAD to -150 mEq/kg. However, we know that acidogenic diets can depress dry matter intake irrespective of the product used to reduce the DCAD. Also, if errors are made during mixing of diets and the DCAD becomes very low (e.g. below -250 mEq/kg), then it is possible that it can interfere with energy metabolism in dairy cows. The compensated metabolic acidosis induced by diets with negative DCAD is highly beneficial to the metabolism of Ca and other minerals, but if it induces severe acidosis, then it can interfere with the action of insulin and potentially predispose cows to ketosis and fatty liver.
On the other hand, if the formulated DCAD is more moderate as you asked (e.g. DCAD of 0 mEq/kg), then it is likely that the incidence of clinical (milk fever) and subclinical hypocalcemia will greater than desired, which will predispose cows to other peripartum problems, retained placenta and metritis.
It is important to remember that forages and byproducts vary in their content of K, Na and Cl, so making sure the formulated DCAD is based on current chemical analyses of dietary ingredients is critical for the success of the program. It is not uncommon for the DCAD of a diet to vary by ± 30 to 50 mEq/kg from that of the formulated value. This means that if the diet is formulated to a more moderate value, e.g. 0 mEq/kg, I would not be surprised if actual DCAD fed is either + 50 or -50 mEq/kg.
Q. What are the limitations of using urine pH as an indicator of acidification or calcium status in the cow?
JS: Urine pH is an excellent tool to help monitor these dietary programs. Urine pH is closely related to the acid-base status of the cow, so it should be used routinely (once or twice weekly) as a monitoring tool to determine if the diet is in fact eliciting the desired effect of inducing a compensated metabolic acidosis. The problem I see with urine pH is when people ignore the mineral composition of the ingredients in the diet and they simply add the acidogenic product and titrate the dose based on urine pH as oppose to making sure the Na, K, S and Cl contents of the diet are known. Urine pH is a monitoring tool, but it should not be used as the only guide to determine whether the diet needs more or needs less amount of the acidogenic product fed per cow.
Q. What other signs should veterinarians or producers look for in evaluating their DCAD programs?
JS: Follow simple steps. First, analyze the mineral composition of the ingredients. In general, forages are the largest contributors to the variation in DCAD and their macro mineral content must be known. The same also applies to byproducts.
- Take a representative sample of the feeds to be analyzed. There are specific techniques to sample forages from a silo or from bales of hay. At the end of the day, you want to make sure the sample collected represents what is fed to the cows;
- Make sure the chemical analyses for minerals is performed by ICP chemistry (Inductively coupled plasma mass spectrometry). Any forage lab will give you choices for mineral analysis and one of them is ICP. In the case of DCAD, the macro mineral you want analyzed is Ca, P, Mg, K, Cl, Na, and S. Do not values generated by NIR (near-infrared reflectance spectroscopy). NIRs is an excellent and fast method of analyzing organic compounds because each component of the feed has a unique near infrared light reflectance detected by the machine based on the molecular structure (the bonds between carbon, nitrogen and hydrogen) that is then contrasted with values from the calibration curves from known samples; however, NIR is not designed to accurately measure minerals because they do not reflect infrared light. Although you receive a value from most laboratories, these values might not reflect the true mineral content of the sample.
- Select ingredients with low Na, K, and P content as the basis of the diet.
- Formulate the diet for the desired DCAD. Make sure P is not high and that Mg is somewhat high. High dietary P can increase the risk of hypocalcemia. Close up cows do not need diets with more than 0.35% P. I suggest you keep dietary P close to 0.30% of the diet dry matter. Dietary magnesium seems to benefit mineral metabolism around parturition, particularly if dietary K is high. Although we do not have clear data demonstrating the benefits of increasing Mg in diets prepartum, the current consensus is that these diets should contain somewhere between 0.35 and 0.45% of the dry matter as Mg. If dietary K is low, which should be the target (e.g. 1.0 to 1.2% diet DM), then the diet can have Mg in the lower value of the suggested range. Conversely, if dietary K is high, which is never desired for close up cows, then one should work with Mg in the upper suggested value.
- Monitor urine pH once, ideally twice a week. Make sure sampled cows have been in the diet for 2 days. The goal is to have most if not all cows with urine pH between 5.0 and 7.0. A good target would be an average urine pH of 6.0 to 6.2; however, it is always ideal that all sampled cows fall within the desired range. If some cows have urine pH of 5.5, while others have 7.5, then you need to investigate the possible reasons for such variability (improper mixing of diets, cows sorting ingredients, cows with high whereas others with low intake, etc). Remember that not every cow will respond to the diet equally. Variability is normal, but too much can indicate problems. For instance, urine pH reflects the absolute amount of strong ions consumed by the cow. Although we are tempted to think that a DCAD of -100 mEq/kg should result in acidic urine (e.g. pH = 6.0), if the cow eats limited amounts of this diet because she is close to calving or some other reason, it would not be surprising if the urine pH is far from the desired value (a low intake cow with urine pH of 7.2).
- Monitor milk fever and other diseases that are related to hypocalcemia. When diets are properly formulated and feeding management is adequate, then milk fever in cows of lactation 2 or greater becomes an unusual disease (< 2%). If milk fever, retained placenta, and metritis increase above the farms alert values, then it is prudent to re-evaluate the prepartum diet, including the DCAD.
- Lastly, if you are unsure, one can take blood samples from cows on the day after calving and submit to a diagnostic laboratory for measurements of serum total Ca, P, and Mg. Laboratories will provide reference values for those analytes.
For more on using DCAD diets, read these articles on BovineVetOnline: