The goal for every cow on every dairy is to have healthy, productive cows that transition well into lactation. About 30 years ago, researchers discovered a way to help cows achieve this goal.
Feeding a diet with a negative dietary cation-anion difference (DCAD) produces a mild metabolic acidosis in prepartum cows which improves cows’ calcium status at calving and helps prevent milk fever.
Since that initial discovery, research has clearly and repeatedly demonstrated the value of feeding a negative-DCAD diet prepartum. Feeding negative DCAD prepartum helps cows “prime the pump” for calcium mobilization at freshening, nearly eliminates milk fever, reduces subclinical hypocalcemia and enables cows to transition with fewer negative outcomes.
The majority of research completed on negative DCAD has used a moderate approach to DCAD. Those low-DCAD diets contained just enough anions to yield a urine pH between 6 and 7, indicating a mild metabolic acidosis. This moderate level of DCAD has been shown repeatedly to yield beneficial results and be safe for the cows.
However, some recent dairy nutrition consulting trends have pushed for a more extreme level of acidification to push cows’ urine pH to 5.5, on average. While this extreme level of acidification does yield more consistent urine pH results than are typically seen from cows with more moderate acidification, extreme acidification has not been scientifically proven to consistently increase the beneficial results of DCAD.
And we don’t know if there are any detrimental effects to the cow when they are pushed to such an extreme level of acidification. More may not always be better.
To understand the potential impact of extreme acidification, it is helpful to understand some basics of biology. Cows and humans alike try to maintain homeostatic normal at the cellular level. When blood pH and electrolyte balance is normal, cellular functions occur freely.
Normal blood pH in cows is generally between 7.35 and 7.45 – a pretty tight range. Normal urine pH in cows is around 8.0 or slightly higher. When a moderate amount of anions are fed to induce a slight metabolic acidosis, urine pH falls to between 6 and 7. Blood pH may not change because the kidneys have removed the extra acid from the system to keep the cellular environment at optimal levels for cellular functions.
Data from a 2004 study illustrates this point well. In the trial, cows were fed a control diet with no anionic supplement. While on the control diet, cows had an average urine pH of 7.97, and blood pH was 7.445.
Then those same cows were fed a diet with enough anionic supplement to lower urine pH (acidogenic diet). While on the acidogenic diet, cows’ urine pH declined dramatically to 5.91, but the blood pH only dropped to 7.430. The kidneys removed the excess acid and protected blood homeostasis from a potentially disruptive change in pH.
Cows on an extremely low-DCAD diet have a urine pH of 5.5 or lower. The lower the pH, the more acidified the cow. But how much acid can the kidneys remove? What level of acidification becomes too much and alters blood pH and compromises cellular function? We don’t know the answers.
Therefore, extreme acidification carries some unknown risk at our current level of understanding. Transition cows already run the gauntlet to successfully transition into lactation. Why add any more risk with an extreme level of acidification?
Whenever we go to extremes, with metabolic acidosis, with electrolyte balance or even with diet, there can be negative consequences. When it comes to extreme DCAD, the science isn’t there. But science has clearly shown the benefits, the efficacy and the safety of moderate DCAD.
Negative DCAD works
Negative-DCAD diets are not one-size-fits-all. They are not just throwing some anionic salts in and hoping for the best. Negative-DCAD diets must be tailored to your farm, to the feedstuffs you have available and to the management ability of the farm.
Success with negative DCAD comes in many forms. Some farms start using anionic supplementation and never check urine pH, and some of these may get by OK. Some farms will test urine pH at the beginning to make sure their target for metabolic acidosis has been achieved and then test occasionally. Other farms will test urine pH weekly. It is really up to you. DCAD can and should be tailored to your dairy.
The first step should always be to select feed ingredients low in potassium, sodium and phosphorus whenever you can. This limits the amount of strong cations in the diet. Invest in mineral composition testing for all feed ingredients used in the prepartum diet. Relying on book values for the mineral composition of forage and byproduct feeds can sabotage your efforts.
Next, incorporate an anionic supplement to create a negative-DCAD diet and test urine pH to determine if the diet changes have acidified the cows. If urine pH has not dropped below 8.0, acidification has not occurred. Sometimes, calculations for the DCAD don’t deliver the intended results. Whether it is from a poor forage sample, inaccurate test or a simple math error, the only way to know if acidification has occurred is to ask the cows.
Testing urine pH is how you check your work. Aim for a urine pH between 6 and 7. Some variability of urine pH between cows is normal. Your goal is for the majority of prepartum cows to have a urine pH between 6 and 7.
Once you confirm the diet changes have induced a slight metabolic acidosis, continue listening to the cows. Testing urine pH is one way to do that. However, producers who have been using negative-DCAD diets for a while tell me when they have cows in the “sweet spot” for metabolic acidosis, they can tell their metabolic status just from cow performance.
Yes, the number of transition cow problems they have to address declines, but it is more than that. Producers report the cows calve easier, their eyes are bright, and they are hungry. Their calves get up more quickly, and they readily drink colostrum from the bottle.
When cows drift from that “sweet spot” of mild metabolic acidosis, producers report they will notice cows that seem just a bit off, have longer deliveries and yield sluggish calves, which is often followed with an increase in transition cow disorders.
Thirty-plus years of research and on-farm experience have demonstrated time and again a moderate level of negative DCAD works. It reduces clinical and subclinical hypocalcemia and noticeably improves the health and profitability of transition cows.
If you’ve gone a bit further and are using extreme DCAD, and having good results, that’s great. But remember, the scientific literature has not shown extreme acidification increases the beneficial results of DCAD, nor has it demonstrated long-term safety of extreme acidification.
No matter what level of DCAD you use, listen to your cows and respond accordingly. Based on what we know today, the most practical advice is to use negative DCAD for prepartum cows. The level of acidification is up to you. 
PHOTO: It has yet to be proven if any additional benefit from extreme metabolic acidification will outweigh the potential negative effects on health and performance. Photo by Mike Dixon.
References omitted but are available upon request. Click here to email an editor.
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Tim Brown
- Director of Technical Support
- SoyChlor
