If we apply the standards of evidence-based medicine to prepartum dairy cows, all close-up cows would be fed a negative DCAD diet. More than 30 years of research, and on-farm application, have repeatedly proven negative DCAD diets work.
When prepartum cows are fed a negative DCAD diet, inducing a mild compensated metabolic acidosis, calcium status is improved, resulting in healthier, more productive cows.
Recently, some nutritionists have been feeding even lower-DCAD diets (more anions) to acidify cows more deeply than the majority of published research. At these higher, more extreme levels of acidification, cows may walk a fine line between being OK and being overwhelmed by uncompensated metabolic acidosis.
So these days, when we discuss negative DCAD diets, we must distinguish the level of acidification – moderate or extreme.
On the farm, the best way to estimate the acid-base status of a group of cows is to measure urine pH. Simply check a few cows in the close-up group, discard the high and the low test results, and calculate the average for the rest of the cows tested.
In general, moderately acidified cows have an average urine pH of 6.0 to 7.0. When urine pH drops below 6.0, cows are considered extremely acidified.
Urine pH is an estimate of acid-base status. More sophisticated tests reveal once urine pH drops below 6.3, the kidneys change how they remove the hydrogen ion (acidity) from the blood, making urine pH a less reliable indicator of the metabolic acid-base status of the cow. Stated simply, urine pH readings less than 6 may not tell us how close the cow is to uncompensated metabolic acidosis.
What the research says
Studies have repeatedly shown feeding a moderately negative DCAD diet during the immediate prepartum period can improve blood calcium status at calving, minimize negative health events and increase milk production. A meta-analysis in 2006 reviewed 22 studies conducted between 1984 and 2003. In those studies, most fed a DCAD diet that resulted in urine pH values between 6 and 7 – a moderate DCAD diet.
Recently, a few studies have fed extreme negative DCAD diets to push urine pH below 6.0. Researchers in 2018 fed pre-fresh cows two levels of negative DCAD (-70 and -180 milliequivalents [mEq] per kilogram [kg] of dry matter [DM]) for either 21 or 42 days before calving.
Cows fed the moderate DCAD diet, regardless of the duration fed, had an average urine pH of about 6.5. In comparison, cows fed the extreme DCAD diet, regardless of the duration fed, had an average urine pH of about 5.5. While the extreme level of DCAD did slightly increase prepartum blood calcium, it did not decrease the incidence of milk fever or subclinical hypocalcemia.
Extreme DCAD did not improve any of the other health parameters studied, nor did it improve milk yield, component yield or reproductive performance. The researchers did, however, discover a couple of negatives associated with feeding an extreme DCAD diet – less colostrum at first milking and decreased DM intake.
Researchers concluded regardless of the blood calcium threshold used to establish hypocalcemia, the incidence of hypocalcemia and all of the health problems that can go with it was not decreased by making cows extremely acidotic. They concluded, “There is no need to reduce the DCAD from -70 to -180 mEq per kg of DM.”
A case study in 2017 tracked urine pH ranges when cows were fed diets with two different DCAD levels (-53 and -143 mEq per kg of DM). The weekly average of urine pH for cows fed a moderate DCAD diet was 6.2 to 6.8. The weekly average for cows fed the extreme DCAD diet was 5.2 to 5.8.
However, on the day of calving, both groups of cows had the same blood calcium concentration of 2.11 millimolars per liter, which is more than adequate relative to the diagnostic standard of 2 millimolars per liter.
Work done by researchers in 2017 compared two levels of DCAD (-30 and -220 mEq per kg of DM), each with either 1 percent or 1.5 percent calcium. Regardless of calcium level, cows fed the -220 DCAD diet ate less feed.
Cows fed the moderate DCAD diet with 1 percent calcium ate the most, just over 38 pounds per day – that’s 5 pounds per day more than cows on the more extreme DCAD diet with the same amount of calcium.
And other researchers in 2017 found feeding an extreme DCAD diet (urine pH 5.96) resulted in increased excretion of the amino acid glutamine in urine when compared to two diets that were substantially less metabolically acidified (urine pH 7.89 or 8.22).
Glutamine is used by the mammary gland for milk protein production as well as by the kidneys to help maintain acid-base balance. The authors hypothesized perhaps the already limited amino acid was in short supply postpartum for protein synthesis by the mammary gland. While milk protein yield was not affected by feeding extreme DCAD prepartum, milk protein percentage was lower.
Pros and cons
Let’s examine the facts. When moderate DCAD diets are fed to produce a urine pH between 6.0 and 7.0, cows respond positively. Benefits include nearly nonexistent milk fevers, reduced subclinical hypocalcemia, fewer retained placentas and metritis. While DM intake declines a little pre-partum from moderate metabolic acidification, the cow responds with increased DM intake postpartum, which allows her to produce more milk.
Urine pH testing is required to confirm cows are acidified. But once you find the level of acidification needed with the feeds available at your dairy, you can test monthly to monitor for DCAD drift. Cow performance will also tell you if you have drifted out of that sweet spot.
With extreme DCAD, research has shown it delivers similar but not more benefits in terms of actual health and production variables compared to moderate DCAD. Extreme DCAD has been shown to decrease prepartum DM intake even more than moderate DCAD, and some studies have shown reduced colostrum yield at first milking.
Urine pH testing will be needed much more frequently because the cows have less safety margin before becoming over-acidified (uncompensated acidosis). One thing proponents do like is: Urine pH of all cows is very similar (in a tight range).
But the goal of a negative DCAD program is not that all cows have nearly identical urine pH values. The goal is a simple, economic nutrition program that improves the health and production of cows transitioning into lactation.
Researchers in 2017 concluded it takes enough metabolic acidification to get urine pH down to at least 7.4 to improve calcium metabolism. Research in 2009 concluded urine pH loses its value as a monitor of acid-base status when urine pH drops below 6.3.
Using these two studies as bookends, a realistic, on-farm target for average urine pH of a group fed a moderate DCAD diet prepartum is 6.0 to 7.0. Keeping close-up cows in this urine pH range will yield the majority of benefits research expects without the added worry of over-acidification.
The limited research completed so far with extreme DCAD diets does not show additional benefits above what we would already expect from feeding moderate DCAD diets. But when we acidify cows more, the kidneys have to work harder to remove excess acid. Current science has not quantified the level of risk from this practice.
So where does that leave us in terms of relative comparisons? Given what we know right now, extreme DCAD certainly requires more intensive management than moderate DCAD.
But there is no clear evidence the extra effort and expense of extreme DCAD yields any improvement in economically important benefits. While more research is needed on this topic, José Santos summed it up well in his 2018 meta-analysis of DCAD research for pre-fresh dairy cows.
“Parous cows should be fed diets with negative DCAD, but data available did not allow for detection of the ideal negative DCAD that optimizes production and minimizes health problems in parous cows.” 
References omitted but are available upon request. Click here to email an editor.
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Tim Brown
- Director of Technical Support
- SoyChlor
