When close-up cows are fed a negative dietary cation-anion difference (DCAD) diet, it induces a metabolic acidosis, which acidifies the urine.
Metabolic acidification of the cow makes more calcium metabolically available to meet the high calcium demand that occurs at calving. A simple urine pH test can indicate whether or not cows are metabolically acidified. A reasonable, conservative group average range for urine pH of 6.0 to 7.0 indicates moderately acidified cows.
DCAD is a simple, straight-forward nutritional tool with proven benefits. However, some people get frustrated when urine pH for each individual cow tested is not in a close, tight range. There is a misconception variability in urine pH is bad. But as long as your feeding management is good, a little variability in urine pH should be expected.
That’s because each individual cow chooses what and how much to eat, when and how often to eat, when and how long to rest and ruminate, and even when to get up to urinate. Pre-fresh cows’ intakes will differ based on pen moves, whether they are a heifer or mature cow, and even their individual bodyweight.
We may manage cows as a group, but we have to remember that each is an individual and, as such, their individual actions affect their individual urine pH.
To better understand, let’s consider two cows fed the same pre-fresh diet with a DCAD of -100 milliequivalent (mEq) per kilogram. Cow A eats 11 kilograms (24.25 pounds), which is 1.1 Eq more anions than cations. Cow B eats 14 kilograms (30.86 pounds), which is 1.4 Eq more anions than cations.
Cow B is more metabolically acidified than Cow A simply because it consumed more net anions. It is the total differential intake of strong ions that determines each cow’s urine pH. This difference in pre-fresh intake contributes greatly to variability in urine pH. How uniform the diet is and how selective the cows are can be issues, but intake differences play a big role in urine pH. That’s why some variability is expected.
When testing urine pH, select cows that have been on the pre-fresh diet for at least three days. For larger herds, we can use a 10-cow example. Test 10 cows, discard the highest and lowest scores, and then average the rest. The remaining eight cows’ urine pH should average between 6.0 and 7.0 when targeting moderate metabolic acidification.
For smaller farms with only a few pre-fresh cows, just spot check urine pH monthly. You’ll get a feel for how the close-up cows are doing – above 8.0 (really high and alkalotic with no health benefits), between 6.0 and 7.0 (on target for moderate DCAD with beneficial results) or below 6.0 (extreme DCAD with higher risk of over-acidification). It really is that simple.
Occasional checks of urine pH can help detect DCAD drift – the unplanned shift in DCAD that typically comes from forage variations. Let’s say you diligently check urine pH every month, and average urine pH is generally between 6.0 and 7.0. Then one day you get a group average above 7.0. Don’t panic. The same holds true if one day the average pH is slightly below 6.0. It’s not an emergency.
If pre-fresh cows’ feed intake and fresh-cow health and performance haven’t changed, you likely haven’t strayed too far from the safe, beneficial zone for moderate DCAD. But this hint of drift should be a heads-up to check pH again soon and then make a decision on whether to adjust the DCAD of the diet.
With moderate DCAD diets, there is no need for knee-jerk ration changes in response to a modest deviation from the ideal pH range. Moderate DCAD includes a safety margin on both sides of your target.
There are some who believe it might be better to feed a more negative DCAD diet that tightly groups all urine pH values below 6.0. But so far, research has not shown extreme acidification yields health or production benefits beyond those seen with moderate DCAD.
New research indicates extreme DCAD may actually be stressing the cow in ways just beginning to be identified – reduced feed intake and more negative energy balance prepartum, reduced colostrum yield and a reduction in milk protein percentage in early lactation.
And research in 2009 shows the useful linear relationship between urine pH and the acid-base status of the cow breaks down when pH drops below 6.3, making urine pH of extremely acidified cows a less reliable indicator of acid-base status.
At this point, science has not determined the ideal negative DCAD to optimize production and minimize health problems in parous transition cows. But 30-plus years of research and on-farm use confirm moderate DCAD delivers beneficial results to cow health, production and longevity in the herd, and you only need a pH between 6.0 and 7.0 to achieve noticeable benefits. 
PHOTO: Feed a negative DCAD diet to pre-fresh cows and measure urine pH to determine if they have become acidified. Getty Images.
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Tim Brown
- Director of Technical Support
- SoyChlor
