Urine pH testing is the most convenient and most common way to monitor pre-fresh cows on a negative DCAD diet. But to effectively manage your herd, it is important to understand the normal variability of pH levels and the limitations of pH testing.
The Skinny
Urine pH testing is a great tool for monitoring the acid-base status in cows on a moderate DCAD diet.
- Urine pH is an indirect reflection of metabolic acid-base status of cows.
- Compensated (mild) acidosis increases calcium mobilization in the body and reduces hypocalcemia at calving.
- Variability in urine pH is not a bad thing.
- But, when cows are extremely acidified (urine pH<6), pH is no longer a good indicator of acid-base status.
- Moderate DCAD works, and inconsistent pH does not mean it’s failing.
- When measuring pH, accuracy matters.
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The Details
Why do urine pH testing?
Urine pH testing is the most common method to measure acid-base status and the best tool we have on-farm. It is important to note that pH testing is only useful over a narrow range, a pH of about 6-8.
Within that range, pH is a good indicator of acid-base status, an indirect reflection of the blood calcium status. That range works well in monitoring cows on a moderate DCAD diet.
Above 8, it simply tells us that the cow has an excess capacity to buffer the blood and prevent changes in the acid-base status. Below 6, it simply tells us that she has used up most of her blood buffering capacity, and could be in danger of over-acidification.
Variability in urine pH
Variability in urine pH is common, even in cows consuming the same moderately negative DCAD diet. Their pH readings can range from the upper 5s to nearly 8.
Urine pH can vary from cow to cow on any given day or from day to day with any given cow. This variability comes from factors such as
- Time since the last anion consumption and the amount of anions consumed
- Time since a cow last emptied her bladder
- The acidity of new urine produced since the last emptying of the bladder
- The time the urine sample was taken
- Intake factors such as size, age, social status, temperature, etc.
These factors and more will cause pH fluctuations as the anion intake changes.
If the cow is extremely acidified – below 6 – the urine will only tell us that she is extremely acidified. It will not reflect the difference in anion consumption. That’s why extremely acidified cows don’t have as much variability in their urine pH.
Moderate vs Extreme DCAD
Based on the available research and case studies, a moderate DCAD diet is sufficient for increasing calcium flux.
In a cow with mild, compensated acidosis, the kidneys act to regulate blood acid-base homeostatis by removing excess acidity from the blood. This lowers urine pH. Once blood homeostasis is achieved, the kidneys quit removing acid from the blood, and newly formed urine will be less acidic.
This shows that the kidneys have not been overwhelmed, and it does not mean calcium flux is decreasing. Metabolic mechanisms for mobilizing bone calcium have already been turned on.
In a cow with extreme acidosis and a pH consistently below 6, the kidneys are removing acid from the blood as fast as they can. The kidneys have resorted to auxiliary methods (ammonium ion, glutamine catabolism) of removing acid from the blood, and urine pH is no longer a good reflection of the degree of metabolic acidosis.
According to a 2018 study from the University of Florida, published in the Journal of Dairy Science (Lopera et al.), a more extreme DCAD diet reduced intakes and colostrum yield in cows and showed no measurable benefits in health or production outcomes. Based on this, an extreme DCAD diet unnecessarily works the kidneys and is not always worth the additional cost and management needs for every producer.
Although urine pH samples may be more consistent when a group of cows are extremely acidified, that does not translate to more consistent health or production benefits.
Which test strips are best?
In 2015, Michigan State undergraduate students tested 10 brands of pH test strips against a calibrated meter and found the most consistently accurate results from the brand “pH ion Balance.” Other brands may well be accurate too, but verification of accuracy is essential if you want to trust the results.
Recommendations
There is no one-size-fits-all program for a DCAD diet. A more moderate approach may be the right fit. A moderate DCAD pre-partum program includes:
- A group average urine pH between 6 and 7 (It’s OK to have some readings outside the target range)
- Avoiding management mistakes that affect intake (cleaning bunks, allowing adequate space, etc.)
- Checking pH at the same time relative to delivering feed to the cows
- Letting cow health and appearance be your guide
- Using reliable pH strips or a calibrated pH meter
