For the past three decades, dairy producers have known that a negative DCAD (Dietary cation-anion difference) diet in close-up cows benefits colostrum production and early lactation. 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 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 hypocalcemia or milk fever.
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, says Tim Brown, PhD, a technical services specialist with Dairy Nutrition Plus (DNP).
Brown says urine pH, while an indirect reflection of blood calcium status, remains the best on-farm metric for monitoring acidification in cows. It is, however, only useful over a narrow range of pH.
At a pH of around 6 to 7, a typical group average for cows on a moderate DCAD diet, Urine pH is a good indicator of the cow’s acidotic state. At higher or lower pH, the relationship begins to break down. Brandi Gednalske, SoyChlor Regional Sales Manager with DNP, says urine pH above 8.0, indicating an alkalotic state, does not tell us how much or what type of blood buffering capacity the cow has. It Just suggests she has an excess for preventing change in blood acid-base homeostasis.
On the other hand, at an extremely acidotic state with pH below 6, urine pH doesn’t say anything about how much or what type of blood buffering capacity the cow has, just that she’s used most of it up.
Brown acknowledges that some farms have success with more extreme DCAD diets, but others could see reductions in daily feed intake, health problems, or at the least, unnecessary expense.
Brown points out that moderate DCAD diets, with a target urine pH range of 6 to 7, can produce fluctuations in pH between cows on a given day or day-to-day changes for an individual cow. This does not indicate the DCAD program is failing, he stresses. Urine Net Base Excretion (NBE), he explains, is complex result of the content and interactions of the following in urine:
- Strong ions –sodium, potassium, calcium, magnesium, chloride, and sulfate
- Volatile buffer ions –bicarbonate and ammonium
- Non-volatile buffer ions –phosphate and creatinine
Urine pH at moderate DCAD levels can fluctuate due to time since last anion consumption and amount of anions, interval since the animal last emptied its bladder, acidity of new urine produced since last emptying bladder and the time the urine sample taken. For cows all consuming the same diet, Gednalske adds, it is not uncommon to have pH readings ranging from upper 5s to nearly 8 for individuals with mild, compensated acidosis.
Gednalske provides an example based on a diet with -100 meq/kg DCAD. If, on one day, a cow eats 10 kg of this diet, she takes in a net of 1000 meq of anions. The next day, she might eat 12 kg of this diet, taking in a net of 1200 meq of anions, and she becomes more acidotic compared with the first day. If this cow is mildly acidified (pH 6 to 7), we should expect urine pH to vary in response to this difference in anion consumption. But, if the cow is extremely acidified (pH<6), urine will only tell us that she is extremely acidified, but it will not reflect this difference in anion consumption.
In a 2018 University of Florida study published in the Journal of Dairy Science (Lopera et al), researchers compared a moderate (-66 DCAD) with a more extreme (-176 DCAD) diet. The moderate DCAD resulted in an average urine pH of 6.46 while the more extreme DCAD produced an average pH of 5.62.
Cows in the moderate group averaged 10.7 kg of daily pre-partum feed intake compared with 10.2 kg for the more extreme group. Colostrum yield for the moderate DCAD group averaged 6.8 kg compared with 4.0 kg for the more extreme DCAD group.
The researchers found that level of acidification did not affect:
- Forty-two-day yield of milk, ECM, 3.5% FCM, fat% and yield, protein % and yield.
- Incidence of retained placenta, metritis, puerperal metritis, mastitis, or displaced abomasum.
- Incidence of hypocalcemia post-partum, or the risk of leaving the herd by 305 days of lactation.
In summary, Gednalske says variability in pH is not a bad thing, and when cows are extremely acidified, pH no longer a good indicator of acid-base status. Moderate DCAD works, and inconsistent pH does not mean it’s failing, and accuracy of pH measurement matters.
For a moderate DCAD pre-partum program, Brown recommends
- Target group average urine pH between 6 and 7.
- Avoid management mistakes that affect intake.
- Check pH at same time relative to delivering feed to the cows.
- Let cow health and appearance guide you.
- Ok to have some individual readings outside target range.
- Use reliable pH strips or a calibrated pH meter.
Dairy Nutrition Plus recently hosted a webinar outlining this topic. The session now is available for viewing on demand.