Subclinical Hypocalcemia: is it a problem or not?

The topic of subclinical hypocalcemia (SCH) deserves some historical context to appreciate the developments in management of hypocalcemia in the last two decades.

With the successful implementation, risk of milk fever in dairy herds across the U.S. plummeted more than 20 years ago - so much so that research on the topic hit a plateau during the late 1990s and early 2000s. There was a resurgence of interest in hypocalcemia research in 2011 when the prevalence and implications of SCH were realized. Essentially, the idea that consequences of hypocalcemia extended beyond milk fever, and that SCH - the occurrence of low blood calcium without clinical symptoms became a hot topic. The relationship between SCH and fresh cow performance is logical because the calcium ion has critical functions in immune, muscle, nerve, and pancreatic cells that arguably underlie risk of infection, poor rumination, weakness, and impaired insulin signaling in fresh cows. 

The critical questions for transition cow management and nutrition were: What is the concentration of calcium in blood that is indicative of poor performance, and what are the best management and prevention strategies for SCH? 

Several thresholds ranging from 1.8 mM to 2.15 mM based on associations with postpartum diseases and cow performance. A key development occurred almost 10 years ago when Dr. McArt’s group revealed that not all SCH is the same - or problematic for that matter. They showed that cows with transient SCH, defined as blood Ca < 1.9 mM on day 1 and < 2.2 mm on day 4 were the top producing cows; whereas, cows with delayed or chronic SCH, defined as blood Ca < 2.2 mM produced less milk and were less likely to become pregnant. A few years ago, the conditions of delayed and chronic, or persistent, SCH were lumped into one category and referred to as dyscalcemic - rightfully so as cows in the delayed or chronic categories were more likely to be less productive. 

During this time, Dr. Erin Horst (a Ph.D. student at the time) and Dr. Lance Baumgard at Iowa State University published a critical review of how transition cow disorders are understood and studied. One of the arguments presented in the review was that SCH was the consequence of diseases like metritis, not necessarily the cause for risk of metritis. Even though the sudden demand of calcium for milk production at the onset of lactation is unquestionably the main driver of postpartum hypocalcemia, the argument that SCH was a consequence of metritis had merit. Multiple datasets showed that cows with metritis had lower blood calcium beginning 2 days after calving when uterine infection is starting to take place. 

The research presented by Fraz et al., at the 2025 ADSA annual meeting (Abstract # 1108) was aimed at teasing apart the associations among postpartum blood calcium status, disease, and production by further differentiating SCH into categories of SCH with clinical disease and SCH with no clinical disease. The dataset had blood Ca and disease data from nearly 1,100 multiparous Holstein cows. It is useful to mention that metritis accounted for 85 % of the clinical disease cases diagnosed in the first 7 DIM. As stated previously, cows with SCH on day 3 (Ca < 2.2 mM) but did not have a clinical disease were the top producers in the herd producing an average of 121 lbs of milk per day in the first 10 weeks of lactation. In contrast, cows without SCH on day 3 but were diagnosed with a clinical disease averaged 116 lbs of milk in the first 10 weeks of lactation. The poorest performing cows were those that had SCH on day 3 and were diagnosed with a clinical disease - they averaged 110 lbs of milk for the first 10 weeks of lactation. There was not an interaction between day 1 SCH and clinical disease, rather, cows with lower calcium on day 1 (older cows) produced more milk.    

Going back to the critical questions for management of hypocalcemia, drop in calcium within 24 hours of calving is most critical. The nadir of blood calcium, and greatest risk for milk fever, occurs within 24 hours of calving. 

In an analysis of blood calcium from 2,252 Holstein cows, the sensitivity and specificity of blood calcium as an indicator for risk of milk fever decreased from 71% and 95% on day 1 to 50% and 86% on day 3 in milk. Blood samples collected on day 3 or day 4 are valuable for assessing transition cow performance, but should not be used to evaluate the prefresh strategy to prevent milk fever. As for day 1 calcium status, blood calcium <1.5 mM was associated with 13-fold increased risk for milk fever and risk did not start to increase until blood calcium was < 1.7 mM. 

The magnitude of the drop in blood Ca at time of calving arguably leads to other fresh cow problems beyond milk fever. Extracellular calcium concentrations are critical for many signaling processes that affect feed intake and immunity. Decreasing incidence of milk fever by feeding prefresh cows a low or slightly negative DCAD results in greater postpartum intakes, fewer retained placentas and less metritis. However, a critical calcium threshold that definitively results in more disease is uncertain. For now, focus on minimizing incidence of milk fever in a way that is not harmful to the cow in other ways (i.e., don’t cause hypophosphatemia or over-acidify).  

Nutritional strategies for prevention of hypocalcemia will be covered in future research updates, but focus should be on cows entering their 3rd or greater lactation as they are the cows at greatest risk of milk fever. Decades of research and practice show that a properly balanced DCAD that targets urine pH of 6.2 to 6.8 results in optimal postpartum Ca status for cows entering 3rd and greater lactation. 

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