FROM THE FARM REPORT: GETTING AHEAD OF HYPOCALCEMIA BEFORE IT STARTS

Subclinical hypocalcemia (SCH) affects upwards of 50% of multiparous cows due to increased calcium demands around calving. Although SCH is generally viewed as undesirable, we have come to learn that short term reductions in blood calcium concentrations, like when a cow has transient SCH (low blood calcium at 1 DIM but normal at 4 DIM), may activate adaptive mechanisms that improve calcium regulation later.

A recent study published in the Journal of Dairy Science tested this thought by inducing SCH during the prepartum period and measuring its effects on postpartum calcium homeostasis, DMI, and milk production.

Researchers enrolled 30 multiparous Holstein cows in a randomized complete block design study. Cows were randomly assigned 1 of 2 treatments beginning 21 days before expected calving date:

CON: intravenous infusion of 0.9% NaCl (saline)

EGTA: intravenous infusion of 5% egtazic acid (EGTA) solution dissolved in 0.9% NaCl

The EGTA is a calcium chelating agent, meaning it is a substance that binds to calcium, reducing its availability. This was previously used in a study at the University of Wisconsin-Madison to successfully induce SCH.

Infusions of the respective treatments began 7 days before the expected calving date and continued for 3-14 days. Limiting the number of infusion days allowed researchers to reduce circulating calcium without inducing clinical hypocalcemia. Infusions happened daily over the course of 6 hours until calving. All cows were fed the same neutral DCAD diet prepartum and a common lactating diet postpartum.

Blood and urine samples were collected daily prior to the start of infusions, beginning 28 days before the expected calving date. Blood samples were taken hourly and urine samples were taken every 6 hours once infusion began. After calving, blood and urine samples were taken daily until 49 DIM. Daily milk yield was recorded through 49 DIM, and milk composition was analyzed from samples taken weekly during the first 3 weeks of lactation.

Prepartum response: successful induction of SCH

The prepartum EGTA infusion worked as intended. Cows receiving the EGTA had reduced calcium concentrations (both total and ionized calcium) during the prepartum period compared to the control. Importantly, these concentrations were within ranges for SCH and did not result in clinical signs.

Interestingly, cows adapted to the infusion over time. Compared to the first day of infusion, on days -3, -2, and -1, relative to expected calving date, a higher infusion rate was required to achieve blood calcium concentrations consistent with SCH. This suggests that cows became somewhat resistant to EGTA and that the repeated challenge served to activate calcium homeostatic mechanisms.

There were also hormonal adaptions. There was an overall treatment effect for a key regulator of calcium homeostasis, parathyroid hormone (PTH). Cows receiving EGTA had increased PTH during the prepartum period and there was a treatment x day x time interaction where PTH was highest on day -2 at hour 0 and day -1 at hour 0 for EGTA cows. Increased concentrations of PTH stimulates increased bone mobilization and intestinal absorption of calcium, and this mechanism seemed to be enhanced in response to the declining blood calcium concentrations experienced by the EGTA cows.

Postpartum response: improved calcium status

Despite having lower calcium concentrations prepartum, EGTA cows had higher ionized calcium concentrations 12 hours after calving, as well as on days 1 and 2, compared to control cows. Total calcium concentrations were also higher for EGTA cows on the day of calving, 12 hours postpartum, and days 1, 2, 3, and 5. And once again there was a treatment effect for PTH, where overall EGTA cows had higher PTH concentrations compared to the control cows.

These results indicate that prepartum SCH induction improved the cow’s ability to regulate calcium during the most vulnerable period, immediately after calving.

Production responses from this study were limited. Although the EGTA cows had lower milk production on days 16, 18, and 24, compared to the control, there was not an overall treatment difference observed in cumulative milk production through 49 DIM.

Take-home message

While infusions of EGTA aren’t a practical on-farm strategy, the physiological responses observed are directly relevant to existing transition cow management programs. Negative DCAD diets and mineral binder strategies aim to stimulate the calcium regulatory pathways before calving, as the EGTA did.

This study showed that transient SCH can enhance calcium homeostasis by activating regulatory mechanisms. Rather than viewing all SCH as inherently negative, these findings help explain why management strategies that stimulate calcium homeostasis before calving reduce the risk of hypocalcemia postpartum.

— Emily Bourdeau

ebourdeau@whminer.com