Globally, whether working in a continuously housed dairy or a grazing herd, it is generally accepted that the transition period brings the complexity of shifting metabolic priorities.

Subclinical hypocalcemia (SCH) has attracted the attention of researchers and producers due to its high prevalence and association with costly health and performance declines. To better manage SCH, several large-scale observational studies were conducted to classify cows into SCH categories based on patterns of blood calcium (Ca) concentrations after calving at one and four days in milk (DIM).

Normal and transient SCH cows (eucalcemic) are understood to have positive health, intake, and production outcomes postpartum, but cows classified in either delayed SCH or persistent SCH groups (dyscalcemic) appear to struggle in their postpartum performance. Dyscalcemic cows do not recover to normal prepartum Ca status by four DIM, suggesting that health and performance are more closely tied to a cow’s ability to rapidly recover from Ca depletions rather than simply any decrease in Ca at a single time-point.

Much of our existing knowledge about SCH in grazing cows is based on work conducted in intensive seasonal calving dairy systems. Though this data has proven valuable to identify additional risk factors of SCH and its associations with performance outcomes, it does not represent the existing heterogeneity of forage, management, breed, purpose (dual or dairy only), or geographic location in grazing systems globally.

South American milk producers have long been strong contributors to the global dairy market. Colombia’s milk production has stayed remarkably steady and even grown over the last decade, despite the challenges of the 2020 pandemic. Much of the Colombian dairy market relies on cows calving year-round and continuously grazing Kikuyu grass (Cenchrus clandestinum), which contains high potassium (K) and oxaloacetate (OA), on a rotational basis. This makes calcium management a particular challenge in this region.

Recognizing the costly losses due to disease and performance issues, we believed that Colombian producers could benefit from learning how prevalent SCH is in their animals. We further asked: If SCH is prevalent, does it present similar risks to performance in smaller grazing systems as has been reported in New Zealand and Ireland?

To answer this, we enrolled 136 multiparous Holstein and Holstein crosses from three farms in Antioquia, Colombia (a prominent dairy region in Colombia) from July 2023 to January 2024 (Figure 1). All farms enrolled in this study were in the same region and managed cows in a rotational grazing system using primarily Kikuyu. Far-off and close-up cows were grouped separately, and lactating cows were milked twice a day. Cows were not fed any anions, mineral binders, or underwent any other Ca management protocol. Blood samples were collected pre and postpartum and measured total Ca (tCa). Cows were later assigned to one of four Ca status groups based on their tCa status on one and four DIM: Normal, transient SCH, delayed SCH, and persistent SCH. Individual milk yields were recorded daily, and milk components were collected once a month from both consecutive milkings, and disease and culling events were documented for nine weeks postpartum.

Across three farms, eucalcemic cows made up 69 percent of enrolled cows (Normal n = 65 versus Transient SCH n = 28) and dyscalcemic cows made up 31 percent of cows (Delayed SCH n = 25 versus Persistent SCH n = 18). Cows in the persistent SCH group had a 1.2 times greater risk of diagnosis of retained placenta, metritis, displaced abomasum, or mastitis and a 7.2 times greater risk of culling during the study period. Cows in the delayed SCH group were not considered different in their risk of disease diagnosis but had a 5.2 greater risk of culling in nine weeks.

Over nine weeks, cows in the transient SCH group did not have increased risk of disease or culling; they did produce more milk and energy-corrected milk (ECM) when compared to other SCH groups as well as cows in the normal group (Figure 2).

The results of this study are comparable to what has been seen in other studies, supporting the idea that some decline in blood Ca concentration at the time of calving may be supportive of a successful lactation if the cow is able to recover to normal Ca status quickly. Those cows that have reached their ‘normal’ Ca status by four DIM seem to have improved health outcomes and perform better overall.

TAKEAWAYS

The challenge of managing SCH through the transition period is not solely a problem of continuously housed dairy cows. Grazing dairy cows may have distinctive risk factors and a different level of prevalence to consider when assessing the challenge of SCH; however they are no less at risk of suffering from similar loss of performance as continuously housed cows. While many research questions remain regarding mechanisms, direct causes, and best management, the work in the current study should emphasize the importance of SCH in grazing systems. To limit losses associated with calcium dysregulation like SCH, it is prudent to pursue interventions that can be applied to a diverse range of systems and management practices.

This article appeared in PRO-DAIRY's The Manager in November 2024. To learn more about Cornell CALS PRO-DAIRY, visit PRO-DAIRY.