All farmers will recognise a downer cow by the symptoms presented and understand that the condition is associated with a low level of calcium in the blood during a stressful period.
This issue is commonly called milk fever, and the affected cow is referred to as a downer cow. What farmers do not always recognise is that cows presenting with clinical symptoms represent only a small part of a much larger issue. This is why most researchers use the term hypocalcaemia. This broader term better conveys the underlying issue – calcium deficiency – and the fact that the condition can take many forms and affect processes critical to early lactation.
For every case of a downer cow, there will be many more cases in which the cow remains standing but suffers the wider effects of calcium deficiency. Calcium deficiency can manifest as calving problems, metritis, retained foetal membranes, mastitis, poor immunity and poor conception. Few of these conditions are as dramatic as a downer cow, yet they are equally important.
The body’s calcium-balancing system
Most feed sources supply more than adequate levels of calcium, yet calcium is among the least efficiently absorbed minerals. The body regulates calcium uptake very tightly, as an imbalance can lead to serious complications. This regulation is achieved through the release of hormones, particularly parathyroid hormone and vitamin D, which functions as a hormone rather than merely as a vitamin. Calcium storage is controlled by another hormone called calcitonin.
The body’s hormonal system responds quickly to regulate the available calcium level according to demand. This system works well, provided the diet is appropriate and the cow has not been subject to long-term deficiencies. It also works best when cows are not under- or over-conditioned, as cows in poor or excessive condition may be less able to regulate these hormones metabolically.
The New Zealand-specific challenge
On New Zealand farms, our reliance on large amounts of pasture during early lactation presents a significant challenge for calcium management. Pasture can supply very high levels of potassium, normally in excess of 3.0%. In contrast, the cow’s nutritional requirement is only in the region of 1.1%.
This large excess of potassium does not simply compete with calcium for absorption; it can also seriously affect the acid–alkaline balance of the blood. An increased blood pH interferes with the release of the hormones required to regulate calcium balance. High blood pH can effectively inhibit the release of parathyroid hormone.
Over the years, farmers have used many strategies to treat milk fever in their dairy herds, with varying levels of success. Many common strategies have produced inconsistent results, and methods that are successful in one situation can fail completely in another.
Here is an overview of common methods used to combat calcium deficiency in dairy cows.
Reducing the level of calcium in the diet
This has probably been the most common method used over the last 40 years. It can work well, provided no pasture is fed. For the approach to work effectively, pasture must be almost entirely removed and replaced with a feed that is low in calcium, potassium and sodium.
Maize silage is one of the few feeds that meets these criteria. Where large quantities of maize silage have been added to the diet, this approach has worked well. However, it is important that these cows receive adequate additional calcium during lactation. Otherwise, the induced calcium deficit can have serious consequences.
Injectable calcium
Calcium borogluconate has traditionally been used as a last line of defence against this condition. This formulation floods the bloodstream with readily available calcium, along with boron, which may further support calcium absorption.
In desperation, some farmers have adopted this method as their primary means of control, treating only cows showing clinical symptoms. Calcium may be administered either intravenously or subcutaneously. This approach is not a satisfactory long-term solution.
While a bottle of calcium borogluconate will usually help a cow to regain her feet, the same cow will often go down again later. In many cases, even when she appears to recover, she may remain affected by subclinical calcium deficiency throughout much of early lactation.
Another issue is that large numbers of subclinical cases of calcium deficiency are not detected. It is common for between 25% and more than 50% of cows to experience subclinical calcium deficiency at calving and well into early lactation. In addition to producing less milk, these cows may be more susceptible to mastitis and uterine infections and may experience poorer reproductive outcomes.
Interestingly, a number of scientists have reported that injecting calcium solutions can seriously interfere with a cow’s ability to regulate her calcium status over the longer term. This may make it more difficult for her to regulate calcium metabolism in subsequent seasons (J. Wilms et al., 2019).
Flooding the body with calcium in the form of lime flour
Another option that has gained some traction in recent years is the idea of supplying the cow with large amounts of calcium in the form of lime flour, sometimes with the option of adding high levels of vitamin D.
This approach can work on some farms. However, success depends heavily on maintaining a high level of continued calcium supplementation. This can be challenging when using lime flour. If supplementation falls below an often poorly defined threshold, the cow may become metabolically unable to mobilise calcium reserves from her body.
High levels of lime flour may also buffer blood pH to the point at which parathyroid hormone release can no longer be activated.
A NZ version of the DCAD diet
So, is there a better option available?
Over the past five years, Agvance Nutrition has worked intensively on developing and applying its own specific version of an anionic, negative-DCAD diet designed to be practical under New Zealand dairy-farming conditions. Based on the many hundreds of farms for which we formulate these blends, we believe we have developed a combination of DCAD salts that suits New Zealand conditions and provides a better option for combating calcium deficiency.
During this time, we have learnt how this system can be applied to local conditions, where pasture normally makes up at least part of the diet. We have also formulated approaches that better match requirements based on the composition of the diet and target specific deficiencies that are more prevalent under different crop-feeding situations.
The poor palatability of these salts is often mentioned in the literature, and considerable time and effort have been devoted to addressing this issue. There is no benefit in formulating an effective mineral blend if the cows will not consume it.
Calcium deficiency is a substantial topic, and we have only touched on it briefly here. For further advice, you are welcome to contact the Agvance Nutrition team.
Article by Chris Balemi, Agvance Nutrition
