Full blown clinical milk fever is a very obvious syndrome which everyone can identify in freshly calved cows, and experts claim it has an incidence rate of around 5-10%. However, as with many production diseases, clinical cases are often the tip of the iceberg indicating a more deepseated imbalance within dry cows in the herd, and perhaps more specifically, transition cows.
The prevalence of sub-clinical hypocalcaemia (milk fever where the blood calcium is low but the cow is not clinically affected) in the transition cow is effectively unknown. Worryingly, one study in 2006 found that 47% of all cows in their second lactation or greater had varying degrees of sub-clinical milk fever which, in some cases, was severe enough to alter physiological and immune functions1.
The effects of milk fever on cows are far reaching. Muscle function is impaired (so they can’t get up) and this also affects the uterus and guts. Frequently this means retained cleansings after calving which have knock-on negative effects on fertility. Failure of the uterus to contract, as it should at parturition, can lead to delayed calving, dystocia and retained placenta, while poor mobility of the guts can lead to bloat with longer term consequences in the recovered cow of reduced feed intake, ketosis and displaced abomasum. These conditions can then have significant impact on milk yield and subsequent fertility.
Mastitis – myth or fact?
So what about this link with mastitis? Well, this could be as simple as cows with milk fever are more likely to get their teat ends contaminated when they are down, which, when coupled with their teat sphincters being more relaxed and open (the effect of low calcium on muscle again), makes them much more prone to mastitis. This simple contamination theory certainly occurs, but we also know the link between milk fever and mastitis can be much more complicated.
Many of these more complex links are to do with the cow’s reduced immunity around the time of calving, and so relate to increased risks from other diseases around calving such as metritis. We know calcium levels fall in cows with milk fever but they also fall (not so much) in cows with sub-clinical milk fever. These cows have low calcium but not low enough to make them obvious in terms of being wobbly or going down. We also know calcium is involved in what is termed ‘intracellular signalling’, a key part of the immune cell function. The reduction in calcium seen in cows with hypocalcaemia (both obvious clinical and sub-clinical milk fever) is likely to lead to a reduced ability to respond to infections.
One study concluded milk fever and retained cleansings are important risk factors for clinical mastitis. However, this conclusion should probably be viewed alongside other risk factors for mastitis which we would all recognise such as older cows in terms of numbers of completed lactations, winter season, the first months of lactation and SCC during the previous lactation – these are all contributing factors.
Unlike some of the other diseases producers see on a daily basis, milk fever is preventable so there is no need to put up with the ‘occasional’ clinical case. Appropriate management of the cow’s diet and use of supplemental calcium in at-risk animals can prevent milk fever. Routine supplementation with a bolus, such as Bovikalc®, can be helpful for cows calving for the third time or more and for those with an increased risk, such as a history of milk fever. The bolus contains calcium chloride and calcium sulphate which help reduce the risk of milk fever in three ways:
■ The bolus dissolves rapidly in the rumen and the calcium chloride provides an immediate source of calcium.
■ The calcium sulphate is broken down by rumen bugs and provides a slow release supply of calcium.
■ In addition, the chloride helps to reduce the pH of the cow’s urine which assists the cow in mobilising her own calcium reserves.
Dry cows and, in particular, the transition cows are increasingly being recognised as the most important cows on your farm. Fertility, mastitis and of course milk yield all start here.
Conditions such as milk fever and displaced abomasums are a sign we have not got things right in the dry period. They should be viewed as a signal things are not well with the whole group, rather than just concentrating on fixing the obvious cows. So talk to your vet about dry cow and transition cow management.