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Time to weigh up risks of milk fever

If you are a dairy farmer with a spring calving herd, now might be a good time to reflect on how many downer cows you’ve had to treat for milk fever (periparturient hypocalcaemia) last spring.
Treating downer cows is not only stressful, it’s expensive in terms of treatment costs, time, energy and the impact on future production, even when cows are treated quickly and look to have made a speedy recovery. In terms of cow and calf health, peace of mind and profitability, the fewer downer cows the better — even at 2-3 percent
downer cows, up to half the herd may have subclinical milk fever.

Risk factors for milk fever include:
• High sodium or potassium intakes pre-calving. These positively charged ions (cations) reduce the mobilisation of calcium from the bones which increases the risk of low blood calcium after calving.
• Inadequate magnesium intakes coming up to and through calving. Magnesium sulphate and magnesium chloride have the advantage that they deliver magnesium and negatively charged ions (anions) that improve the mobilisation of calcium from bones.
• Channel Island breeds including Jerseys are about two times more likely to get milk fever than Friesians.
• Older cows, especially aged 7 years and above, appear to have a decreased capacity to mobilise calcium from bones and possibly decreased ability to absorb calcium from the intestines in response to the increased demand for colostrum production.
• High phosphorous intakes in the last 2 to 3 weeks before calving. Avoid feeding too much high P feeds such as PKE and DDGS.
• Over-conditioned cows that come into milk faster may be at risk but also under-conditioned cows may be at more risk of getting milk fever, perhaps because of compromised immunity to diseases.

Simply feeding some maize silage, grass silage or a lot of magnesium oxide (over 40g elemental magnesium/ cow/day) doesn’t seem to eliminate sub-clinical milk fever. In an observational study of 76 spring calving herds in the Waikato, Roberts and McDougall (2019) found the prevalence of sub-clinical milk fever was greater in herds feeding grass silage or maize silage pre-calving than those that did not. They suggested that grass silage in the region could be high in potassium. Maize silage is normally considered to lower the risk of milk fever because it lowers the DCAD (dietary cation anion difference) compared with pasture. However it was suggested that management factors associated with feeding of maize silage could also increase the risk of milk fever. It could
be that historically high rates of milk fever have encouraged farmers to feed silage and high rates of magnesium oxide and while this has taken farmers in the right direction, they have not reached the end-point of eliminating milk fever and need to look at a Plan B. Ironically, cows with milk fever normally have a lower than normal body temperature. If you want to avoid getting hot under the collar next spring, explore the products now available that can help reduce the risks.

For more information, contact your Farmlands Technical Field Officer or the friendly team at your local Farmlands store.

Article supplied by Dr. Rob DerrickHead of Product Development (Nutrition)

KI Roberts & S McDougall (2019) Risk Factors for
subclinical hypocalcaemia, and associations between
subclinical hypocalcaemia and reproductive performance,
in pasture-based dairy herds in New Zealand,
New Zealand Veterinary Journal, 67:1, 12-19, DOI: