Milk fever and grass tetany, what’s the difference?

09 Aug 2021

By Julie Dart - Senior Land Services Officer, Coffs Harbour

Metabolic disorders caused by an imbalance of critical minerals in the blood can occur in ruminants, such as cattle, sheep, and goats. Deficiencies in calcium (Ca) or magnesium (Mg) is the most common and can significantly affect herd health as they impact muscles and the nervous system.

Milk Fever (hypocalcaemia/ parturient paresis) is caused by a sudden drop of blood plasma calcium levels near birth, as calcium is required to make colostrum and milk. It causes muscle weakness and reduced contraction of smooth muscles, usually within 72 hours of calving. It is most commonly seen in older, high producing dairy cows. Jersey & Guernsey breeds are the most susceptible, but milk fever can affect any birthing mammal.

Early stage acute cows can appear excitable/twitchy/restless and may have tremors. As the disorder progresses to muscle weakness, cows become unable to stand. Down cows may lie on their breastbone with their head turned to the flanks and have a cool dry muzzle. Cows will be reluctant to eat.

If left untreated, cows will become floppy, lay on their side, bloat, develop a weak pulse, and become unconscious.

Treatment by a vet will usually include intravenous calcium to down cows. This treatment can affect the heart rhythm, so it must be done very carefully. Down cows will require special nursing treatment to prevent muscle & nerve damage whilst immobile. They must be provided with feed, water, shade and shelter. These cows are likely to suffer from secondary infections such as mastitis and metritis.

Less severe (sub-clinical) cases occur where the cow is still mobile. Cows that appeared to be “lazy” calving or required birth assistance but did not collapse are likely to be subclinical cases. They should be checked and treated for milk fever and monitored for follow-on health issues. Sub-clinical cows can be treated with slower release subcutaneous injections that contain both calcium & magnesium.

The problem can be greatly prevented on dairies by keeping heavily pregnant cows from high potassium (K) pastures such as Kikuyu. High potassium levels affect blood pH which affects calcium mobilization. Cows are fed other forages and a pellet with specialised mineral supplements for 21 – 28 days prior to calving. Dairy Australia delivers Transition Cow Management workshops to registered dairy farmers and their workers.

Oxalates and Calcium

Other summer growing tropical grasses such as Setaria, Sorghum and Kikuyu are also naturally high in oxalates that can bind to calcium to reduce availability. Oxalate induced hypocalcaemia most frequently occurs on Setaria sphacelata pastures, especially the older variety ‘Kazungula’. The improved varieties ‘Narok’ and ‘Solander’ have moderate levels of oxalate.

The problem usually occurs on dairies with Jersey herds, when milkers that have not grazed Setaria over winter are re-introduced to it in spring, and the regrowth is short and lush. As oxalate levels in pasture increase overnight, the problem is seen most in the morning as downer cows. These animals show acute milk fever symptoms, but they have not recently calved. Dairy farmers will often describe the problem as ‘late lactation milk fever’. Affected cows are managed as milk fever cases.

Specialised microbes can break down oxalates, but it takes up to four weeks to build up the numbers in the rumen. Risky pastures should be re-introduced slowly to the herd through time-controlled grazing or the use of conserved hays and silages. Year-round feeding of some Setaria will maintain a base level of the useful rumen microbes.

Farmers who have significant areas of high oxalate grasses can often offer extra minerals to stock, when moving to risky paddocks after a break on other feeds. Supplements should contain calcium, magnesium and sodium (salt).

Oxalates are particularly toxic to horses and donkeys which can’t break them down, causing “big head” syndrome. Horses should not graze any type of Setaria pasture for more than one month.

Grass Tetany (Hypomagnasemic Tetany) is a shortage of magnesium in the blood. It causes excitability, muscle spasms (tetany), convulsions, collapse and death. This can affect all ruminants of various ages and is often caused by grazing immature pastures and immature grain crops that are low in magnesium, or after a stressor that reduces feed intake.

In the early stages of grass tetany animals are very excitable or acting “crazy”. They may have a stiff or staggery walk, are twitchy and have muscle spasms. They are difficult to treat at this stage as they are hard to restrain.

If left untreated animals collapse and have convulsions. Scrape marks may also be found near dead animals’ feet from leg paddling movements.

Grass Tetany is life threatening, and will affect the whole mob, so it’s crucial to call in the vet for treatment advice when it is first noticed. Your vet may use intravenous calcium & magnesium therapy and subcutaneous injections. Less affected stock should be moved off the affected pasture. Treated animals must not be disturbed until recovery as excitement can cause further seizures. They should be moved once they recover.

Magnesium feed supplements such as magnesium oxide should be fed to protect stock from further attacks. Magnesium oxide is not palatable, and cattle will sort it out of their feed if they can. It can be mixed with molasses or added to a manufactured pellet with a flavouring agent.

Use of magnesium sulphate (epsom salts) in stock water may be effective when water access is controlled in troughs. Any supplementation programme should ensure that all cattle consume the recommended dose of Magnesium. On acid soils where soil tests show magnesium deficiency, dolomite or magnesium fertilizers can be used.

Other factors that can be involved are

• Age of the cow- older, lactating cows are more vulnerable
• Feeding grass dominant pastures or cereal crops at an immature growth stage
• Anything that reduces feed intake (stress, feed shortage, advanced pregnancy, yarding & transport)
• Magnesium deficiency on some types of acid soils
• Environmental effects. A cold snap or cold wet days followed by good weather, that influences plant nutrient uptake.
• High potassium levels in soils and pastures interfere with magnesium uptake.
• Recent applications of Nitrogen or Potash fertilizers

In the North Coast region, we usually see Grass Tetany that is caused by excessive levels of other minerals such as potassium that interfere with the uptake of magnesium and calcium in pastures.

Some tropical pastures such as Kikuyu are naturally higher in potassium which can accumulate in excess. The levels increase when grown on soil with high potassium levels, such as an effluent re-use area, former dairy night paddock or on the site of an old laneway. The use of nitrogen fertilizer also increases potassium uptake.

The use of mineral supplements in the milker ration is an important management tool on dairy farms where high potassium/low magnesium pastures can’t be avoided. Adding legumes to the pasture mix can help reduce the risk of grass tetany as they contain more magnesium than grasses.

If your herd experiences Milk Fever or Grass Tetany type symptoms, a team approach is best used to correct it. Your vet is the first point of call in the crisis. Follow this up with an agronomy check. Use both soil and feed testing to identify high risk paddocks and feeds so you can better manage the risks.

Local Land Services vets and agricultural extension staff can work together to assist you to develop a management strategy.

Jersey cows are more susceptible to milk fever