Hypocalcaemia in sheep

This article has been compiled from a webinar and discussion with David Masters and Victorian advisers (through the Bestwool/Bestlamb co-ordinators) in 2019.

Hypocalcaemia is a metabolic disease in sheep that is primarily due to a lack of available calcium. This is not usually a simple deficiency. Risk may be increased on pastures or forages with high potassium and/or low sodium and magnesium. Low vitamin D also increases the risk. Because of the interactions between minerals in the forage and digestion, absorption and the metabolic state within the animal it is difficult to manage.

Sheep are most susceptible in late pregnancy, but hypocalcaemia may also occur during lactation or in young rapidly growing animals. The incidence is greatly increased when at-risk sheep are stressed through handling or changes in weather. There are numerous differences between sheep and cattle in susceptibility to deficiencies; peak timing of requirements; differences in the reproduction cycle and a greater variation in reproductive rates. Therefore while the sheep industry often looks to the dairy industry for solutions, recommendations on managing and preventing the disease should be considered separately.

There is reportedly increasing incidence of the disease in the sheep industry, although this can be difficult to measure given the ability to diagnose hypocalcaemia and often associated diseases (e.g. hypomagnesaemia; pregnancy toxaemia). While MLA report the incidence of the disease as of relatively low importance with incidence of 0.2–0.4% nationally or a value of $11M, Ivan Caple (1988) estimated that between 100,000 and 300,000 pregnant ewes die from hypocalcaemia in Victoria alone. The Victorian sheep industry has substantially shifted since then from primarily a merino wool-based industry to a more diverse crossbreed lamb industry focussing on reproduction and increased growth rates, which would logically put more pressure on the mineral requirements, absorption and recovery.

Current recommendations for prevention of hypocalcaemia

1. For sheep being fed cereal grain supplements (ewes and growing stock) in summer and autumn add 0.5% salt and 2% limestone (of the rations) to the diet. Mixing in the diet is preferable but an alternative is to provide limestone and salt ad lib as a loose lick.

Cereal grains are low in calcium (Ca) and sodium (Na). The purpose of this supplementation is to ensure bone reserves are not depleted prior to pregnancy and young growing sheep have adequate calcium for bone development. Salt provides sodium, but also is used to increase intake of the limestone.

2. For all pregnant ewes, from mid-pregnancy, keep stress events and time off feed to a minimum (i.e. yarding for shearing; crutching etc). Avoid yarding ewes during the last 4 weeks of pregnancy.

Stress can cause a redistribution of calcium and magnesium (Mg) in the body. It is likely stress can induce hypocalcaemia even in ewes that are otherwise managed to have an adequate calcium status. The higher the risk status of the ewes, the higher attention required.

3. For sheep grazing vegetative cereal crops, provide a mixture of causmag:limestone:salt (2:2:1) at an allowance of 30 g/head/day.

Young cereals are often low in magnesium, sodium and calcium but high in potassium (K). The risk of mineral imbalance is high.

4. Continue to provide calcium supplements to ewes grazing lush pastures post droughts (or after high rates of grain feeding).

Past research has also indicated that high incidence of hypocalcaemia has occurred in ewes grazing lush green pastures post drought (and after high cereal grain diets). Continuing to offer calcium supplementation after grain feeding has ceased, is the current recommendation rather than removing the supplement to ‘induce’ bone mobilization, as sometimes practised in the dairyindustry. Ensuring supplements are consumed at the amount required (and by the animals that require it) in a paddock situation via a lick, can be unreliable due to significant variation in individual animal consumption within and between mobs. In a Charles Sturt University study, where ewes in paddocks were given access to a low DCAD (dietary cation-anion difference) supplement (supplied in troughs) only 2 out of 5 mobs consumed amounts considered to be effective.

High DCAD reduces bone mineralisation and hence the use of anionic salts that aim to aid mobilisation of bone stores, is gaining interest and some promotion in these situations, but are yet to be researched as to if, when and how they play a role in prevention in sheep. Part of the reasoning for using anionic salts rather than continuing to provide access to calcium supplements is due to the changing blood pH status to more alkaline on green pastures reducing effectiveness of Ca absorption. Promoting Ca mobilisation in sheep would require some confidence that bone supplies have been repleted while sheep have been on grain and are adequate.

There are also reports that indicate the incidence of hypocalcaemia is an increasing issue on green pastures (not associated with drought) and possibly related to an increased focus on reproduction and growth rates.

1. Identify and manage separately high-risk ewes grazing pastures pre-lambing and consider options for prevention as outlined in Table 3 – given no current guarantees. High risk ewes are older ewes (>3 years old) carrying more than one foetus.

Ewes carrying twins or triplets have a higher calcium requirement and older ewes tend to have lower bone calcium turnover. Cyclical seasonal deficiencies of Ca and P, together with losses associated with pregnancy and lactation, could lead to progressive depletion of bone Ca reserves in ewes. Replenishment of partly depleted skeleton takes longer in older animals which have lower rates of accretion of Ca into bone than young. Mg and Ca have a synergistic interaction, (and high K reduces Mg absorption) and while the incidence of hypomagnesemia is considered to be an issue in Victoria, the Charles Sturt study reported in a one-year study, 6 out of 15 farms across southern Australia had low (but not critical) plasma Mg concentrations (but mean plasma and urine concentrations on all farms did not indicate a risk).

Other indicators?

While grasses are lower in calcium and often higher in potassium than legumes, pasture composition may provide some clues to identifying high risk pastures. However, this is not consistently supported. Pastures associated with hypocalcaemia post the 1982 drought were dominated by sub clover and capeweed. Simple testing of pastures for mineral status has also been proposed as a potential indicator of high risk and hence provide some triggers for enacting more preventative options. However, this also has not been consistently supported. Larsen (1983) reported annual monitoring and comparison of minerals in pastures between years was considered more useful than looking at individual years and Caple also suggested where pasture levels indicated adequate levels, there may be some issue with the dietary availability for the animal. The Charles Sturt University study did not find mineral levels of Ca or Mg in pastures were consistent with urinary pH and Ca and Mg concentrations in pregnant ewes. There do not therefore appear to be any simple and reliable pasture indicators of potential risk.

Hypocalcaemia and Pregnancy Toxaemia

Hypocalcaemia can be confused with pregnancy toxaemia in late pregnant ewes. Both are related to nutrition, symptoms and treatments can also be similar (treatment is often aimed to treat both). Table 4 outlines some of the key differences in the 2 diseases. One of the key differences however is that treatment of pregnancy toxaemia does not often lead to recovery.

Table 3. Options for prevention of hypocalcaemia for high-risk ewes on green pasture

Table 4 Comparison between pregnancy toxaemia and hypocalcaemia

Source: Department of Primary Industries and Regional Development, Government of Western Australia
agric.wa.gov.au/livestock-biosecurity/pregnancy-toxaemia-and -hypocalcaemia-ewes