Calcium disorders of ewes and lambs: hypocalcaemia and bone diseases
Colin Trengove, University of Adelaide
Hypocalcaemia is the most common calcium disorder of ewes and is frequently seen at lambing time. It also occurs in transported sheep and in other stressful situations, such as when yarded for shearing in winter. Fortunately, if recognised early, a simple treatment with a calcium solution injected under the skin and oral supplements will effect a recovery in most sheep. Other calcium disorders include osteomalacia in lambs and osteoporosis in ewes. Why are ewes so susceptible to hypocalcaemia and osteoporosis?
During pregnancy and the first few weeks of life the lamb is entirely dependent on the ewe to provide calcium to build strong bones. The peak demand for calcium by the lamb is at 10 days of age, but that demand on the ewe steadily increases from midway through pregnancy, as the lamb's bones begin to calcify. It is the bone development of the lamb that determines future bone calcium storage; and the bones are a major source of calcium for the ewe to provide the lamb. During pregnancy approximately 20% of the ewes' total bone calcium is mobilised to supplement the calcium in the diet to meet the lamb's demand for calcium. This demand on the ewe's bone stores increases to 70% in early lactation with the remaining calcium coming from pasture.
Inadequate calcium supply by a ewe to her lamb means the lamb has lower bone calcium stores, and as an adult will be more at risk of hypocalcaemia when lambing, perpetuating the risk into the next generation.
Farmers can influence the risk of hypocalcaemia by ensuring the ewes get fed a balanced diet which contains at least 3 gm/kg DM calcium, and up to 5 gm/kg in ewes with multiple lambs. Legume forages are great sources of calcium, and cereal grains are poor. The text box below lists some of the risk factors that can be assessed and corrected with the provision of stock lime – the simplest form of supplemented calcium. Measures of calcium in the forage are included such as the DCAD ratio (dietary cation and anion difference) and the calcium to phosphorous ratio. These and other calcium ratios are currently being assessed for their validity and predictive value, so watch this space.
Potential risk factors for hypocalcaemia at lambing time
- Ewes fed on cereal grains without lime supplementation for more than 6 weeks.
- Supplementation with lime is particularly important if feeding cereal grains in mid-pregnancy
- Pasture analysis suggest either:
- DCAD > 300 meq/kg DM or
- dietary Ca:P < 1:1
- Ewes are grazing abundant lush green feed, or conversely
- Their diet consists of limited dry feed only
(e.g. < 1,000 kg DM/Ha), or
- Older ewes (over 4 years of age).
Case study: hypocalcaemia in twin bearing ewes
There were 14 cases of hypocalcaemia in a mob of 200 twin-bearing, July lambing ewes. Ewes responded to treatment, but ten dead ewes were recorded. Hypocalcaemia was not observed in any of the other mobs in this flock of 2000 ewes.
All ewes had been fed barley for eight weeks in a containment area after mating, before being depastured. Limestone was not provided in the stock containment area. At lambing time these ewes were grazing an Italian ryegrass pasture with over 2500 kg/ha DM which was tested at 13.2 MJ ME/kg DM and 32.4% CP. Comparative mineral analysis was done on this pasture and another: both had Ca:P ratios of 1:1, but this pasture had a poor result on the DCAD calculation.
As pasture and fodder tops up the supply of calcium to the lamb during pregnancy and early lactation, anything that disrupts the supply or absorption of calcium from the feed puts ewes at risk of hypocalcaemia. This is where mustering or transport have their effects, through disruption to the supply of calcium.
Diseases such as 'worms' which affect the absorption of nutrients, or copper deficiency, which interferes with calcium metabolism can produce osteomalacia ('soft bones', or rickets) in lambs. Some pastures lack calcium or contain chemicals that interfere with calcium metabolism. Examples include oxalate containing weeds, green cereal crops, and rye-grasses which may contain compounds that have an anti-vitamin D effect. Vitamin D is a hormone formed in the body through exposure to sunlight and is important in assisting the uptake of calcium from the diet. Prolonged cloudy weather can also limit the supply of vitamin D to lambs and induce osteomalacia, especially when grazing pastures with inadequate calcium levels.
Osteoporosis is where ewe bones contain insufficient calcium, which is drawn down with each lactation. Older ewes are more at risk of both osteoporosis and hypocalcaemia because their bone calcium reserves are more likely to be depleted . Provision of a balanced diet after weaning such as adequate green feed or legume forages for 2-3 months will restore bone calcium stores provided uptake is not restricted by the factors mentioned earlier.
This article is a summary of a presentation made at the recent Best Wool Best Lamb conference in Bendigo and is part of a PhD currently being undertaken by the author.