Do calcium and magnesium deficiencies in reproducing ewes contribute to high lamb mortality?

Abstract

High lamb mortality continues to be a significant economic and welfare problem within the Australian sheep industry, with 20–30% of lambs born in commercial flocks dying mostly within 3 days of birth. Clinical hypocalcaemia and hypomagnesaemia cause ewe mortality, and, subsequently, either fetal or lamb death, but it is not known whether subclinical deficiencies of calcium (Ca) and magnesium (Mg) compromise lamb survival. This review considers the potential mechanisms through which Ca and Mg deficiencies may influence lamb survival, and factors influencing the risk of deficiency. Pastures grazed by lambing ewes may be marginal in calcium (Ca; <4 g/kg DM) and magnesium (Mg; <0.9 g/kg DM) but also have a high dietary cation–anion difference (>12 meq/100 g DM) and high concentrations of potassium (K; >30 g/kg DM) and nitrogen. In young cereal crops, sodium concentrations are also often low (<0.9 g/kg DM). This combination of minerals and other nutrients creates an imbalance in supply and increases susceptibility to acute Ca (hypocalcaemia) and Mg (hypomagnesaemia) deficiency. Calcium is required for smooth muscle function and has a direct role in uterine contraction, so may influence the duration of parturition. Low Ca and Mg intake both influence insulin release and sensitivity, low Mg results in poor glycaemic control and insulin resistance by impairing both insulin secretion and its action on peripheral tissues, also potentially altering the duration of parturition as well as risk of metabolic disease. Magnesium is also a neuroprotectant that slows the neuronal damage during hypoxia and has been linked with thermogenesis in offspring and increased immunoglobulins in colostrum. These functions indicate potential importance in improving the ease of parturition and improved ability of the newborn lamb to thermoregulate and survive after birth. Subclinical Ca and Mg deficiencies commonly occur in 20% of lambing ewes grazing temperate pastures, so further studies are warranted to investigate whether correction of these deficiencies can improve lamb survival.

Reproductive Performance of Triplet-Bearing Ewes on Commercial Farms and Research Priorities Identified by Sheep Producers to Improve the Survival of Triplet-Bearing Ewes and Their Lambs

“…Little is known regarding whether mineral supplementation of lambing ewes can reduce ewe and lamb mortality. Subclinical deficiencies of calcium and magnesium are common in lambing ewes in Australia due to imbalances in pasture grazed by the ewes [ 34 ]. Mineral imbalances in vegetative cereal crops also present a risk of low calcium status in ewes grazing these crops in late pregnancy [ 35 ].…”

“…Mineral imbalances in vegetative cereal crops also present a risk of low calcium status in ewes grazing these crops in late pregnancy [ 35 ]. Subclinical deficiencies in calcium and magnesium may increase the risk of dystocia and related issues, including hypothermia in lambs and poor ewe-lamb behaviour [ 34 ]. Providing lambing ewes with ad libitum access to mineral supplements containing magnesium, sodium and calcium has been reported to reduce the risk of ewe mortality when grazing cereal crops [ 36 ].…”

The effects of maternal calcium status during late gestation on lamb growth and survival in twin‐bearing Merino ewes grazing pasture

“…The potential mechanisms whereby subclinical calcium deficiencies influence lamb survival has been reviewed previously. 8 Calcium is an important mineral required for smooth muscle contraction, with deficiencies in calcium leading to weaker uterine contractions, which, in turn, can extend parturition duration and result in dystocia 17,18 and, thus, lamb death. 8 The degree of lesions/haemorrhaging to the CNS observed during lamb necropsies can indicate whether parturition-induced hypoxia, resulting from dystocia, is the cause of lamb death.…”

Calcium and magnesium supplementation of ewes grazing pasture did not improve lamb survival

“…Subclinical calcium deficiency is defined as blood Ca of <90 mg/L (2.0 mmol/L) and not showing clinical signs (Suttle 2010). However, it is unclear whether subclinical deficiencies elevate perinatal lamb mortality (Friend et al 2020).…”

“…Supplementation with Ca and Mg in either lime/Causmag or low-dietary cation-anion difference (DCAD) forms has been shown to improve the mineral status of late-pregnant twin-bearing ewes grazing cereal crops (Masters et al 2019), but that study did not measure lamb survival. The low-DCAD form of supplementation is thought to increase Ca mobilisation from bones, or increase absorption from the intestines (see review by Friend et al 2020). However, there are no studies in sheep evaluating the effectiveness of low-DCAD supplements to increase lamb survival, to our knowledge, and only one study has evaluated lime/Causmag supplementation for lamb survival, with no increase reported (McGrath et al 2015).…”