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

Abstract

Context. Clinical deficiencies of calcium and magnesium may result in the metabolic disorders hypocalcaemia and hypomagnesaemia, resulting in ewe and lamb mortality. However, the contribution of subclinical deficiencies to perinatal lamb mortality in grazing flocks is unclear. Aims. To test the hypothesis that calcium and magnesium supplementation during the lambing period would increase lamb survival to marking age. Methods. In 2017, an on-farm study used five flocks across New South Wales, South Australia and Western Australia. On each farm, twin-bearing mature Merino ewes (n = 400-600) grazing pasture were allocated to two replicates of control and supplemented treatments. The supplemented groups were offered 30 g/ewe per day of a loose lick containing magnesium chloride (MgCl 2 (H 2 O) 6 ), calcium sulfate (CaSO 4 •(H 2 O) 2 , and salt (NaCl), in the ratio 12.5:32.5:55.0, designed to have a low dietary cation-anion difference (−390 meq/100 g). A second study was conducted in 2018 on one farm to test the form of supplement. This study used two replicates of three treatments: control; a low-dietary cation-anion difference supplement as used in 2017; and a standard lime, Causmag (calcined MgO) and salt loose mix (ratio 1:1:1). Mature twin-bearing composite ewes (n = 600) were allocated to groups and those supplemented were offered minerals for the last month of pregnancy and during the lambing period. Blood and urine samples were collected in both experiments for analyses of mineral concentrations. Key results. In the 2017 study, only two flocks consumed >10 g/ewe of supplement per day, and supplementation did not increase lamb survival to marking age in these flocks. In the 2018 study, the mean consumption of supplement was 18 or 20 g/ewe per day. Of non-supplemented ewes, 61% were deficient in plasma calcium (≤90 mg/L) and 17% were deficient in magnesium (≤18 mg/L) at Day 140 after the start of joining. Lamb survival was not increased by supplementation and was 77 ± 3.8% in both treatments. Conclusions. Calcium and magnesium supplementation did not increase lamb survival. Implications. Lamb survival was not increased by calcium and magnesium supplementation; however, evaluation under a wider range of grazing conditions with adequate supplement intake is required.