Mineral supplements improve the calcium status of pregnant ewes grazing vegetative cereals

Abstract

Grazing vegetative wheat, barley and oats (both dual-purpose and traditional spring varieties) is becoming an important strategic and tactical grazing option on farms where crops and livestock coexist. However, metabolic disorders have been reported and many producers avoid grazing these crops with reproducing ewes because of the risk. A recent survey of 18 farms grazing crops with pregnant ewes indicated a high proportion of grazed crops had forage calcium (Ca), sodium (Na) and magnesium (Mg) below published requirements and potassium (K) above the published maximum tolerable level. Many ewes grazing these crops had concentrations of Ca in plasma and urine indicative of a low or marginal Ca status. The aims of the present study were to further investigate the changes in mineral status that occur in ewes grazing vegetative crops during late pregnancy and to measure the effectiveness of mineral supplements for improving Ca and Mg status. On each of six farms, a paddock growing cereals in the vegetative stage was subdivided into three plots. Three groups of 30 pregnant ewes were selected and each group allocated to one plot. Forage was provided as wheat on four farms, barley on one farm and oats on one farm. Ewes were mature (3–7 years), in the last third of pregnancy (between 108 and 129 days after the start of mating) and were mostly twin-bearing. They grazed the crops for 21 days. One group of ewes was given no supplement (Control), a second was provided with an industry standard supplement (Standard) [Causmag (MgO):limestone (CaCO3):salt (NaCl), 40:40:20] at 30 g/day, whereas the third group was provided with a low dietary cation-anion difference (DCAD) supplement (New) (MgCl2.6H2O:CaSO4.2H2O:NaCl, 12.5:32.5:55.0) also at 30 g/day. Both Ca supplements improved Ca and, to a lesser extent, Mg status. The supplemented ewes showed significant increases in Ca concentration in urine, plasma and Ca fractional excretion on all but one of the six farms. There were no consistent differences between the two supplemented groups of ewes. It is concluded that the Ca status of ewes grazing vegetative cereal crops in late pregnancy can be improved by providing supplements containing Ca, Mg and Na. As the literature indicates the relationship between Ca status and susceptibility to hypocalcaemia is still inconclusive, additional research on a commercial scale is required to determine if supplements decrease the incidence of metabolic disorders when pregnant ewes graze cereal crops.

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

“…Lucerne is a known natrophobe with low accumulation of Na in leaf, particularly under dryland conditions (Hall 1982;Champness et al 2021), so salt-based supplements may be attractive to sheep grazing lucerne, consistent with the intake recorded in Experiment 2. High intake of the same low-DCAD supplement (21-30 g/day) has been reported previously for ewes grazing cereal crops, which are also often deficient in Na, although more variable intake (mean 22, range 4.9-30 g/day) was recorded for a lime/Causmag/ salt loose lick (Masters et al 2019). Alternatively, supplement intake may be reduced where high quantities of live pasture are present, since intake was minimal at three sites in Experiment 1 when the quantity of live pasture was ≥1300 kg DM/ha.…”

“…There is large variability among sheep in intake of block or liquid supplements (coefficient of variation <100%) (Mulholland and Coombe 1979) and of loose-lick mineral supplements (1.6-fold) (White et al 1992). Despite the variable intake, loose-lick mineral supplementation has increased weaner weight gain (White et al 1992) and, elsewhere, plasma Ca and Mg concentrations in late-gestation ewes (Masters et al 2019). These indicate that variable intake among ewes should not prevent a response in lamb survival if the response to supplementation is large, unless intake is low or few ewes consume the supplement.…”

“…After sampling, each group of ewes was placed into a lambing paddock (10-60 ha), and mineral supplementation was commenced. Post-lambing blood and urine samples were not collected because previous studies (Masters et al 2019) have shown that a similar type and method of supplementation improved the mineral status of late pregnant ewes. The loose lick of minerals was offered to the supplemented groups from the day of pre-lambing sampling until the end of the lambing period.…”

“…The grab samples were sent to a commercial laboratory (CSBP Soil and Plant Analysis Laboratory, Bibra Lake, WA, Australia) for standard mineral analyses. The percentages of minerals in the pasture DM were used to calculate the Ca:P ratio, K:Na ratio and tetany index = (K/0.039)/((Mg/0.012) + (Ca/0.02)), and were converted to molecular equivalents/100 g DM to calculate DCAD = (Na/0.023 + K/0.039) -(Cl/0.0355 + S/0.016; Masters et al 2019). The concentrations of Mg, Ca and phosphorus (P) in plasma and urine samples were analysed using kits (photometric colour-test method).…”

“…The evidence therefore suggests potential for subclinical plasma Ca and Mg concentrations to reduce lamb survival. 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).…”