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.