Liveweight gains of young sheep grazing dual-purpose wheat with sodium and magnesium supplied as direct supplement, or with magnesium supplied as fertiliser

Abstract

In four experiments conducted in 2006–2008 near Canberra, Australia, young sheep grazed vegetative dual-purpose wheat (cv. Mackellar) while receiving either no mineral supplement or receiving extra sodium (Na) and/or magnesium (Mg). In the first two experiments, supplements were fed to the sheep whereas in Experiments 3 and 4, the extra Mg was applied to the crop just before grazing commenced, either as MgSO4 (Experiment 3, 4) or Causmag (MgO; Experiment 4). Direct supplements of Na resulted in increases of liveweight gain of 20–25% (36–42 g/day). In Experiment 2, sheep consumed significantly less of the mixed Na/Mg supplement than of salt alone (12.0 vs 24.8 g/day; P < 0.05) but still grew significantly faster (61 g/day; P < 0.05) than unsupplemented sheep. Application of supplementary Mg to the crop resulted in significant increases in forage Mg, from levels close to animal requirement (0.10% DM) to levels well above this. It also improved a range of mineral indices related to Mg status. Increases in liveweight gain occurred in response to top-dressing of the crop, but compared with feeding Mg directly to the sheep, these were transient and strongly interacted with stocking rate. Estimates of the cost of supplementation or top dressing, and of the value of the extra liveweight gain obtained, indicated that direct supplementation would be economic, but that top-dressing costs would exceed the value of the extra liveweight gain obtained.