Impact of magnesium–sodium supplementation on liveweight gains of young sheep grazing dual-purpose cereal or canola crops

Previous experiments have shown that liveweight gains of livestock grazing dual-purpose wheats were increased by 15–60%, by supplementing animals with a 1 : 1 mixture of Causmag (MgO) : salt (NaCl). The supplement appears to overcome both an Na deficiency in wheat forage, plus a reduced rumen Mg absorption due to a high forage (and thus rumen) K : Na ratio. In crop–livestock systems, there is also renewed interest in grazing forage oats and, more recently, barley and dual-purpose canola. The possible need for Mg–Na supplements for sheep grazing these last three crops was investigated in two experiments near Canberra, ACT. In Experiment 1, sheep grazing wheat, oats, barley and canola were either unsupplemented or received a Mg–Na supplement. There was no significant response to the supplement in sheep grazing oats. After adjustment by covariance for differences in sheep numbers per plot, the difference in weight gain/ha of supplemented and unsupplemented sheep grazing barley approached significance (P = 0.068). For the first time in our experience, in this experiment there was also no significant response in sheep grazing wheat. There was a significant interaction between crop type and supplement, because of a 20% depression in liveweight gain in supplemented sheep grazing canola. This negative effect of supplementation on canola was further investigated in Experiment 2, in which sheep grazing only canola were either supplemented or not supplemented. Supplementation in this case had no significant effect on liveweight gain. Our results suggest that there is no need to provide mineral supplements for sheep grazing dual-purpose oats; they also suggest supplementation may be contraindicated for sheep grazing canola. Further work is needed to confirm this and to clarify supplement responses in sheep grazing barley.