Diet selection, herbage intake and liveweight gain in young sheep grazing dual-purpose wheats and sheep responses to mineral supplements

Young crossbred sheep grazed dual-purpose wheat crops in three experiments evaluating the factors affecting variability in liveweight gain. All crops had high crude protein contents [22-33% dry matter (DM)] and in vitro DM digestibilities (0.754-0.876), which would not have limited liveweight gains. Digestibilities measured in vivo in grazing animals were even higher (0.839-0.854). No significant differences were found in preference of sheep for six different wheat cultivars. The diet composition and herbage intake of the sheep were measured in all experiments using plant wax marker profiles. Wheat forage made up 0.88-0.98 of the diet in sheep supplemented with unchopped oaten hay and 0.97-0.99 of the diet in unsupplemented sheep. Total intakes in the first two experiments were similar to 1380 g DM/day (3.7-4.0% of liveweight). Intakes in Experiment 3 were lower (960-1180 g DM/day; 2.6-3.1% of liveweight) because drought conditions reduced the amount of forage available. There were marked liveweight gain responses (30-50%) to mineral supplements based on NaCl, CaCO(3) and MgO. Evidence is presented that these were mainly due to the Na and Mg, since the consumed diet was more than adequate for Ca. Relative to the requirements of young sheep for growth, wheat forage had high K levels (6-8 times requirement) and very low Na levels (as low as 0.02-0.03 of requirement) and thus very high K: Na ratios. Since high dietary K: Na ratios can greatly reduce Mg absorption from the rumen, the response to Na may have been partly mediated through improved Mg absorption. The liveweight gain responses to Na/Mg were large and economic, so it is recommended that young sheep grazing dual-purpose wheat be routinely supplemented with a 1 : 1 mix of NaCl : MgO.