Cattle performed as well as sheep when grazing a river saltbush (Atriplex amnicola)-based pasture

Abstract

In Mediterranean-type climates, saltbush grown on saline land offers producers valuable feed during the autumn feed gap. Saltbush has high crude protein concentrations, moderate to low energy concentrations and accumulates very high concentrations of soluble salt. As ruminants cannot store quantities of salt, intake of saltbush is limited by the individuals' ability to excrete salt. The aim of this 6-week study was to compare the relative performance, diet selection and urine concentrating ability of mature sheep and cattle grazing a young stand of river saltbush (Atriplex amnicola) with a volunteer annual understorey, consisting of senesced and germinating annual grasses.We measured liveweight and condition score of 30 mature Merino wethers and 15 mature Hereford-cross cows grazing the same saltbush stand, with a volunteer understorey of predominantly annual grasses, for 6 weeks during late autumn. Saltbush intake and diet selection were estimated at the same time using sodium (Na):creatinine ratios in urine and carbon isotope ratios in faeces. During the first 3 weeks of grazing, the data indicated that cattle significantly outperformed sheep with a 15% increase in liveweight compared with sheep with an average increase of 4%. It is possible that some of this difference may be associated with different rates of loss of digesta associated with restricted access to food in the 2 h before weighing. No differences were found between the proportion of plants with a C4 photosynthetic pathway (saltbushes) in the diets selected by sheep and cattle for the majority of grazing. The specific gravity and Na concentration of urine was similar across livestock species. While this provided no indication of a difference in ability to concentrate or excrete Na, sheep did have a consistently higher Na:creatinine ratio in urine. The higher ratio is consistent with more efficient Na excretion and this may be significant when the species graze saltbush without low salt supplement or if water is restricted. Historically, it has been thought that cattle would not perform as well as sheep when grazing saltbush, but under the conditions of this study, when feed on offer is not limiting dry matter intake and fresh water is available, cattle are able to utilise this out of season feed source as well as, or possibly better than, sheep.