Prolonged exposure to sheep grazing reduces the palatability of Australian saltbush populations

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Abstract
This study examined the variability in palatability of populations of Atriplex vesicaria and the influence of past grazing history on mean palatability to sheep. Twenty saltbush plants (10 male, 10 female) were collected from each of six locations across chenopod shrubland of South Australia; three populations came from areas with long records of intensive sheep pastoralism and three from areas with negligible stock grazing in the past. The plants were transplanted into an experimental paddock at Middleback Station, near Whyalla, South Australia and the paddock was grazed by three sheep for nine days. Relative palatability of the populations was assessed by their rates of decline in bush biomass, as indexed by loss of tagged terminal shoots.Overall accumulated tag loss after 9 days was significantly different between populations. These differences were enhanced when dung drop in paddock subdivisions, flowering status of bushes, and leaf sodium and potassium concentrations were included as covariates in tag loss models. Although there was no clear separation of grazed versus ungrazed populations by tag loss alone, the covariate models demonstrated a significant difference between grazing history types, with grazed populations being less palatable. Examination of model residuals showed heterogeneity of palatability to be higher in the grazed populations. This study indicates that the piosphere is a gradient of variability in palatability as well as a gradient of floristic composition.

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