Revegetated Saline Pastures as a Forage Reserve for Sheep: 1. Effects of Season and Grazing on Morphology and Nutritive Value of Saltbush.

Abstract

This study determined the impact of grazing on the kind of material selected by sheep from a saltbush (Atriplex amnicola) plantation and the changes in the chemical composition of saltbush plants associated with season. Saltbush plants in four plots of about 0.9 hectares each were scored from 1 to 5 on the basis of leafiness. Ten Merino wethers were grazed on each plot, giving a stocking rate of about 11 per hectare. The results demonstrated that sheep selected only stem material of less than 1.5 mm diameter, while the grazing pressure on saltbush plants, as described by the number of stems eaten per 0.1 m2, increased from less than one to an average of 18 to 20 stems in 5 to 7 weeks. Grazing had a significant effect on dry matter digestibility (DMD) and nitrogen concentration of whole plant samples. During six weeks of grazing the DMD of whole plant samples cut 10 cm from the tips of the branches dropped from 0.53 to 0.25 (P<0.05), whereas the nitrogen content declined from 11 g/kg to 8 g/kg (P<0.05). At the end of grazing the nutritional value of whole plant samples was very poor and sheep refused to eat stem that was thicker than 1.5 mm. These findings question the grazable fraction reported in the literature for saltbush plantation. when the stem diameters used in calculations are not reported or thicker stems were assumed grazable. Season also had a significant effect on the ratio of leaf to stem and the mineral content. The concentration of sodium in saltbush leaf was negatively correlated (r = -0.93) with both nitrogen and potassium.