Factors contributing to differences in forage yield in the semi-arid woodlands.

Abstract

A series of experimental sites, which were all on the same land unit but differed in present condition, were established in the semi-arid woodlands near Cobar, N.S.W. The forage yield on these plots was measured quarterly over a two year period. Associated measurements were made of species composition, forage quality, shrub and tree cover, area of eroded surface and soil fertility (available phosphorus, organic carbon, organic nitrogen and available nitrogen). Mean forage yield varied from 34 to 237 kgha. A major proportion of this variation was explained by total shrub and tree cover, organic nitrogen and organic carbon in the surface soil. Differences in nitrogen were considered to have arisen by differential erosion, but this was more accurately characterized by laboratory measures than by field observations of the extent of soil erosion. Some differences were recorded in the composition of the herbaceous layer between plots, including a dominance of perennial grasses on plots that had been exclosed from grazing. However, nearly all species were edible so that composition differences were not a major contributor to variation in digestible forage yield. Perennial forbs (e.g. Sclerolaena diacantha) had the highest digestibility and biennial grasses (e.g. Stipa variabilis) the lowest. Total shrub and tree cover and the extent and severity of erosion are recommended as the most useful variables for monitoring changes in pastoral productivity of the semi-arid woodlands.