A study of sheep grazing a belah (Casuarina cristata)–rosewood (Heterodendrum oleifolium) shrub woodland in western New South Wales

Abstract

A 5-year study (1969–1974) was made of the effects of the different grazing pressures on animal productivity and the vegetation of a belah (Casuarina cristata Miq.)–rosewood (Heterodendrum oleifolium Desf.) shrub woodland set-stocked by Merino wethers at three stocking rates (0.16, 0.25, and 0.5 sheep/ha).

Most of the pasture species were short-lived and the plant biomass present fluctuated considerably from year to year (200–2000 kg/ha), depending on the amount of rainfall received. The two principal components of the pasture were Stipa variabilis Hughes and Sclerolaena diacantha (Nees) Benth., both of which were dependent on seedling recruitment rather than on perenniality for their continued existence within the pasture.

The animals preferred forbs and green grass when these were available; at other times Sclerolaena spp. became the major part of the diet. There were few species that were unacceptable to the sheep. Analyses of extrusa collected from fistulated animals showed that the nitrogen content of the diet was generally 2% or above, and in vitvo digestibility within the range 52–67%, falling below that range only when forage availability was lower than about 50 kg/ha. Wool production reached a peak in spring of 16–18 g clean wool per day with a plateau of 12–14 g/day at other times, unless influenced by low levels of forage availability.

Of the three stocking rates imposed, only the low rate (0.16 sheep/ha) proved capable of sustaining the animals at a satisfactory level of production throughout the 5-year period. At 0.25 sheep/ha, it was considered that the sheep could be maintained satisfactorily for extended periods, whilst at the high rate (0.5 sheep/ha) productivity could only be maintained for relatively short terms.

Despite the difference in grazing pressures, few permanent differences were observed in the vegetation composition. The data suggest that it is unlikely that any form of sheep management other than the traditional continuous grazing would result in higher, or more stable, animal productivity or be of any permanent benefit to the pasture.