The Effects of Grazing Exclusion and Blade-Ploughing on Semi-Arid Woodland Vegetation in North-Western New South Wales Over 30 Months.

Abstract

Inedible native shrubs are considered to be a primary cause of lost pastoral productivity in the semi- arid woodlands of north-western New South Wales. Most rehabilitation programs have focussed on reducing shrub density. In recent years blade-ploughs have been tested for this purpose. Results have been variable and most programs have brought neither lasting shrub control nor improved production for pastoralism over the medium to long term. Failure to control grazing after initial treatment is suspected as one reason for the low success rate. An experiment involving blade-ploughing and grazing exclusion was established in July 1990 in sandplains supporting semi-arid woodlands, near Bourke New South Wales (NSW). The treatment factors were crossed and each was imposed at two levels: present and absent. Shrub density, pasture composition and pasture biomass variables were selected and measured before, and over a 30 month period following treatment. Data were analysed using repeated measures ANOVA. The densities of Eremophila sturtii, E. deserti, E. gilesii and Dodonaea viscosa ssp. angustissima as well as that of 'Total shrubs', all increased after ploughing, irrespective of the presence or absence of grazing. In all cases except that of E, sturtii, there was no significant difference between shrub density in ploughed and unploughed plots 30 months after treatment. While E. sturtii density increased significantly to begin with, it remained significantly lower in ploughed plots compared to unploughed plots at the end of the study period. This suggests that blade-ploughing offered only temporary control of these shrubs on this land type. Pasture biomass was significantly greater in ploughed/ungrazed plots (1300 kg/ha) compared with other treatments at the end of the study. Ploughed/ungrazed plots were the only ones where fuel had accumulated to a level which offered any possibility of burning to control shrub regeneration. The same treatment also contained a significantly greater proportion of desirable pasture species than any other treatment at the end of the study period.