The influence of kangaroos and forage on sheep productivity in the semi-arid woodlands.

Abstract

Problems of pasture degradation and competition from kangaroos in the semi-arid woodlands were investigated in a grazing trial conducted in western New South Wales. The trial area of 200 ha was subdivided into 12 paddocks, varying in size from 7.5 to 30 ha and grazed with matched groups of six merino hoggetts, with and without a similar number of western grey kangaroos. Stocking rates were 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8 sheep per hectare and 0.2, 0.27, 0.33, 0.4, 0.47 and 0.53 sheep per hectare, with an equal number of western grey kangaroos. Wool growth and weight gain of the sheep, and the yield of forage species and components, were recorded quarterly over a three year period. Rainfall received over these years was close to the 30, 90 and 50 percentile levels respectively. The results were analysed by regression modelling. Both wool growth and sheep weight gain were reduced by increasing stocking rates of sheep or kangaroos, except when forage was abundant. The relative effect of kangaroo grazing was greatest in the year of lowest rainfall, with an equivalence of one sheep: 0.6 kangaroo of equal weight. Forage biomass declined as stocking rates increased with kangaroos having the equivalent effect of 0.75 sheep on a weight for weight basis. More than 80 per cent of the variation in clean fleece weight (CFW, kg/year) and annual sheep weight gain (WG, kg/year) was explained by the average yield of green herbage leaf (GL, kgtha) in the expressions and CFW = 1.30 + 0.72 In GL WG = 5.6 In GL These results show that sheep productivity can be increased by controlling kangaroo numbers in the drier years. They also indicate that productivity can be raised by increasing the abundance of palatable perennial species, which maintain green leaf during periods of moderate and low rainfall.