The effects of perennial grasses, stocking rate and rainfall on sheep production in a semi-arid woodland of eastern Australia

A grazing study was conducted in a semi-arid mulga (Acacia aneura) woodland to determine the relative importance of variables controlling sheep production in this environment. The study was based on six sheep stocking rates (0.3-0.8 sheep/ha) with the exclusion of other large herbivores including kangaroos and feral goats. The availability of forage and its components, together with the wool growth and live weight of the sheep were measured every three months for seven years from October 1986. Annual rainfall accounted for 60% of the annual variation in live weight gain per head and 79% of the variation in wool growth per head. Stocking rate accounted for an additional 10% of the variation. Rainfall accounted for this large proportion of variation by its strong influence on the yield of green leaf which accounted for 78% of the variation in live weight gain and 66% of the variation in wool growth. Green leaf was a determinant of sheep production because of its superior crude protein content and digestibility compared to dry leaf and stem. The species comprising this green leaf had an influence on sheep production only during drier years when the presence of palatable perennial grasses became important to sheep production. The frequency and yield of perennial grasses were greatly diminished at high stocking rates. This change in forage composition caused a decline in sheep productivity as detected by a loss of linearity in the relationship between stocking rate and live weight gain/head. We conclude that adjusting stocking rates is required in this environment for maintenance of long-term sheep productivity and landscape function. The loss of palatable perennial C(3) grasses over much of the region has reduced livestock productivity and increased the variability in forage availability. Stock numbers should be adjusted at critical times to avoid losing perennial grasses that are important for sheep production in drier years. New grazing strategies are required to restore these grasses to the region.