Feed intake of sheep grazing Astrebla pastures of different condition

Abstract

Proc. Amt. Sot. Anim. Prod. 1994 Vol. 20 FEED INTAKE OF SHEEP GRAZING ASTREBLA PASTURES OF DIFFERENT CONDITION D.H. COBONA, J.V. BAILEYB and S.L. PEART B AQld. Dept of Primary Industries, PO Box 5 19, Long-each, Qld. 4730 BQld. Dept of Primary Industries, Toorak, Julia Creek, Qld. 4823 Vast areas (43%) of Mitchell grass (Astrebla spp) in Australia have been degraded (Tothill and Gillies 1992). Set stocking of these areas in western Queensland has contributed to a loss of desirable species and reduced productivity of sheep. Astrebla is the indicator of pasture sustainability. The climate is characterised by predominantly summer rainfall which produces a short growing season. The high variability of mean annual rainfall (coefficient of variation, 49%) contributes to large interseasonal changes in forage production. An alternative to set stocking is setting stock numbers based on the quantity of available biomass present after the growing season. Stock numbers in paddocks can be further adjusted to allow for the condition of pasture and the quantities grazing animals consume. This strategy would reduce overgrazing of Astrebla pastures and improve their sustainability. The aim of this research was to determine the intake of sheep grazing pastures in different condition. S, S, Experiments at Rosebank (RB; 23' 144'E) and Toorak (TK; 21' 142'E) were conducted in November 1991 (dry stem; end of season pasture) and June 1993 (dry leaf, green stem grasses and forbs; mid season pasture) respectively. Relative abundance (RA) of Astrebla was used to determine condition of pastures (~35% RA, low condition (L); 35-55% RA, medium condition (M)). Peppin Merino wethers (RB 7 months old, 30 kg liveweight (LW); TK 3 years old, 43 kg LW) were randomly allocated to pastures in different condition (RB, 1 L, 2M x 2 replicates of each, 5 sheep/replicate; TK, lL, 1M x 3 replicates of each, 4 sheep/replicate). Dry matter (DM) yield and botanical composition, RA of species, irz vitro DM digestibility of plucked pasture samples and intake (chromium sesquioxide control release,see Costigan and Ellis 1987) were determined during a 10 day sampling period which was preceded by a 14 day lead up period. Faecal samples were taken at 0900 and 1700 hours daily. Table 1. Pasture condition (L-low, M-medium), relative abundance (RA) of Astrebla (%), DM yield (kg/ha), in vitro DM digestibility (IVDMD) (%) and intake (g/kg liveweight, LW) by wethers grazing end of (RB) and mid season (TK) mitchell grass pastures Wethers grazing dry, end of season pastures at RB with a low RA of Astrebla (21%, low condition) had reduced intake compared to those grazing paddocks with a higher RA (40-45%, medium condition). The intake of wethers grazing mid season pastures of medium condition at TK however, was lower than the intake of low condition pastures. This may have been due to a higher level of annual grasses and forbs in the low compared to the medium condition pastures (Iseilema, 215 vs 120 kg/ha; Ipomoea, 10 vs 0 kg/ha). COSTIGAN, P. and ELLIS, K.J. (1987). N-2. J. Tech. 3: 89-92. TOTHILL, J.C. and GILLIES, C. (1992). Tropical Grassland Society of Australia, Occasional Publication No. 5 (Cranbrook Press: Toowoomba). 399