The long-term influence of superphosphate and stocking rate on the production of spring-lambing Merino sheep in the high rainfall zone of southern Australia

The productivity of spring-lambing fine wool Merino sheep grazing pastures sown in 1977 to perennial ryegrass and subterranean clover was assessed from 1989 to 1998. The pastures were fertilised each autumn with single superphosphate at 6 levels, and were stocked at a low, medium, or high stocking rate (SR) at each level of fertiliser. The average phosphorus (P) applied annually since sowing ((P) over bar) ranged from 1.6 to 32.9 kg/ha. The SRs used varied with fertiliser level in that they were higher where more fertiliser had been applied, so that the highest SR at each level of fertiliser ensured that the pastures were well utilised. Each ewe raised 1 lamb, which was removed at weaning. The influence of fertiliser on the productivity of the sheep at 4 classes (1-4) of SR (mean SR = 7.1, 10.1, 12.6, and 18.2 ewes/ha for classes 1-4, respectively) was described by: y =A-BC(P) over bar, where y represents production per sheep (kg), and A, B, and C are constants. For greasy fleece weight, estimates of B and C were 1.59 and 0.84; and for SR classes 1-4, the estimates of A were 5.06, 4.89, 4.78, and 4.46, respectively. For weaning weight of lambs, estimates of B and C were 8.4 and 0.82, and estimates of A were 23.5, 22.7, 21.5, and 20.9 for SR classes 1-4. The mean fibre diameter (mu m) of the wool was described by: (D) over bar = 14.18+1.48 <(GW)over bar>, where <(GW)over bar> is the mean greasy wool produced annually per sheep (kg) averaged over all sheep and years for each of the 18 treatments. The price (cents/kg) of wool with a fibre diameter D (P-D) was given by: P-D = 12197+4.94P(2) + 688D - 0.1945P(20)D - 5810 root D, where 20 mu m wool is P-20 cents/kg. Supplements were fed if the body condition of ewes fell to a predetermined level. The supplement fed per ewe each year (S), expressed as metabolisable energy (in MJ) was described by: S = - 602 - 44.1S - R - + 178.5P - + 8.71S P+539 root<(SR)over bar>-338.5 root P- 70.8P root SR, where <(SR)over bar> - and P - represent the mean stocking rate (ewes/ha) and mean P applied annually. When a current set of costs and prices was applied to these equations, the maximum gross margin for a SR of 7.1 ewes/ha was $AU119/ha with 8.6 kg P/ha applied annually, and $AU262/ha for SR of 18.2 ewes/ha with 17.6 kg P/ha applied annually. If income derived from sheep is maintained constant, intensifying the sheep enterprise from the low to the high SR system would involve increasing sheep numbers by about 17%, but would release about 55% of the farm's area for another purpose.