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 fro m 1989 to 1998. The pastures were fertilised each autumn with single superp hosphate at 6 levels, and were stocked at a low, medium, or high stocking r ate (SR) at each level of fertiliser. The average phosphorus (P) applied an nually since sowing ((P) over bar) ranged from 1.6 to 32.9 kg/ha. The SRs u sed varied with fertiliser level in that they were higher where more fertil iser had been applied, so that the highest SR at each level of fertiliser e nsured 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 ewe s/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 fo r SR classes 1-4, the estimates of A were 5.06, 4.89, 4.78, and 4.46, respe ctively. For weaning weight of lambs, estimates of B and C were 8.4 and 0.8 2, and estimates of A were 23.5, 22.7, 21.5, and 20.9 for SR classes 1-4. T he 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 pro duced 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 cond ition of ewes fell to a predetermined level. The supplement fed per ewe eac h 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 stocki ng 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 a pplied annually. If income derived from sheep is maintained constant, inten sifying the sheep enterprise from the low to the high SR system would invol ve increasing sheep numbers by about 17%, but would release about 55% of th e farm's area for another purpose.