EFFECTS OF PHOSPHORUS-FERTILIZER AND RATE OF STOCKING ON THE SEASONALPASTURE PRODUCTION OF PERENNIAL RYEGRASS-SUBTERRANEAN CLOVER PASTURE

The response of pastures based on Lolium perenne L. and Trifolium subt erraneum L. to single superphosphate was assessed at Hamilton, Victori a, by measuring the growth of pastures during winter, spring, and summ er over 7 years from 1979 to 1987. The seasons were defined by the pat tern of pasture production, rather than by calendar months. Winter was the period of constant growth rate following the autumn rain; spring was the period of accelerating growth rate until growth rate changed a bruptly following the onset of dry summer weather. Pastures were graze d with sheep at a low, medium, or high grazing pressure, corresponding generally to stocking rates of 10, 14, or 18 sheep/ha. At each level of grazing pressure, single superphosphate was applied at 5 rates from 1979 to 1982; the highest rate, expressed as elemental phosphorus (P) , was reduced from 100 to 40 kg/ha during this time. In addition there was an unfertilised treatment. In 1984, fertiliser was applied at 6 r ates from 4 to 40 kg P/ha. No fertiliser was applied in the remaining years, including 1983. Pasture production was measured from 1979 to 19 82 and from 1985 to 1987. Total pasture dry matter (DM) accumulation p er year at the highest stocking rate was less than the other treatment s in 4 of the years. Averaged over all years and fertiliser treatments , the annual net production was 10.1, 10.1, and 9.0 t DM/ha (P < 0.05) for plots grazed at low, medium, and high stocking rates, respectivel y. The amount of fertiliser required to reach a given proportion of ma ximum yield response did not vary between winter and spring in any yea r, but the greater potential yield in spring (P < 0.001) meant that as more fertiliser was applied, the disparity between pasture grown in w inter and pasture grown in spring increased. Differences in this dispa rity between extreme levels of P ranged from 1.4 t DM/ha in a drought to about 7 t DM/ha in a good season.The implications for managing farm s when pastures are fertilised at higher rates than currently practise d by district farmers are that systems of animal production with a req uirement for plentiful good quality pasture in spring, such as ewes la mbing in spring, should be used. The benefit of spring lambing over au tumn lambing was supported when the 2 systems were compared over 26 ye ars using the GrassGro decision support system. Well fertilised pastur e systems will also allow more scope for conserving pasture as hay or silage, and increase opportunities for diversification in the farming enterprise, such as spring-growing crops.