Effects of superphosphate fertiliser and stocking rate on the nutritive value of perennial ryegrass and subterranean clover herbage

The effect of superphosphate fertiliser on digestible dry matter (DDM) and crude protein (CP) percentages of perennial ryegrass and subterranean clove r was assessed on a long-term, grazed experiment in western Victoria. CP of both species increased significantly (3-6 units) where long-term average p hosphorus (P) applications were 33 kg/ha.year compared with 1 kg/ha.year. C P of herbage was also greater on paddocks grazed at higher (9-23 ewes/ha) t han lower (5-13 ewes/ha) stocking rates (1-3 units). DDM of subterranean cl over significantly increased (8-10 units) with higher superphosphate applic ations but there was little effect on the DDM of perennial ryegrass. Long-t erm applications of superphosphate were associated with larger increases in both DDM and CP than if the same rate was applied in autumn of the year of measurement. For subterranean clover, 30 kg/ha of P applied to infertile s oils (Olsen P 5 mg/kg) in autumn increased the DDM of herbage collected in September by only 4-6 units. The nutritive value of most volunteer pasture species also increased with higher superphosphate applications but the DDM of these species was usually inferior to perennial ryegrass. The results hi ghlight the importance of monitoring nutritive value, pasture composition, and herbage production to quantify fully the likely effect of fertiliser ap plications on animal production.