PHOSPHORUS EFFICIENCY IN PASTURE SPECIES .6. A COMPARISON OF ITALIAN RYEGRASS, PHALARIS, RED-CLOVER AND WHITE CLOVER OVER TIME

Two problems in comparative studies of the P efficiency of pasture species are the choice of measurement to define P efficiency and the basis of comparison to use. Therefore, the degree to which the level of P supply and plant age, or time of harvest, influence a variety of measures of P efficiency was examined in four temperate pasture species grown in a soil at a range of P levels in a glass house. The four species were Italian ryegrass, phalaris, red clover, and white clover. Italian ryegrass was the most P efficient and red clover the least P efficient species in terms of the broad scale measures of P effiency, shoot yield per unit of P applied and total P uptake over the first 40 days after sowing (DAS), and shoot P concentration at 68 DAS. Nevertheless, by 68 DAS the shoot fresh weight yield of the four species was similar at both the low (5 kgP ha-1) and the high (80 kg P ha-1) P rate. The superior shoot biomass production at lower P rates over 40 DAS of Italian ryegrass and phalaris compared with red and white clovers was largely the result of a greater root weight and P uptake per plant. Early root weight was determined by the relationship between seed weight and root relative growth rate (PGR), whereas P uptake per plant was influenced by root weight and plant P demand as measured by relative P uptake rate (RPR). The understanding of such relationships was shown to be essential to the effective interpretation of broad scale measures of P efficiency. The comparative broad scale P efficiences of the four species changed over time. When Italian ryegrass and phalaris were compared on an ontogenetic basis, number of leaves per plant, shoot yield and P uptake per plant were similar. The comparison of the P efficiency of species on both an ontogenetic and temporal basis is proposed.