Responses of Italian ryegrass to phosphorus in highly-weathered soils.

Abstract

Italian ryegrass (Lolium multiflorum cv. Midmar) responses to fertilizer P were studied in field experiments under irrigation on highly-weathered ferrallitic soils at two localities in Natal. Each experiment was conducted over two seasons (1985 and 1986), the pasture being re-established after the first season. Large dry matter yield responses to P were measured on both soils. Phosphorus effects were greatest during the establishment phase (autumn) and, on the colder site, in spring. Concentrations of P in herbage were much lower in winter than in autumn or summer. The greater responsiveness to P in spring and the lower P concentrations in herbage during the colder months suggested that restricted P uptake contributed to yield reductions in winter. Relationships between ryegrass P concentrations and yield indicated substantial within-season variations in internal P requirement for maximum yield. This detracts from the usefulness of routine herbage P analyses for assessing ryegrass P sufficiency. However, soil analysis using an alkaline extractant (NH4HCO3/EDTA/NH4F at pH 8.0), showed promise for predicting responsiveness to P. Concentrations of P in Italian ryegrass were well below the requirements reported in the literature for most classes of grazing ruminants, particularly during the colder months.

Keywords: dry matter yield; fertilizer; grazing; herbage phosphorus; italian ryegrass; lolium multiflorum; natal; pasture; ph; phosphorus; seasonal dry matter responses; soil analysis; soil test calibrations; south africa; yield