Responses of Italian ryegrass to phosphorus on highly‐weathered soils

Abstract

Italian ryegrass (Loliwn 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.

Italiaanse raaigras (Loliwn multiflorum cv. Midmar) se reaksie op toegediende P is met besproeide perseelproewe op ferrallitiese gronde by twee lokaliteite in Natal bestudeer. Elk van die proewe het oor twee seisoene (1985 en 1986) gestrek met die weiding hervestig na die eerste seisoen. Groot droemateriaalreaksies op P is op beide gronde gemeet. Fosfor effekte was die grootste gedurende die vestigingsfase (herfs) en, op die kouer rigging, in die lente. Konsentra‐sies van P in die blaarmassa was baie laer in die winter as in die herfs of somer. Die groter reaksievermoë op P in die lente en die laer P konsentrasies in die blaarmassa gedurende die kouer maande dui daarop dat beperkte P opname bydraend kan wees tot die laer opbrengste in die winter. Die verwantskap tussen raaigras P konsentrasies en opbrengs toon aansienlike binne‐seisoen variasies in interne P behoefte vir maksimum opbrengs. Dit verlaag die nuttigheid van roetine P blaarontledings om raaigras P toereikendheid aan te slaan. Grondontledings met ‘n alkaliese ekstraheermiddel (NH₄HCO₃/EDTA/NH₄F by pH 8.0) toon egter belofte as voorspeller van P reaksievermoe. Konsentrasies van P in Italiaanse raaigras was ver onder die behoeftes wat in die literatuur vir meeste klasse weidende herkouers rapporteer word, veral gedurende die kouer maande.

Additional index words:

• Herbage phosphorus
• seasonal dry matter responses
• soil test calibrations