Strategic nitrogen fertilisation of kikuyu-based pastures over-sown with legumes

[摘要] ENGLISH ABSTRACT: Kikuyu (Pennisetum clandestinum)-ryegrass (Lolium spp.) pastures for dairy production in thesouthern Cape of South Africa often receive nitrogen (N) fertilisation rates in excess of 500 kg ha-1yr-1. To reduce the high N fertilisation inputs of the region, legumes have been incorporated intothese grass pasture systems. Legumes fix atmospheric N and can improve nutritive value of thepasture. However, research concerning N fertilisation guidelines of grass-legume pastures in thesouthern Cape is limited. As result of this limited research, producers may be applying varying Nrates to grass-legume pastures, which may result in these systems being environmentally andeconomically unsustainable. The aim of this study was to determine the effect of N fertilisationregimes on soil and pasture parameters of a kikuyu-perennial ryegrass (L. perenne)-clover(Trifolium spp.) (KPrCl) and a kikuyu-lucerne (Medicago sativa) (KL) pastures under grazing andirrigation. Five N fertilisation regimes were used as treatments and were applied after each grazingevent [0 (N0), 20 (N20), 40 (N40), 60 kg N ha-1 (N60) and a variable N application (Nvar)]. Nitrateconcentration measured from wetting front detectors as well the distribution of total soil N betweenthe depths 0-100, 100-200 and 200-300 mm indicated that a significant amount of leaching mayhave occurred throughout the study. However, leaching was constant for all treatments on bothpasture types. Furthermore N treatments had an minimal influence on total soil N, potentialmineralisable N and urease activity. The only soil parameter that showed a significant reaction to Nfertilisation regimes was total inorganic soil N. For both pasture types, total inorganic soil Nincreased under the N60 fertilisation regime. Seasonal herbage yield for the KPrCl type was onlyaffected by N treatment in the spring (year one), where N60 produced a higher yield than N0 and Nvar. Treatment had no effect on total annual yield of KPrCl pastures. For the KL type, treatmentN60 produced the highest or similar to the highest yield in the spring, summer and autumn. Theannual yield under treatment N20, N40 and N60 were higher than N0 and Nvar. Botanicalcomposition for both pasture types were affected differently by N treatments. For the KPrCl type,kikuyu was positively related to N fertilisation rates in autumn, whilst ryegrass was positivelyrelated to N rates in the winter and spring. For most seasons the clover component respondednegatively to N fertilisation. For the KL pasture, kikuyu was not affected by N treatments. Lucerneonly responded to N fertilisation in the summer, during which N40 had the highest lucernecomponent. The crude protein content of the KPrCl pasture type was highest or similar to thehighest under treatment N60 for all months, whilst crude protein for the KL type was highest underN60 from February to June.

[发布机构] Stellenbosch University