Optimising nitrogen fertilisation of kikuyu and kikuyu-ryegrass pastures

[摘要] ENGLISH ABSTRACT: Dairy production in the southern Cape is mostly based on irrigated planted pastures. Pure grass pastures [kikuyu (Pennisetum clandestinum) and ryegrass (Lolium spp.)] are often established by using minimum-tillage methods. One of the most important management practices of kikuyu-ryegrass pastures is nitrogen (N) fertilisation. The current N fertilisation guidelines often recommend more than 500 kg N ha-1 yr-1, which is possibly too high. The guidelines need to be re-evaluated, since it was developed under cutting conditions using conventional-tillage, and may not be accurate for the minimum-tilled and grazed systems. The aim of this study was to determine an optimum rate of N application of kikuyu and kikuyu-ryegrass pastures, either by a fixed N fertilisation rate or a variable rate according to the demand of the plant in a specific season. Six N fertilisation treatments, one variable rate (Nvar) and five fixed rates (0, 20, 40, 60 and 80 kg N ha-1 grazing cycle-1) were used in the current study. Treatment Nvar was based on the soil water nitrate concentration obtained from using wetting front detectors (WFD). The nitrate concentrations and total soil mineral N indicated that a major pool of N is vulnerable to potential leaching losses. In both kikuyu and kikuyu-ryegrass systems, applications above 40 kg N ha-1 grazing cycle-1 indicated a build-up of total mineral N in soil. No difference between the lower N treatments (≤ 40 kg N ha-1) was found in terms of total mineral N. Mineral N and urease enzyme activity were the only soil parameters that were affected by treatments. Urease activity of the control treatment (no N) was mostly higher (P≤0.05) compared to the 80 kg N ha-1 treatment. Total soil N resulted in seasonal differences and was considered to be related to variation in seasonal herbage production. For example, during periods of high pasture production total soil N in soil was low, but increased during periods of low pasture production. For both kikuyu and kikuyu-ryegrass pastures, the highest herbage production was during spring and summer, while the lowest total soil N was found during summer and autumn. On the kikuyu site, N treatments had an effect on the herbage production during all the seasons of year one, but not during year two. On the kikuyu-ryegrass site, N treatment affected the production during winter, spring and summer of year one, and during the summer of year two. As N treatments increased on both the study sites, the self-sown clover component decreased. Agronomic N use efficiency was similar across treatments and seasons on the kikuyu and kikuyu-ryegrass site, with the exception of winter in the first year in the kikuyu-ryegrass site. This supports the notion that the soil is saturated with N. Crude protein (CP) content of herbage increased with an increase in N, to a point where CP was too high for milk production for some treatments. It is concluded that the current N guidelines needs to be revisited as they pose a risk to the environment and farm economics.

[发布机构] Stellenbosch University