Optimising nitrogen fertilisation of kikuyu and kikuyu-ryegrass pastures

Viljoen, Charne (2018-12)

Thesis (MA)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Suiwelproduksie in die Suid-Kaap is meestal op aangeplante weidings onder besproeiing. Suiwer grasweidings [kikoejoe (Pennisetum clandestinum) en raaigrass (Lolium spp.)] word deur minimum-bewerkingsmetodes gevestig. Een van die belangrikste bestuurspraktyke van kikoejoe-raaigras weidings is stikstof-(N)-bemesting. Die huidige N-bemestingsriglyne beveel dikwels N-peile van meer as 500 kg N ha-1 jaar-1 aan, wat moontlik te veel is. Hierdie riglyne behoort weer ondersoek te word, aangesien dit op snyproewe en konvensionele bewerkingstelsels ontwikkel is. Die doel van hierdie studie was om 'n optimale N-peil vir kikoejoe- en kikoejoe-raaigrasweidings te bepaal, hetsy deur vaste N-bemestingspeile of 'n veranderlike peil volgens die behoefte van die weiding in 'n spesifieke seisoen. Ses N-bemestingsbehandelinge, een veranderlike peil (Nvar) en vyf vaste peile (0, 20, 40, 60 en 80 kg N ha-1 weisiklus-1) was in die huidige studie gebruik. Die behandeling Nvar, was op nitraatkonsentrasies van die grond gebaseer, wat deur benattingsfrontaanwysers bepaal is. Die nitraatkonsentrasies en die totale minerale N van die grond, het 'n groot poel potensiële loogbare N aangedui. In beide kikoejoe- en kikoejoe-raaigrasstelsels het N-toedienings bo 40 kg N ha-1 weisiklus-1, 'n opbou van totale minerale N aangedui. Daar was geen verskille tussen die laer N-behandelings (≤ N40) nie. Minerale N en urease-aktiwiteit was die enigste grondparameters wat deur N-behandeling beïnvloed is. Urease-aktiwiteit van die kontrole (geen N) was meestal hoër in vergelyking met die van 80 kg N ha-1 weisiklus-1. Totale grond N het seisoenale verskille tot gevolg gehad en dit word vermoed dat dit met seisoenale weidingsproduksie verband hou. Byvoorbeeld, gedurende periodes van hoë weidingproduksie was totale grond N laag, terwyl dit gestyg het gedurende periodes van lae weidingproduksie. Op beide kikoejoe- en kikoejoe-raaigrasstelsels was die hoogste weidingsproduksie gedurende lente en somer, terwyl die laagste totale grond N gedurende somer en herfs gevind is. Op die kikoejoe-weiding het N behandelinge 'n effek op die weidingsproduksie gedurende al die seisoene van jaar een, maar nie gedurende die tweede jaar gehad nie. Op die kikoejoe-raaigrasweiding het N-behandelinge die produksie gedurende winter, lente en somer van jaar een, en gedurende die somer van die tweede jaar beïnvloed. Op beide weidings, namate die N-behandelinge toegeneem het, was daar 'n afname in die klawerbydrae. Agronomiese N-doeltreffendheid was soortgelyk oor behandelings en seisoene op die kikoejoe- en kikoejoe-raaigrasweidings, met die uitsondering van winter in die eerste jaar op die kikoejoe-raaigrasweidings. Hierdie versterk die stelling dat die grond N versadig is. Die inhoud van ru-proteïen (CP) van albei weidings het toegeneem met 'n toename in N-behandeling tot ‘n punt waar CP te hoog geraak het vir melkproduksie in sommige behandelings. Die gevolgtrekking word daarom gemaak dat die huidige N riglyne heroorweeg moet word aangesien dit waarskynlik tot omgewings- en finansiële verliese sal lei.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/104964
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