Discipline: grazing/forages; Key words: kikuyu-ryegrass pasture, minimum tillage, nitrogen fertilization.
The sustainability of current nitrogen (N) fertilization regimes for minimum tillage kikuyu-ryegrass (Pennisetum clandestinum, Lolium multiflorum) pastures in the southern Cape is questioned. The guidelines, originally, were developed for conventional tillage and non-grazing systems. It may well be that these guidelines may not apply any more. In fact, there is the possibility that the N levels applied are too high with implications to both economics and leaching of unutilised N. Thus, the aim of the investigation of the researchers referenced below was to study the effects of low to high N fertilization rates on soil and pasture characteristics.
Nitrogen treatments were applied to a kikuyu-annual ryegrass trial site at the Outeniqua Research Farm for two years. Nitrogen fertilizer was applied after each cow grazing cycle of 28 to 35 days at five fixed rates namely 0, 20, 40, 60 and 80 kg N per hectare (N0, N20, N40, N60 and N80). Prior to each grazing cycle, soil and pasture production characteristics were determined, while botanical composition was determined seasonally.
The response of the total mineral N in the soil to N treatments varied (p <0.05) between grazing cycles. The total mineral N in the 0–100 mm soil depth increased (p <0.05) over time in high N treatments (N60 and N80) compared to N0, where it remained relatively constant throughout the trial period. This was also evident for the 100–200 mm and 200–300 mm depths. These findings indicate a build-up of N in soil to a point beyond what can be utilized by pasture, and an increased risk of leaching when N is applied at more than 60 kg per hectare per grazing cycle. The pasture production response to N fertilization was similar for all grazing cycles. Treatments N60 and N80 had, on average, a greater (p <0.05) pasture production compared to the N0 treatment; however, this was not the case during all seasons. Pasture production was most negatively affected (p <0.05) during winter and autumn season. The ryegrass component was greatest (p <0.05) during winter and spring. All N-containing treatments had a greater (p <0.05) ryegrass component than the N0 treatment. Season and treatment affected (p < 0.05) the pasture crude protein (CP) content, where high N treatments were greater (p < 0.05) compared to low N treatments.
The results suggest that N fertilization rates should be adjusted according to season. Furthermore, in order to prevent N losses through leaching, while maintaining a high pasture production and quality, it is advisable to apply no more than N40 after each grazing during winter and spring. Interestingly, applying N20 after each grazing during summer and autumn will probably result in a similar pasture production as when applying N80. In addition, N20 should ensure more acceptable pasture CP content for grazing dairy cows and induce a mineral N content in the soil that is less likely to result in N leaching. Lower N rates during summer might be due to the kikuyu component, which was greater (p < 0.05) during summer and autumn, regardless of the N application. This may indicate sufficient soil N for pasture growth and is supported by a volunteer legumes component that was present in low N treatments, particularly in spring and summer. This, together with a potential to mineralise N (15-70 kg per hectare per grazing cycle) could have contributed to adequate N for pasture growth during summer.
C. Viljoen, J. Van Der Colf & P. Swanepoel, 2018.A NEW NITROGEN FERTILISATION REGIME FOR MINIMUM-TILLAGE KIKUYU-RYEGRASS PASTURE IN THE SOUTHERN CAPE. In: Proc of the 53rdGSSA Annual Congress, Roodeplaat, Pretoria, 22-27 July 2018.