Abstract:
Predominant and adaptable grasses utilized for ruminant production in the rangelands of South Eastern Kenya are low in biomass yield and quality while their methane emission and carbon sequestration are not documented. These grasses include Eragrostis superba (E. superba), Cenchrus ciliaris (C. ciliaris), Panicum maximum (P. maximum), and Enteropogon macrostachyus (E. macrostachyus). Towards realizing improved ruminant productivity in the rangelands, some higher biomass-yielding grasses with better quality and low moisture demand (Brachiaria hybrid variety Brachiaria cayman, Brachiaria cobra and Chloris gayana varieties Boma rhodes and Extozi rhodes) have been introduced. However, this intervention is being implemented without the knowledge of their potential in mitigating greenhouse gas emissions in the rangeland ecosystems to support sustainable ruminant production. To resolve this knowledge gap, this study assessed the utilization of three indigenous and two introduced grass species to determine i) digestibility and methane emission using an in-vitro gas production technique in which rumen fluid was incubated for 72 hours and gas was sampled after every three (3) hours to determine methane concentration using gas chromatograph; ii) carbon sequestration using carbon stock method, and iii) enteric methane emission estimates from ruminant livestock grazing rangeland ecosystems using IPCC Tier I and Tier II approaches. Field and experimental data were analyzed with general linear models and the means were separated with Tukey’s HSD test. Relative to the indigenous grasses, the introduced grasses had higher crude protein content (74.05 g/Kg DM vs. 52.11 g/Kg DM), dry matter digestibility (34.13% vs 31.43%), organic matter digestibility (31.70% vs 29.27%) and NDF (712.7 g/Kg DM vs. 708.0 g/Kg DM). But they also had higher methane emission (25.61 ml vs 15.93 ml) and 24% lower in total carbon stock sequestration (9.2 tons C/ha vs 11.3 tons C/ha) and 23% lower in dry matter production (14.0 tons vs 17.3 tons /ha). The estimated methane emission and associated Global Warming Potential with Tier II were 4.4% higher than the estimates with the Tier I approach (Kg CH4/year of 9,279,526.80 vs 8,889,997; kg CO2eq of 259,826,750.4 vs. 248,919,916). Results suggest that the production of ruminants utilizing introduced grasses would achieve increased productivity with trade-offs on sustainability, at increased methane emission and low carbon sequestration. Use of IPCC Tier II marginally improved estimates of enteric methane emission and associated global warming potential for zebu cattle population grazing in the rangelands ecosystems of South Eastern Kenya.
