Estimation of tier 2 enteric methane emission factors for cattle in past oral and agro-pastoral production systems in Uganda

Abstract

Three inter-related studies (Papers 1, 2 & 3) were conducted to develop enteric methane (CH4) emission factors (EFs) for pastoral and agro-pastoral cattle in Uganda using the Intergovernmental Panel on Climate Change (IPCC) Tier 2 EF model based on gross energy (GE) intake. For Paper 1, 16 sites of Uganda’s pastoral and agro-pastoral natural pasturelands were sampled during the dry and rainy seasons, and herbage examined for yield, as well as chemical composition and energy content. For Paper 2, herbage dry matter (DM) intake of cattle in the two production systems was estimated using internal and external markers, and subsequently enteric CH4 EFs were developed using the IPCC Tier 2 EF model based on GE intake. Finally, for Paper 3, empirical equations for prediction of pasture herbage DM intake of cattle were developed based on live body weight (BW), diet chemical composition, and animal-diet interaction data of Papers 1 & 2. The results showed no effects (P > 0.05) of production system, season or their interaction on herbage DM yield (averaging 1.78 ton DM/ha. No effects (P > 0.05) of production system or its interaction with season on chemical and energy content existed, except for CP composition, which was higher (P < 0.05) in agro-pastoral than in pastoral in the rainy season (8.5 versus 6.1% DM) but not in the dry season (4.0 versus 3.8% DM). The estimated average DM intakes for mature females, mature males, heifers, young bulls and calves in the two production systems ranged from 1.0 to 6.67 kg/head/day. As a % of BW, average DM intake was 42.3% greater in the rainy than in the dry season (1.69 versus 1.18% BW) for “other” cattle in the pastoral system. In the agro-pastoral system, DM intake was 24.6 and 117.0% greater in the rainy than in the dry season for “dairy” (1.52 versus 1.22% BW) and “other” (2.30 versus 1.06% BW) cattle, respectively. Regarding Tier 2 EFs, the averages derived from estimated GE intake ranged from 10.5 to 30.7 kg CH4/head/year for the different “other” cattle sub-categories in pastoral and agro-pastoral systems, as well as 9.6 to 43.0 kg CH4/head/year for “dairy” cattle sub-categories in the agro-pastoral system. The weighted average EFs (population-weighted mean based on the average % contribution of each each sub-category to the herd) for “other” cattle in pastoral (19.1 kg CH4/head/year) and agro-pastoral (19.7 kg CH4/head/year) systems were lower than that for “dairy” cattle in the agro-pastoral system (25.0 kg CH4/head/year). The percentage differences between the weighted average EFs and the IPCC Tier 1 default values proposed for low productivity systems in Africa were 47.5% (19.1 vs. 31 kg CH4/head/year for “other” cattle under pastoral), 44.6% (19.7 vs. 31 kg CH4/head/year for “other” cattle under agro-pastoral), and 59.2% (25.0 vs. 46 kg CH4/head/year for “dairy” cattle under agro-pastoral). Finally, seven linear empirical regression equations were developed for prediction of herbage DM intake from the variables of CP, Fat, Ash, DM digestibility (DMD), digestible energy (DE), and DE as a percentage of GE (DE%), with BW as the primary predictor. However, only the equation: DM intake (kg/head/day) = 0.0073 × BW (kg) + 0.093 × CP (%) + 0.22 × DE (%) – 0.078 × Ash (%) + 0.32 × Fat (%) – 0.48 (R2 = 0.71) had an acceptable prediction accuracy (with a relative prediction error (RPE) value of 23.3%). Using BW as the sole predictor: DM intake (kg/head/day) = 0.0076 × BW (kg) + 0.80 (R2 = 0.56) values gave the least satisfactory prediction (RPE = 28.7%). The use of metabolic body size (BW0.75) did not improve the prediction accuracy neither did quadratic polynomial regression. In conclusion, the Tier 2 enteric CH4 EFs estimated in this study for all sub-categories of the “other” cattle type in the pastoral production system are comparable to those in the agro-pastoral system, but not with the “dairy” cattle type in the agro-pastoral system. When compared with the Tier 2 enteric CH4 EFs estimated in this study, the 2006 IPCC Tier 1 default enteric CH4 EFs overestimate Uganda’s enteric CH4 emissions from cattle in pastoral and agro-pastoral production systems by over 44%.