Development of methane conversion factor models for Zebu beef cattle fed low-quality crop residues and by-products in tropical regions.

Chatchai Kaewpila,Kritapon Sommart

doi:10.1002/ece3.2500

Abstract

The enteric methane conversion factor (Y m) is an important country‐specific value for the provision of precise enteric methane emissions inventory reports. The objectives of this meta‐analysis were to develop and evaluate the empirical Y m models for the national level and the farm level for tropical developing countries according to the IPCC's categorization. We used datasets derived from 18 in vivo feeding experiments from 1999 to 2015 of Zebu beef cattle breeds fed low‐quality crop residues and by‐products. We found that the observed Y m value was 8.2% gross energy (GE) intake (~120 g methane emission head−1 day−1) and ranged from 4.8% to 13.7% GE intake. The IPCC default model (tier 2, Y m = 6.5% ± 1.0% GE intake) underestimated the Y m values by up to 26.1% compared with its refinement of 8.4% ± 0.4% GE intake for the national‐level estimate. Both the IPCC default model and the refined model performed worse in predicting Y m trends at the farm level (root mean square prediction error [MSPE] = 15.1%–23.1%, concordance correlation coefficient [CCC] = 0.16–0.18, R 2 = .32). Seven of the extant Y m models based on a linear regression approach also showed inaccurately estimated Y m values (root MSPE = 16.2%–36.0%, CCC = 0.02–0.27, R 2 < .37). However, one of the developed models, which related to the complexity of the energy use efficiencies of the diet consumed to Y m, showed adequate accuracy at the farm level (root MSPE = 9.1%, CCC = 0.75, R 2 = .67). Our results thus suggest a new Y m model and future challenges for estimating Zebu beef cattle production in tropical developing countries.

Highlights

With an estimated 999 million tons of carbon dioxide equivalent per annum, methane emissions from the enteric fermentation of beef cattle are a major human-induced greenhouse gas emission (Opio et al, 2013)
Enteric methane emissions represent a loss in the range of 2%– 12% of the gross energy (GE) intake; that is, MJ methane energy loss per 100 MJ GE consumed by cattle, directly reducing the energy use efficiency of the diet consumed (Johnson & Johnson, 1995)
Enteric methane emissions for cattle globally are estimated from energy requirements using the enteric methane conversion factor (Ym, % of GE intake), according to the IPCC (2006) standard

Summary

| INTRODUCTION

With an estimated 999 million tons of carbon dioxide equivalent per annum, methane emissions from the enteric fermentation of beef cattle are a major human-induced greenhouse gas emission (Opio et al, 2013). Extant models for estimating enteric methane emissions can be classified into two principal groups: empirical (statistical) or dynamic mechanistic models (Kebreab, Johnson, Archibeque, Pape, & Wirth, 2008) In terms of the former, independent variables such as animal and diet as well as energy utilization efficiency have been selected to develop empirical Ym models (Blaxter & Clapperton, 1965; IPCC, 2006, Jaurena et al, 2015). As the diets fed to these Zebu beef cattle typically consist of poor- quality crop residues and by-products compared with those fed to B. taurus (Kearl, 1982; NRC, 2000, WTSR, 2010), extant Ym models may be inaccurate for the Zebu beef population Based on this gap in the body of knowledge on this topic, this meta-analysis aimed to develop new and evaluate existing regional diet-specific empirical Ym models for Zebu beef cattle production in tropical regions at the national level and the farm level from on-farm accessible data

| MATERIALS AND METHODS

Findings

| DISCUSSION