Comparison of open-circuit respiration chambers with a micrometeorological method for determining methane emissions from beef cattle grazing a tropical pasture

Abstract

Emissions of CH4 methane from beef cattle grazing pastures characteristic of northern Australia are yet to be reliably quantified. Poor quality pastures and low animal productivity are typically associated with high CH4 emissions/unit animal product. Our study reports two independent methodologies to quantify CH4 emissions from beef cattle at herd scale and from individual animals. The first method identifies each animal within a herd (n=18) as a point source of CH4. Emissions at the herd scale were estimated with a backward Lagrangian stochastic (bLs) dispersion model. This model used atmospheric turbulence data and line-averaged concentrations of CH4 measured with an open-path laser and inputs (ppm) from 5 paths across 1ha Rhodes grass (Chloris gayana) dominated pastures that were flat and free of obstructions. The second method used open circuit respiration chambers to measure 24h CH4 emissions from 18 steers fed freshly cut forage from the same pasture. The CH4 emissions determined using the dispersion method were 136.1±21.5g/d or 29.7±3.70g/kg dry matter (DM) intake and 114±5.1g/d or 30.1±2.19g/kg DM intake using open circuit respiration chambers. Estimates of CH4 emissions based on predictive algorithms for beef cattle grazing similar pastures range from 82.7±3.98g/d to 112.7±2.57g/d. Additional studies with different tropical pastures and herds are required to obtain estimated CH4 emission values for further comparisons with predictions from the Australian Greenhouse Gas Inventory and those currently reported in the literature for beef cattle.This paper is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson.