Comparison of active flux and passive concentration measurements of methane emissions from cattle

Abstract Text:

There are two new measurement techniques to measure emitted methane (CH4) and carbon dioxide (CO2) from cattle in production systems, the passive concentration measurement method (PCM) and the active gas capture method (AGC). Both systems estimate cattle muzzle CH₄ and CO₂emissions for short term periods (3-15 min) while cattle visit a feeding station multiple times daily.   The objective was to determine if the two techniques yielded comparable results under farm conditions.

A GreenFeed (GF) system was used (C-Lock Inc, Rapid City, SD) that measures individual animal emissions over a feed trough.   For AGC, an active airflow (2,000 l/min) was induced around the animal’s muzzle that attracted emissions into a air collection pipe where airflow and CH₄ and CO₂ concentrations were measured and the average flux was calculated for each visit.  For PCM, a concentration sampling intake (at 1 l/min) was placed inside the feed trough, no active airflow was used, and the average CH₄ and CO₂concentrations for each visit were calculated. 32 Swedish Red dairy cows (BW 664±72 kg, MY 30.2±6.3 kg/d, and DMI 20.1±2.8 kg/d) housed in a free-stall barn had an  access to  two separate GF units.  The diets were fed ad libitum as TMR (60% forages, 40% concentrates on DM basis). The GF were configured for 10 day sampling periods using PCM and AGC repeated twice.  The data was analyzed with linear mixed models using the MIXED procedure in SAS. Repeatability (R) was calculated as R = δ²_Animal / (δ²_Ánimal + δ²_Residual).

The cows visited GF on average 2.85 ± 0.95 times per day.  For CH₄, the between animal coefficient of variation (CV) was greater (11.0 vs. 17.6%) with PMC compared to AGC.  Comparing CH₄ results for individual animals to determine if ranking was consistent between AGC and PCM, a weak correlation was found between CH₄ concentration with PCM and CH₄ flux with AGC:  CH₄ Flux (g/d) = 363 ± 30.5 + 0.058 ± 0.0214 × CH₄ (ppm) (R² = 0.13; RMSE = 52.1). For CH₄/CO₂ ratio, CV values were similar (6.4 and 6.6%) but averaged CH₄/CO₂ ratio was greater (P = 0.001) with PMC (0.107) compared with AGC (0.094).  The repeatability for AGC and PCM were high (0.72- 0.74). 

It is concluded that PCM methods are not sufficient for ranking animal’s emissions on farms.  Measuring concentration passively is not the same as measuring fluxes.

Keywords: Methane, cattle, emissions