Methane emissions from dairy cows measured using the sulfur hexafluoride (SF6) tracer and chamber techniques.

Our study compared methane (CH4) emissions from lactating dairy cows measured using the sulfur hexafluoride (SF6) tracer and open-circuit respiration chamber techniques. The study was conducted using 16 lactating Holstein-Friesian cows. In each chamber, the cow was fitted with the SF6 tracer apparatus to measure total CH4 emissions, including emissions from the rectum. Fresh ryegrass pasture was harvested daily and fed ad libitum to each cow with a supplement of 5 kg of grain/d. The CH4 emissions measured using the SF6 tracer technique were similar to those using the chamber technique: 331 vs. 322 g of CH4/d per cow. The accuracy of the SF6 tracer technique was indicated by considering the ratio of the CH4 emission measured using the SF6 tracer to the emission measured using the chamber for each cow on each day. The calculated ratio of 102.3% (SE = 1.51) was not different from 100%. A higher variability within cow between days was found for the SF6 tracer technique [coefficient of variation (CV) = 6.1%] than for the chamber technique (CV = 4.3%). The variability among cows was substantially higher than within cows, and was higher for the SF6 technique (CV = 19.6%) than for the chamber technique (CV = 17.8%). Our CH4 emission data were compared with whole-animal chamber studies conducted in Canada and Ireland. In the Canadian study the SF6 technique did not measure CH4 emissions from the rectum and emissions were 8% lower than those measured using the chamber, indicating that emissions from the rectum may be greater than previously measured (1%). The relationship between CH4 emission and dry matter intake was examined for our data and for that reported in the Canadian study. There was a difference in the slopes of the regressions derived from our data and that from Canada; 17.1 vs. 20.8 g of CH4/kg of dry matter intake. A difference between the 2 locations was expected based on the difference in diet composition for these 2 studies. The SF6 tracer technique is reasonably accurate for inventory purposes and for evaluating the effects of mitigation strategies on CH4 emissions.