Enteric methane emission can be reliably measured by the GreenFeed monitoring unit

Ruminants contribute to global warming by releasing methane (CH₄) gas to the atmosphere. This has increased interest among animal scientists to develop and improve methods measuring CH₄ production in dairy cows. The GreenFeed emission monitoring unit (GEM) was introduced to estimate CH₄ production by measuring gas concentration and flux when cattle visit a GEM. The objective of the present study was to compare CH₄ production measured by the GEM with equations predicting CH₄ production. Evaluation was based on 83 treatment means from dairy (n = 65) and growing cattle (n = 18) studies, in which CH₄ production was measured by GEM. Methane production was predicted from intake and nutrient composition data with 18 empirical equations derived mainly from respiration chamber (RC) datasets. A comparison of observed and predicted values were performed for all equations using fixed and mixed regression models. The evaluation was based on root mean squared prediction error (RMSPE) expressed as a proportion of observed mean. All equations were precise in terms of high R² values (in most cases > 0.90), but there were considerable differences in RMSPE. Generally, the equations based on CH₄ yield and dry matter or gross energy intake resulted in the smallest RMSPE. When expressed as a proportion of observed mean, RMSPE for the 18 equations was 11.2%, and it ranged from 6.9 to 28.4%. Twelve equations had RMSPE less than 10% of observed mean. Ranking of the models remained rather similar when the relationships between predicted and measured CH₄ production was estimated using the mixed model regression analysis. Following the exclusion of 2 equations with large mean bias, RMSPE adjusted from random study effects was on average 6.2% of observed mean. Root MSPE were smaller than the corresponding errors in development of the equations, probably reflecting more standardized calibrations of the GEM system between laboratories compared with RC. In direct comparisons (n = 20) there was a good relationship in CH₄ production measured by RC and GEM (R² = 0.92). Root MSPE was 35.7 g/d (12.9% of the observed) with mean bias, slope bias and random error being 12, 0 and 88% of MSPE, respectively. Results from the current analysis indicated that CH₄ emissions measured by the GEM system agreed well with values predicted by empirical models derived from RC data suggesting indirectly that enteric CH₄ emission can be reliably measured by the GEM system.