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Since heritability of CH 4 emissions in ruminants was demonstrated, various attempts to generate large individual animal CH 4 data sets have been initiated. Predicting individual CH 4 emissions based on equations using milk mid-infrared (MIR) spectra is currently considered promising as a low-cost proxy. However, the CH 4 emission predicted by MIR in individuals still has to be confirmed by measurements. In addition, it remains unclear how low CH 4 emitting cows differ in intake, digestion, and efficiency from high CH 4 emitters. In the current study, putatively low and putatively high CH 4 emitting Brown Swiss cows were selected from the entire Swiss herdbook population (176,611 cows), using an MIR-based prediction equation. Eventually, 15 low and 15 high CH 4 emitters from 29 different farms were chosen for a respiration chamber (RC) experiment in which all cows were fed the same forage-based diet. Several traits related to intake, digestion, and efficiency were quantified over 8 d, and CH 4 emission was measured in 4 open circuit RC. Daily CH 4 emissions were also estimated using data from 2 laser CH 4 detectors (LMD). The MIR-predicted CH 4 production (g/d) was quite constant in low and high emission categories, in individuals across sites (home farm, experimental station), and within equations (first available and refined versions). The variation of the MIR-predicted values was substantially lower using the refined equation. However, the predicted low and high emitting cows (n = 28) did not differ on average in daily CH 4 emissions measured either with RC or estimated using LMD, and no correlation was found between CH 4 predictions (MIR) and CH 4 emissions measured in RC. When individuals were recategorized based on CH 4 yield measured in RC, differences between categories of 10 low and 10 high CH 4 emitters were about 20%. Low CH 4 emitting cows had a higher feed intake, milk yield, and residual feed intake, but they differed only weakly in eating pattern and digesta mean retention times. Low CH 4 emitters were characterized by lower acetate and higher propionate proportions of total ruminal volatile fatty acids. We concluded that the current MIR-based CH 4 predictions are not accurate enough to be implemented in breeding programs for cows fed forage-based diets. In addition, low CH 4 emitting cows have to be characterized in more detail using mechanistic studies to clarify in more detail the properties that explain the functional differences found in comparison with other cows. [ABSTRACT FROM AUTHOR]

• Acacia mearnsii mitigates methane from the first hour of feeding. • Acacia mearnsii modifies milk fatty acids from the first day of feeding. • Acacia mearnsii is effective in low and high methane emitting cows. • Distinct milk fatty acids were related with methane emission. The effects of dietary supplements for modifying cattle digestion and metabolism are typically measured after one or more weeks of adaptation. Consequently, how quickly the effects occur remains unknown. The long-term efficacy of Acacia mearnssii bark tannins (Acacia) on methane mitigation has been previously demonstrated. The present study, therefore, investigated the time it took for the extract to affect methane emissions and milk fatty acid profiles. Twenty lactating Brown Swiss dairy cows, categorized as 10 low- and 10 high methane emitters (average difference in methane yield: 0.10 of total), were housed in respiration chambers for 4 days. A control diet consisting of a mixed ration supplemented with grass pellets and concentrate pellets was fed initially (Day 0). The original pellets were then replaced with pellets containing 141 g Acacia/kg, providing 30 g Acacia/kg of dietary dry matter (DM) (Days 1–3). Methane emissions were measured every 10 min and gas chromatography was used to analyze individual fatty acids from daily milk samples. A significant decline in methane production was detected 20 min after starting supplementation, with methane production (g/day) and methane yield (g/kg DM intake) decreasing in a linear fashion from Day 0 to Day 3 by up to proportionately 0.18 and 0.16, respectively. Additionally, changes in proportions of various milk fatty acids occurred within 3 days of Acacia feeding. Using stepwise multiple regression analysis, several milk fatty acids were identified as being related to methane emissions. Applicable equations from the literature also showed relationships to methane emissions of high and low emitters as well as to the Acacia diet effect on methane emissions. The equations with close relationships all included minor and non-dietary milk fatty acids like odd-chain fatty acids. These specific fatty acids originate from rumen microbial activity. In conclusion, A. mearnsii extract is an immediately acting methane mitigating supplement. Certain milk fatty acids are related to methane emission in dairy cows and may assist in determining whether and when tanniferous supplements will act against enteric methane formation. [ABSTRACT FROM AUTHOR]