Phenotypic traits related to methane emissions from Holstein dairy cows challenged by low or high forage proportion

Limited literature is available identifying phenotypical traits related to enteric methane (CH₄) production from dairy cows, despite its relevance in relation to breeding for animals with a low CH₄ yield (g/kg DMI) and the derived consequences hereof. This study aimed to investigate the relationships between CH₄ yield and different animal phenotypes when 16 second-parity dairy cows, fitted with a ruminal cannula, were fed 2 diets differing in forage/concentrate ratio in a crossover design. The diets had either a low forage proportion (35% on DM basis; F35) or a high forage proportion (63% on DM basis; F63). Gas exchange was measured by means of indirect calorimetry. Spot samples of feces were collected, and indigestible NDF (INDF) was used as an internal marker to determine total-tract digestibility. In addition, ruminal evacuations, monitoring of chewing activity, determination of ruminal VFA concentration, analysis of relative abundance of methanogens, and measurement of liquid passage rate were performed. Statistical differences were analyzed by a linear mixed model with diet, DIM, and period as fixed effects, and cow as random effect. The random cow estimates (RCE) were extracted from the model to get the Pearson correlations (r) between RCE of CH₄ yield and RCE of all other variables measured, in order to identify possible phenotypes related to CH₄ yield. Significant correlations were observed between RCE of CH₄ yield and RCE of OM digestibility (r = 0.63) and ruminal concentrations of valeric acid (r = -0.61), acetic acid (r = 0.54), ammonium (r = 0.55), and lactic acid (r = ‒0.53). Additionally, tendencies were observed for correlations between RCE of CH₄ yield and RCE of H₂ yield in g/kg DM (r = 0.47, P = 0.07), and ruminal isobutyric acid concentration (r = 0.43, P = 0.09). No correlations were observed between RCE of CH₄ yield and RCE of ruminal pool sizes, milk data, urinary measurements, or chewing activity. Cows had a lower DMI and ECM when they were fed F63 compared with F35. Cows fed F63 had higher NDF digestibility, CH₄ emissions (g/d, g/kg of DMI, and g/kg of ECM), ruminal concentration of acetic acid, ruminal pH, degradation rate of digestible NDF (DNDF, %/h), and longer rumen retention time (h). Rumination and total chewing time (min/kg DMI) were also higher for cows fed F63. The results in the present study emphasize the positive relation between a cow's ability to digest OM and their CH₄ emissions. The derived consequences of breeding for lower CH₄ emission might be cows with lower ability to digest OM, but more studies are warranted for further documentation of this relationship.