Evaluating enteric methane emissions within a herd of genetically divergent grazing dairy cows.

Enteric methane (CH) emissions of 3 genetic groups (GG) of dairy cows were recorded across the grazing season (early March to late October). The 3 GG were (1) high economic breeding index (EBI) Holstein-Friesian (HF) representative of the top 1% of dairy cows in Ireland at the time of the study (elite), (2) national average (NA) EBI, which were representative of the average HF dairy cow in Ireland, and (3) purebred Jersey (JE) cows. Enteric CH was recorded using GreenFeed technology. Seasonal variation in CH was observed, with the lowest daily CH emissions and CH expressed per unit of dry matter intake occurring in spring (253 g/d and 15.56 g/kg, respectively), intermediate in summer (303 g/d and 18.26 g/kg, respectively), and greatest in autumn (324 g/d and 19.80 g/kg, respectively). Seasonal variation was also observed in the proportion of gross energy intake converted to CH (Y); in the spring the Y was lowest at 0.046, increasing to 0.053 and 0.058 in the summer and autumn, respectively. There was no difference in daily CH between the elite and NA, whereas JE had lower CH emissions compared with the elite. When expressed per unit of milk solids (fat + protein yield; MS), the elite and JE produced 6.8% and 9.7% less CH per kilogram of MS, respectively, compared with NA. There was no difference between the GG for CH per unit of DMI or the Y. This research emphasizes the variation in CH emissions across the grazing season and among cows of differing genetic merit for CH emission intensities but not for CH per unit of DMI or the Y.