Evaluating the effect of white clover inclusion and dairy cow genetic merit on enteric methane emissions within rotational grazing systems

This study aimed to quantify and compare the enteric methane emissions from dairy cows grazing either perennial ryegrass (Lolium perenne L.; PRG) or PRG with white clover (Trifolium repens L.; clover) in a farm systems experiment. A study was established with 2 sward treatments consisting of PRG receiving 225 kg of nitrogen (N) per hectare per year (grass-only) and PRG and clover receiving 150 kg N/ha per year (grass-clover). Two dairy cow genetic groups, high and low Economic Breeding Index (EBI), were randomly assigned across the 2 sward treatments, which were managed as self-contained farmlets. Enteric methane emissions were measured throughout the study (late March to late October) using GreenFeed units (C-Lock Inc.). Both swards had similar total herbage production (11.8 t DM/ha) during the study, and the average clover content of grass-clover was 23.0% (SD = 16.51). Cows in the grass-clover treatment had greater DMI and consequentially had greater daily methane emissions (341 g/d) compared with the cows in the grass-only treatment (308 g/d). However, there was no difference in methane yield (methane per kg DMI) between the 2 treatments. There was also no treatment effect when methane emissions were expressed per kilogram of milk solids (milk fat + protein; MSo), but methane per kilogram of fat- and protein-corrected milk tended to be greater from cows grazing grass-clover. There was no effect of EBI group on daily MSo production, methane emissions, methane yield, or methane per kilogram of MSo. Cows in the high EBI group had lower milk yield and greater milk fat and protein content compared with the cows in the low EBI group. The ability to reduce artificial N fertilizer inputs in a grass-clover system benefits nitrous oxide emissions and farm-gate N use efficiency. However, the greater enteric methane emissions from cows grazing grass-clover highlight the need for balanced consideration when implementing environmental measures on farms to avoid pollution swapping.