Individual methane emissions (and other gas flows) are repeatable and their relationships with feed efficiency are similar across two contrasting diets in growing bulls

•Individual methane emissions are repeatable with high-starch or high-fibre diets.•Most efficient animals produce less methane (g/d).•No relationships exist between alternative methane metrics and residual feed intake.•Co-selection of low-methane emitters and efficient animals is possible.•The optimal CH4 metric for recording phenotypes remains challenging.