Table 4.

Summary of the mixed effects models for CH₄ production (L/d) using O₂ (L/d) as a fixed effect with and without dietary variables, experiment and ”cow within experiment” as random effects.

Model	AIC1	ΔAIC1	Fixed effects and significance2
			O2	Phys	FP	ADF	NDF	ME	FP × O2
O1	8,770	0	+***	+*	+**			ns
O2	8,772	2	+***	+*	+***
O3	8,790	20	+***	+*		ns		ns
O4	8,791	21	+***	+*			ns	ns
O5	8,793	23	+***	+*		ns
O6	8,794	24	+***	+*			ns
O7	8,799	29	+***	+*				ns
O8	8,802	32	+***	+*
O9	8,802	32	+***	+*	+***				ns

FP = Forage proportion, ADF = acid detergent fibre, NDF = neutral detergent fibre, ME = metabolisable energy, Phys = animal physiological state (dry and young animals or lactating cows).

ns = P > 0.05; *P < 0.05; **P < 0.01; ***P < 0.001, Wald tests.

¹AIC = Akaike information criterion, with ΔAIC corresponding to the difference between the AIC of each model and the AIC of model O1.

²Sign of the estimated coefficients is also given.