Table 2.

Summary of the mixed effects models for CH₄ production (L/d) using CO₂ (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
			CO2	Phys3	FP	ADF	NDF	ME	FP × CO2
C1	8,323	0	+***	+*	+***			ns
C2	8,324	1	+***	+*	+***
C3	8,341	18	+***	+*	+***				+**
C4	8,343	20	+***	+*			+***	ns
C5	8,345	22	+***	+*			+***
C6	8,347	24	+***	+*		+***		ns
C7	8,349	26	+***	+*		+***
C8	8,371	48	+***	+*				ns
C9	8,373	50	+***	+*

FP = forage proportion, ME = metabolisable energy, ADF = acid detergent fibre, NDF = neutral detergent fibre.

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 C1.

²The sign of the estimated coefficients is also given.

³Phys = animal physiological state (dry and young animals vs. lactating cows).