Table 3.

Comparison and evaluation of strategies to predict VFA concentrations from pH or from diet and time postfeeding¹

Item	Acetate, mM	Propionate, mM	Butyrate, mM	Isobutyrate, mM	Isovalerate, mM	Total VFA, mM
pH-based model
P-value2
Linear pH	0.032	0.015	0.028	0.241	0.322	0.030
Quadratic pH	0.037	0.019	0.036	0.253	0.343	0.034
Fit statistic
$\widehat{{\sigma }_E},$ mM	21.6	2.07	1.49	0.255	0.195	25.5
$\widehat{{\sigma }_E},$ %	37.8	31.7	32.0	21.0	29.6	36.1
Residuals analysis for pH-based model predictions vs. F:C ratio
Estimated marginal means (SEM)
100:0	−6.44 (3.1)	−0.95 (0.28)	−0.724 (0.23)	−0.23 (0.05)	−0.23 (0.04)	−7.66 (3.7)
85:15	−10.8 (3.1)	−1.1 (0.28)	−0.528 (0.23)	−0.22 (0.05)	−0.21 (0.04)	−12.4 (3.7)
70:30	−0.44 (3.1)	−0.11 (0.28)	−0.074 (0.23)	−0.16 (0.05)	−0.15 (0.04)	0.16 (3.7)
55:45	17.2 (3.1)	1.97 (0.28)	1.34 (0.23)	0.13 (0.05)	0.065 (0.04)	19.9 (3.7)
P-value2	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
F:C ratio
Diet-based model
P-value2
F:C ratio	0.060	0.077	0.069	0.033	0.001	0.059
Time	0.011	0.002	0.018	<0.001	<0.001	0.008
F:C ratio × time	0.269	0.001	0.008	0.236	0.566	0.173
Fit statistic
$\widehat{{\sigma }_E},$ mM	19.0	1.65	1.23	0.22	0.16	22.1
$\widehat{{\sigma }_E},$ %	33.3	25.3	26.4	18.4	24.2	31.3

¹pH-based models used point-in-time measurements of pH and VFA to estimate VFA concentrations based on linear and quadratic relationships with measured pH. $\widehat{{\sigma }_E},$ = residual error standard deviation expressed in mM or %; F:C ratio = forage-to-concentrate ratio used in treatment diets.

²Significance was declared at P < 0.05, and statistical tendency was declared when 0.05 ≤ P < 0.10.