Depression in fiber digestibility in lactating dairy cows has been widely associated with increases in dietary starch (DS) due to reductions in rumen pH. However, greater starch intake (SI) can increase fiber passage rate (kp) that may also depress NDF digestibility. The objective of this study was to infer if the depression in NDF digestibility associated with DS is due to pH or fiber kp. Two structural equation models were built using 2 research trials (n = 64, Latin square design) in which DS, SI, total-tract NDF digestibility (TTNDFD), pH, and kp of potentially digestible NDF (pdNDF) were measured. All variables were adjusted for the fixed effect of diet and random effects of period within square, cow within square, square and trial. The first model (M1) tested the direct effects of DS on TTNDFD and SI on TTNDFD, and the indirect effect of DS on TTNDFD. The indirect effect of DS on TTNDFD was composed by the effects of DS on SI, SI on pH, and then pH on TTNDFD. The same approach was used in the second model (M2), but pH was replaced with kp pdNDF. Results (Table 1) suggest that there is an indirect effect of DS on TTNDFD through the increase in kp pdNDF caused by greater SI (P < 0.05). However, there is no evidence of causal effect of DS on TTNDFD due to changes in rumen pH (P > 0.05). The chi-squared test (χ²), comparative fit index (CFI), Akaike information criteria (AIC), and root mean square of the approximation (RMSEA) for M1 were: χ² (P = 0.07), CFI = 0.89, AIC = 685, and RMSEA = 0.31, while for M2 were: χ² (P = 0.95), CFI = 1.0, AIC = 660, and RMSEA = < 0.001. Increase in passage rate is the main factor that depresses TTNDFD when SI increases in diets with DS ranging from 16 to 27%. Table 1. Direct and indirect effects of dietary starch (DS, % of DM) on total tract NDF digestibility (TTNDFD, %)

Item1	Estimate	SE	P-value
Direct effects
DS on TTNDFD	−0.18	0.15	0.24
SI on TTNDFD	0.24	0.16	0.13
Indirect effect - M1
DS on SI	0.74	0.08	<0.001
SI on pH	−0.31	0.11	0.01
pH on TTNDFD	0.04	0.13	0.72
Indirect effect - M2
DS on SI	0.74	0.08	<0.001
SI on kp pdNDF	0.42	0.11	<0.001
kp pdNDF on TTNDFD	−0.56	0.11	<0.001

¹SI = starch intake (kg/d), pH = rumen pH, kp pdNDF = passage rate of potentially digestible NDF (%/h).