The objective of this study was to evaluate the effect of feeding different dietary concentrations of 240-h undigested neutral detergent fiber (uNDF240) and physically effective neutral detergent fiber (peNDF) on ruminal pH, volatile fatty acid (VFA) concentrations and ruminal digesta characteristics. Sixteen Holstein cows, 8 ruminally cannulated, averaging 123 (SD = 9) days in milk (DIM) were used in a replicated 4 × 4 Latin square design with 4-wk periods. Cows were fed diets formulated to be isoenergetic and isonitrogenous but varying in uNDF240 and peNDF by changing forage to concentrate ratio and particle length of hay. Treatments were 1) 8.6% uNDF240 and 18.4% peNDF (LULP), 2) 8.6% uNDF240 and 19.9% peNDF (LUHP), 3) 10.5% uNDF240 and 16.2% peNDF (HULP), and 4) 10.5% uNDF240 and 19.0% peNDF (HUHP). Cows were housed in individual tie stalls and fed TMR once daily. Indwelling pH loggers recorded ruminal pH of cannulated cows on d 23–26 and ruminal fluid samples were collected every 4 h on d 26 and analyzed for VFA and NH₃-N concentrations. Ruminal evacuations occurred on d 27 and 28 to determine ruminal digesta characteristics. All data were analyzed as a replicated Latin square design using ANOVA and the MIXED procedure of SAS (version 9.4). Models included the fixed effects of diet, period, replicate, and time (as appropriate) and the random effect of cow within replicate. Greater forage particle size with low uNDF240 diets resulted in ruminal pH, VFA concentration, and ruminal digesta characteristics not different to higher uNDF240 diets with smaller particle size (Table 1). Table 1.

Item	LULP	LUHP	HULP	HUHP	SE	P-value
Daily mean pH	6.11b	6.17ab	6.22ab	6.24a	0.05	0.03
Daily standard deviation pH	0.29	0.29	0.24	0.28	0.03	0.10
Area under curve1 < 5.8	52.0	49.6	33.5	30.0	15.0	0.29
Total VFA, mM	122.8a	120.6ab	118.3ab	112.3b	4.1	0.05
Acetate (A), %	63.4	63.8	63.9	64.1	0.94	0.18
Propionate (P), %	22.7a	22.5a	21.5b	21.6b	0.83	<0.01
A:P	2.83c	2.89bc	3.04a	3.01ab	0.15	<0.01
Ammonia-N, mg/dL	4.38	5.04	4.72	4.93	0.61	0.45
Ruminal digesta volume, L	110	111	116	111	4	0.42
Ruminal digesta mass, kg	95	95	100	97	4	0.34

^a–cWithin a row, values with different superscripts differ (P ≤ 0.05). ¹Area < 5.8 = ruminal pH units below 5.8 by hour.