Evaluating the interaction of stocking density and the feeding environment is the next step in furthering dairy cow well-being and ruminal health. The objective of this study was to determine the short-term effects of stocking density and feed access on ruminal pH. Multiparous (n = 16, 4 cows/pen) ruminally cannulated Holstein cows were assigned to 1 of 4 pens as a part of a larger study (n = 17 cows/pen). Treatments were assigned to pens in a 4 × 4 Latin square with 14-d periods using a 2 × 2 factorial arrangement. Two stocking densities (STKD; 100 or 142% of stalls and headlocks) and 2 levels of feed access (FA; no restriction; NR and 5 h restriction from 19 to 24 h post-feeding; R) resulted in 4 treatments: (1) 100NR, (2) 100R, (3) 142NR, and (4) 142R. A total mixed ration was delivered 1×/day at approximately 0600 h. Ruminal pH was measured on d 12–14 of each period using indwelling pH loggers. Data were averaged into 10-min intervals across days and among cows into a pen average. Data were analyzed using a mixed model in JMP with pen as the experiment unit. Overstocking significantly reduced daily time spent below pH 5.8 and tended to increase area under the curve (AUC) below pH 5.8. While time spent below pH 5.8 tended to increase with overstocking 9–16 h post-feed delivery, no other time periods were significantly different. This indicates no singular time period accounted for the increased daily time spent below pH 5.8 with overstocking but rather a culmination throughout the day. Daily ruminal pH was not affected by FA, but an interaction was found between STKD and FA on time spent below pH 5.8. Overstocking negatively impacts ruminal pH and R exacerbates this effect. Table 1.

Item	100%			142%		SEM	P-value
	NR	R		NR	R		STKD	FA	STKD × FA
Mean pH	5.96	6.03		5.98	5.89	0.06	0.14	0.80	0.08
Minimum pH	5.42	5.50		5.51	5.39	0.07	0.81	0.78	0.12
Maximum pH	6.49	6.61		6.48	6.53	0.04	0.25	0.06	0.29
AUC <5.8
pH units × h/d	1.66	1.24		1.73	2.55	0.63	0.09	0.52	0.11
pH < 5.8, h/d	6.62	5.23		6.78	8.77	1.27	0.02	0.49	0.02
pH < 5.8, h
0-8 h post-feed delivery	1.79	1.31		1.56	2.12	0.31	0.32	0.88	0.12
9-16 h post-feed delivery	2.28	1.75		2.56	3.25	0.59	0.08	0.82	0.17
17-24 h post-feed delivery	2.55	2.44		2.67	3.11	0.56	0.16	0.49	0.27