Understanding the interaction of stocking density and diet is vital for dairy cow well-being and ruminal health. Multiparous (n = 11) and primiparous (n = 5) ruminally cannulated Holstein cows (116 ± 84 d in milk and 46 ± 9 kg milk/d) were assigned to 1 of 4 pens to determine the short-term effects of stocking density and source of forage fiber on ruminal fermentation and behavior. Pens were assigned to treatments 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 diets (straw; S and no straw; NS) resulted in 4 treatments: (1) 100NS, (2) 100S, (3) 142NS, and (4) 142S. Dietary forage content consisted of 39.7% corn silage and 6.9% haycrop silage versus 39.7% corn silage, 2.3% haycrop silage, and 3.5% chopped straw (dry matter; DM basis) for NS and S, respectively. Both diets were formulated for 16% crude protein, 28% neutral detergent fiber (NDF), and 28% starch (DM basis). Alterations in forage fiber source resulted in physically effective NDF values of 18.8% and 20.1% for NS and S, respectively. Ruminal pH measurements were recorded on d 12–14 of each period using indwelling pH loggers. Time spent feeding and ruminating were measured using 72-h direct observation on d 8–10 of each period. Data were analyzed using the MIXED procedure in SAS with pen as the experiment unit. Eating time (248 min/d, SEM = 9) and rumination time (496 min/d, SEM = 23) did not differ (P > 0.05) among treatments. Higher STKD tended (P < 0.07) to have a lower mean and maximum pH and significantly reduced (P < 0.01) time that pH <5.8. Area under the curve (AUC) and time spent below pH 5.8 were significantly reduced with S. Higher STKD reduces ruminal pH and S tends to counteract this effect. Table 1. Ruminal pH in Holstein cows fed 2 stocking densities (STKD; 100 or 142% of stalls and headlocks) and 2 diets (straw; S and no straw; NS)

Item	100%			142%		SEM	P-value
NS	S	NS	S	STKD	DIET	STKD × DIET
Mean pH	6.17	6.13		6.09	6.10	0.03	0.07	0.62	0.39
Minimum pH	5.70	5.67		5.62	5.59	0.05	0.11	0.53	0.95
Maximum pH	6.63	6.58		6.56	6.53	0.04	0.07	0.22	0.68
ph <5.8, h/d	2.29	1.90		4.12	2.77	0.41	<0.01	0.01	0.10
AUC <5.8, pH units × min/d	0.38	0.19		0.58	0.34	0.10	0.06	0.03	0.75