Understanding the interaction of stocking density and diet is vital for the improvement of dairy cow well-being and productivity. Multiparous (n = 48) and primiparous (n = 20) Holstein cows were assigned to 1 of 4 pens (n = 17 cows/pen, based on previous response variability) to determine the short-term effects of stocking density and source of forage fiber on behavior and lactational performance. 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. Pen intake and milk yield were measured on d 8–14 of each period. Time spent feeding, ruminating, and lying were measured using 72-h direct observation on d 8–10 of each period. Milk component yields were quantified for 6 milkings on d 13 and 14 of each period. Data were analyzed using the MIXED procedure in SAS with pen as the experiment unit. Eating time (238 min/d, SEM = 4), rumination time (493 min/d, SEM = 9), and pen intake (25.3 kg/cow/d, SEM = 0.4) did not differ (P > 0.10) among treatments. Milk, true protein and lactose yields tended to decrease with S. Although higher STKD decreased lying time, diet had no effect. The dietary addition of straw had minimal effects on short-term production and behavioral responses at varying STKD. Table 1. Production of Holstein cows assigned to 2 stocking densities (STKD; 100 or 142% of stalls and headlocks) and 2 diets (straw; S and no straw; NS)

Item	100% STKD			142% STKD		SEM	P-value
NS	S	NS	S		STKD	Diet	STKD × Diet
Milk, kg/d	41.2	40.4		40.7	40.0	0.7	0.21	0.06	0.79
Fat, kg/d	1.29	1.30		1.30	1.25	0.02	0.48	0.12	0.08
True protein, kg/d	1.02	1.01		1.02	0.99	0.01	0.25	0.07	0.42
Lactose, kg/d	1.41	1.39		1.40	1.37	0.04	0.25	0.08	0.61
Lying, min/d	832	827		779	796	11	<0.01	0.56	0.31