The objectives of this study were to evaluate the effects of particle size and source of undigested neutral detergent fiber (uNDF) on dry matter intake (DMI), milk production and composition, and chewing behaviors. Fifty-six Holstein cows averaging 102 (SD = 28) DIM were fed one of 3 brown midrib (BMR) corn silage based diets balanced for equal uNDF240 intake (2.32 kg/d) and varying straw length (0.82 vs 0.66 physical effectiveness factor;pef) in a completely randomized block design with a 2-wk covariate period and 4-wk treatment period. Treatments were 1) 42% BMR corn silage (1.01 kg/d of uNDF240) and 7% long straw (0.74 kg/d of uNDF240;LS), 2) 42% BMR corn silage (1 kg/d of uNDF240) and 7% short straw (0.73 kg/d of uNDF240;SS), and 3) 60% BMR corn silage (1.54 kg/d of uNDF240) and 2% long straw (0.23 kg/d of uNDF240;BMR). Cows were housed in freestall pen, fed TMR daily with Calan bins, and milked 3x/d. Feed intake and milk yield were collected daily. Milk composition and body weight were collected weekly. Chewing behavior was collected for 72 h the last week of both periods. Data were summarized by week and analyzed using MIXED procedure in SAS with cow as experimental unit. The uNDF240 intake for the LS, SS, and BMR diets were 2.32, 2.28, and 2.36 kg/d. Results reveal that particle size and source of uNDF will affect milk fatty acid profile and chewing behaviors. Table 1.

Item	LS	SS	BMR	SE	P-value
DMI, kg/d	28.6x	28.2xy	27.8y	0.3	0.07
DMI, % of BW	4.05	4.03	3.99	0.04	0.50
Milk, kg/d	49.5	48.4	47.7	0.7	0.16
De novo FA, g/100 g FA	26.2abx	25.6ay	26.6bxy	0.2	0.002
Mixed origin, g/100 g FA	42.9a	42.5a	43.6b	0.2	0.001
Preformed FA, g/100 g FA	31.0a	31.8a	29.9b	0.3	<0.001
Eating, min/d	233x	216y	233x	5	0.04
Rumination, min/d	504	495	516	7	0.12

^abWithin a row, different subscripts differ at (P ≤ 0.05). ^xyWithin a row, different subscripts differ at (P ≤ 0.10) .