This study was conducted to evaluate the effects of the particle size (PS) of alfalfa hay (AH) and soybean hull (SH) onmilk production of dairy cows and the population of major cellulolytic bacteria in the rumen. Eight lactating Holstein cows, averaging590±33 kg BW and 47±13 days in milk (DIM ), were assigned in a 4×4 Latin square design to a 2×2 factorial arrangement of treatments:alfalfa hay particle size (fine vs. coarse) combined with soy hull (zero or substituted as 50% of AH). The cows were fed diets formulatedaccording to NRC (2001). Physically effective factor (pef) and physically effective fiber (peNDF) contents of diets increased by increasing AH particle size and inclusion of SH in the diets (p<0.01). Dry matter intake was not significantly affected by treatments butintake of peNDF was increased marginally by increasing the PS of AH (p = 0.08) and by SH inclusion (p<0.01) in the diets. Milkproduction was increased by feeding diets containing SH (p = 0.04), but it was not affected by the dietary PS. Milk fat content wasincreased by increasing AH particle size (p = 0.03) and decreased by SH substitution for a portion of AH (p<0.01). The numbers of totalbacteria and cellulolytic species were not affected by PS of AH or by SH. F. succinogenes was the most abundant species in the rumen followed by R. albus and R. flavefaciens (p<0.01). This study showed that SH cannot replace the physically effective fiber in AH havingeither coarse or fine particle size. In diets containing SH, increasing of diet PS using coarse AH can maintain milk fat content similar to diets without SH. Particle size and peNDF content of diets did not affect the number of total or fibrolytic bacteria in the rumen.

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