Adequate long fibre, in a form that is physically effective, is necessary in dairy cow diets to maintain proper rumen function. The term physically effective fibre (peNDF; Mertens, Feedstuffs, 2000) is used as a means of formulating diets to provide adequate particle size. Physically effective NDF stimulates chewing activity which increases saliva secretion and rumen pH. The Penn State Particle Separator (Lammers et al. J. Dairy Sci. 1996) is a quick and cost-effective method to estimate particle size of forage and total mixed rations and can be used on-farm to evaluate peNDF. The objectives of our study were to determine whether the peNDF content of forage influences how much feed the cow will consume and the digestion of feed in the rumen and in the total tract. Corn silage and barley silage, chopped fine, medium and coarse were each used in a separate study designed as a double 3x3 Latin square. The fine, medium and coarse silages were combined with a barley-based concentrate to provide three levels of peNDF in each study. The peNDF contents were 8.9, 10.3 and 11.5% of dry matter (DM) for the corn silage-based diets, and 10.5, 11.8 and 13.8% for the barley silage-based diets.
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