Impact of forage fiber content on digestion and digesta passage in lactating dairy cows.

Five Holstein cows (5 wk postpartum) were used in a Latin square design (15-d periods) to determine rumen fill and fractional rates of ruminal digestion and passage. Treatments consisted of prebloom, midbloom, and full bloom alfalfa hay, mature bromegrass hay, and corn silage fed in diets containing forage: concentrate in a 60:40 ratio (DM basis) formulated to be isonitrogenous. Intake of DM averaged 4.0% of body weight for prebloom alfalfa and corn silage. Milk yield and DM intake were lower for full bloom alfalfa and bromegrass than for prebloom alfalfa. Digestibility of organic matter was 7.5 percentage units lower for full bloom than for prebloom alfalfa. Weight of DM in the rumen was higher for midbloom and full bloom alfalfa and bromegrass than with prebloom alfalfa. Ruminal retention time of Yb applied to forage was longer for bromegrass than for prebloom alfalfa. Fractional rates of in situ NDF digestion were slower for full bloom alfalfa and bromegrass than for prebloom alfalfa. Results suggest that the point of limitation of feed intake due to gut fill is dependent on forage quality as well as energy demand of the animal. Dry matter fill of the rumen was more closely related to rates of ruminal digestion and passage than to total tract digestibility or maximum digestibility after lengthy in situ fermentation.