EFFECTS OF INERT RUMEN BULK ON DRY-MATTER INTAKE IN EARLY AND MIDLACTATION COWS FED DIETS DIFFERING IN FORAGE CONTENT

The effects of ratio of forage to concentrate and replacement of rumen contents with water-filled bladders on DMI and digesta kinetics were assessed in two trials. In trial 1, eight midlactation, rumen-cannulated cows were fed a 53:47 forage to concentrate diet. Treatment was replacement of 19.6 L of rumen contents with water-filled bladders. Bladders had no effect on intake, milk yield, or composition. In trial 2, four early and four midlactation cows received a 74:26 (33.2% NDF) or 50:50 (27.3% NDF) forage to concentrate diet with or without 23-L rumen bladders in a 2 x 2 factorial. Cows fed the 50:50 ratio of forage to concentrate consumed more DM, yielded more milk, and retained a greater amount of energy than those fed a 74:26 forage to concentrate diet. Bladders had no effect on intake or milk yield regardless of stage of lactation or diet fed. Rumen fluid pH and acetate to propionate ratio were increased by bladders and the 74:26 forage to concentrate diet. Bladders had no effect on chewing time per kilogram of NDF intake or mean reticular motility in either trial. Cows compensated for inert rumen bulk by an expansion of rumen volume and a reduction in rumen retention time. Reduced intake of the high forage diet in trial 2 was because of factors other than distension of the reticulorumen.