Effects of prepartum diet, inert rumen bulk, and dietary polyethylene glycol on dry matter intake of lactating dairy cows.

Effects of prepartum energy intake, replacement of 25% of reticulorumen contents with water-filled bladders, and feeding of polyethylene glycol on DM intake during lactation were assessed in two trials. In trial 1, six rumen-cannulated cows were assigned to diets of either 1.50 or 1.68 Mcal NEL/kg 70 d prepartum. Animals fed the higher energy diet had greater NEL intake and tended to gain more BW prepartum. Intake from 28 to 70 d postpartum was not affected by prepartum diet but was reduced by 24.2 L of bladders placed in the rumen. Milk production tended to be increased for cows fed high energy prepartum and to be reduced by bladders. Total reticulorumen volume, digesta fractional passage rates, and acetate:propionate ratio were increased by bladders. In trial 2, eight rumen-cannulated cows, 28 d postpartum, received polyethylene glycol, 4% of diet DM, and 21.4 L of bladders in the rumen in a 2 x 2 factorial arrangement. Polyethylene glycol did not affect DM intake but reduced DM, CP, ADF, and NDF digestibilities. Bladders increased total reticulorumen volume, rumen fluid pH, and acetate:propionate ratio but decreased DM intake. Intake of DM was reduced .099 kg/L added bulk in trial 1 and .130 kg/L in trial 2. Compensation for replacement of rumen contents with inert bulk occurred by expansion of organ volume and, in trial 1, by a reduction in rumen retention time. These factors may explain why reductions in voluntary intake after addition of inert bulk to the reticulorumen often are proportionally lower than would be predicted from the volume of bulk added.