LAUSR

To assess the effect of physically effective fiber (peNDF) on ruminal pH, 8 Simmental heifers with 258±5.7kg BW were randomly assigned to 1 of 4 treatments in a replicated 4x4 Latin square design. Diets were formulated with an 83 to 17 concentrate to straw ratio, and offered ad libitum as total mixed ration. Barley straw was chopped with a feeder wagon, and then processed either by: a) sifting with the 3-screen Penn State Particle Separator to obtain straw particle size greater than 4 mm, or b) grinding in a hammer mill through a 2-mm screen to obtain straw particle size lesser than 4 mm. Treatments after mixing the processed straws to obtain different peNDF proportions were: 5%, 7%, 9%, and 11%. Concentrate was ground through a 3-mm screen and manually mixed with the straw. The study was performed in 4 3-wk periods: 2-wk diet adaptation and 1-wk sampling. A ruminal bolus (smaXtec, Graz, Austria) was orally inserted in each heifer to measure ruminal pH every 10 min. Dry matter intake was individually measured. Data were analyzed using the mixed procedure of SAS. No significant effects were found in average and minimum pH (6.66 and 5.87 on average, respectively). Time under pH lesser than 5.8 was significantly longer (P < 0.01) in 5% (134 min/d) than 7%, 9% and 11% (63 min/d on average). Time under pH lesser than 5.6 tended to be longer (P = 0.074) in 5% (65 min/d), and shorter in 9% and 11% (25 min/d on average). Time under pH lesser than 5.5 was unaffected by treatment (23 min/d on average). Intake of DM was greater (P < 0.001) in 5% and 7% than 9% and 11%. These results suggest that the risk of subclinical acidosis decreases from 7 to 11% of peNDF, but DMI also decreases in 9% and 11%.