Influence of Sodium Bicarbonate and Magnesium Oxide on Digestion and Metabolism in Yearling Beef Steers Abruptly Changed from High Forage to High Energy Diets2

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Abstract
Eight rumen-cannulated steers (initial wt 330kg) were adapted to a mixed alfalfa-grass hay diet for 30 d and abruptly changed to complete mixed diets of corn silage, snapped ear corn and a corn-based supplement to determine the effects of buffers on diet adaptation, digestion and ruminal metabolism. The diets contained: 1) no buffer, 2) .5% magnesium oxide (MgO), 3) 1.0% sodium bicarbonate (NaHCO3) and 4) .5% MgO and 1.0% NaHCO3, as a percentage of diet dry matter (DM). Digestion, metabolism and ruminal characteristics were measured in each of 2 wk immediately after the diet change. The animals were then adapted to the mixed alfalfa-grass hay diet, re-randomized and assigned to the four diets, thus four steers consumed each diet. Intakes and digestibilities of DM were generally greater for the diets containing buffers. The most notable differences were a greater DM intake with added NaHCO3 and an improved DM digestibility with added MgO. The higher DM digestibility with MgO was apparently related to improved neutral detergent fiber (NDF) and starch digestion. Fecal pH was significantly increased with MgO addition. Because of the greater intake and digestibilities, the amount of DM, NDF, and starch digested tended to be greater for the buffered diets. There were no diet × week interactions for intake and digestibilities, thus the responses observed existed during both wk 1 and 2 after the change in diets. In general, intake and digestibilities were greater in wk 2 than in wk 1 for all diets. Ruminal pH and acetate.-propionate ratio (A/P) determined on samples collected at 0, 2, 4 and 6h postfeeding were not different between diets. A higher ruminal pH occurred in wk 2 than in wk 1 for all diets, although ruminal pH was not significantly affected by buffers. Ruminal dilution rate was lower with added MgO and pool size was higher with added NaHCO3. Dilution rate was higher in wk 2 than in wk 1 for all diets. Copyright © 1983. American Society of Animal Science . Copyright 1983 by American Society of Animal Science.

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