Effect of Fiber Source and Ruminal Starch Degradability on Site and Extent of Digestion in Dairy Cows

¹ Department of Animal Sciences, University of Arizona, Tucson 85721

Four duodenally cannulated Holstein cows (166 DIM) were used in a 4 x 4 Latin square with a 2 x 2 factorial arrangement of treatments to study effects of forage fiber source and ruminal starch degradability on digestion events. Forage NDF (21% of dietary DM) was from either alfalfa hay or a 1:2 (substituted on an NDF basis) ratio of alfalfa hay to wheat straw. Low and high ruminal starch degradabilities were achieved by use of either dry-rolled or steam-flaked sorghum grain, respectively. Dry matter intake did not differ among treatments (21 kg/d). Ruminal and total tract digestibilities of dietary OM, N, NDF, and ADF and flow of N fractions to the duodenum were not affected by fiber source. Diets with steam-flaked grain had higher ruminal (74 vs. 48%) and total tract (98 vs. 83%) starch digestibilities than did those diets with dry-rolled grain. Digestibilities of feed OM (63 vs. 51%) and CP (61 vs. 53%) and duodenal flows of nonammonia N (120 vs. 110% of intake) and bacterial N (.45 vs. .35 kg/d) also were higher, but microbial yield efficiency was not influenced by degradability of starch. Steam-flaked sorghum grain improved ruminal and total tract digestion of starch by lactating dairy cows and enhanced bacterial protein synthesis. Wheat straw NDF was substituted for two-thirds of the alfalfa hay NDF in diets for midlactation cows without adversely affecting digestion events or nument flow to the duodenum.

Key Words: fiber • starch degradability • digestion

Submitted on October 19, 1992
Accepted on February 24, 1993

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