INTERACTIONS OF FIBER AND NONSTRUCTURAL CARBOHYDRATES ON LACTATION AND RUMINAL FUNCTION

Four Holstein cows averaging 147 DIM and fitted with ruminal and duode nal cannulas were used in a 4 x 4 Latin square trial to determine diet effects on DMI, milk production, and ruminal metabolism. Diets contai ned either rapidly or slowly degraded NDF, referred to as low fill and high fill, respectively, combined with two percentages of nonstructur al carbohydrate. Treatments were 39% nonstructural carbohydrate (low o r high fill) and 29% nonstructural carbohydrate (low or high fill). In take of DM was not affected by either fill or nonstructural carbohydra te. Ruminal NDF digestibilities averaged 43.1 and 35.6% for the low fi ll and high fill diets, respectively. Ruminally digested nonstructural and total carbohydrate increased, but milk production decreased, as n onstructural carbohydrate increased from 29 to 39% in diets. Liquid an d solid ruminal passage rates, as measured by Co-EDTA and Yb, respecti vely, were reduced by either 39% nonstructural carbohydrate or low fil l diets. Lower microbial N flow to the duodenum and lower efficiency o f microbial growth also were observed for diets with 39% nonstructural carbohydrate. The combination of 39% nonstructural carbohydrate and r apidly degraded fiber gave the highest DM and nonstructural carbohydra te digestion in the rumen but resulted in low microbial N synthesis pe r day and the least microbial N per kilogram of OM digested.