Nutrient digestibility of diets with different fiber to starch ratios when fed to lactating dairy cows.

The objective of this experiment was to determine whether increasing the dietary neutral detergent fiber (NDF):starch ratio affected NDF digestibility when diets were formulated to have equal in situ NDF digestibility. Six lactating Holstein cows were fed 1 of 3 diets in a replicated 3 x 3 Latin square. All diets had 41.5% of the dry matter (DM) as corn silage but the concentration of corn grain varied from 23.3 to 34.8% and the NDF:starch ratios were 0.74, 0.95, and 1.27. As corn grain increased, the concentration of a mixture of 54% soyhulls and 46% cottonseed hulls decreased. The soyhull:cottonseed hull mixture had the same in situ NDF digestibility as the corn silage. All diets had 18% forage NDF but starch concentration varied from 25.4 to 33.3% and NDF varied from 24.7 to 32.2%. Intake tended to increase as the NDF:starch ratio increased. Total tract digestibility (measured by total collection of feces and urine) of dry matter and energy decreased linearly as the NDF:starch ratio increased, but NDF digestibility was not affected by treatment. Retention of N increased linearly as the NDF:starch ratio increased. As dietary NDF:starch ratio increased, ruminal pH was not affected, but the concentration of total volatile fatty acids (VFA) decreased and the VFA profile was altered by diet. Consistent with the observed changes in ruminal VFA, milk fat percentage increased with increasing dietary NDF:starch. Intake of digestible energy and output of energy in milk and body weight change was not affected by treatment.