The proportion of the diet to which fibrolytic enzymes are added affects nutrient digestion by lactating dairy cows.

Eight lactating Holstein cows, four with ruminal cannulas, were used in a duplicated 4 x 4 Latin square design to investigate a fibrolytic enzyme product characterized by xylanase and cellulase activities (Promote N.E.T. Agribrands International, St. Louis, MO). The diet consisted of concentrate containing rolled barley and supplement, barley silage and alfalfa haylage (55% to 45% DM basis, forage to concentrate ratio) and differed in enzyme application: 1) control, 2) enzyme applied to concentrate (45% of TMR), 3) enzyme applied to supplement (4% of TMR), and 4) enzyme applied to premix (0.2% of TMR). All diets that were supplemented with the enzyme product delivered about 1.0 grams per cow per day. Digestibility of OM, NDF and ADF in the total tract was increased in comparison to the control when enzymes were added to the entire concentrate. Enzyme treatments that were applied to a smaller portion of the diet showed only numerical increases in digestibility over the control. However, there was an increase in microbial N synthesis for cows fed enzymes added to the premix. The effects of enzyme supplementation on milk production and composition were not statistically significant, but cows receiving the enzyme product added to the concentrate had a numerically higher FCM compared to the control cows. These results indicate that enzyme supplementation increases total tract digestibility of organic matter and fiber. The proportion of the diet to which the enzyme is applied must be maximized to ensure a beneficial response.