Estimation of microbial nitrogen flow to the duodenum of cattle based on dry matter intake and diet composition

The objectives of this study were: 1) to evaluate the National Research Council equation used to predict microbial N flow to the duodenum in lactating cows, and 2) to determine whether improved equations could be developed by using dietary parameters used in the field. Treatment means from 55 trials with lactating and nonlactating cattle with duodenal cannulas were subjected to the backward elimination procedure of multiple regression. Variation within and among trials was accounted for by weighting the observations and including trial effects in all models. The equations to predict microbial N flow based on net energy for lactation (NEL) intake were different from the equation based on NEL intake used by the dairy National Research Council. Dry matter intake (DMI) estimated microbial N flow as well as did NEL intake, indicating that DMI drives predictions based on NEL intake. When multiple dietary factors [i.e., DMI; dietary percentages of crude protein, forage, and neutral detergent fiber; and all two-way interactions] were included, the resulting equation [microbial N (grams per day) = 16.1 + 22.9 x DMI (kilograms per day) - 0.365 x DMI2 - 1.74 x dietary neutral detergent fiber (percentage of dry matter)] tended to fit the data better than the equations based on NEL intake but not better than the equation based on DMI alone. The multiple-factor equation appeared to be the best overall equation for prediction; in contrast to the equation based on DMI, this equation is sensitive to diet composition. An asymptotic multiple-factor equation was developed, which may be more appropriate when extrapolating beyond the data range.