Effects of canola meal or soybean meal on duodenal flow of nitrogen fractions in dairy cows

Abstract

Canola meal (CM) is a protein supplement increasingly used in dairy cow diets. Substituting soybean meal (SBM) with CM has been shown to have a positive effect on milk and milk protein yields. This positive response on milk production occurs even though the NRC predicted a reduction in MP supply when CM replaces SBM in dairy rations. The objective of this project was to evaluate the ¹⁵N kinetics in 5 N fractions from duodenal samples after a ruminal bolus ¹⁵N-labeled CM or SBM to determine the effects on MP supply. The ¹⁵N fractions were as follows: total duodenal, small particles, fluid phase, particle-associated bacteria (PAB), and fluid-associated bacteria (FAB). Four cows fitted with a ruminal and a duodenal cannula were used in a crossover design with diets including CM or SBM. On d 18 or 19 of each period, a ruminal bolus of ¹⁵N-labeled CM or SMB was administered and duodenal digesta samples were collected regularly for 72 h. The proportion of FAB in the duodenal digesta was higher when cows were fed CM than SBM. The areas under the curve of the enrichment of ¹⁵N as a function of time after the ruminal bolus were greater for CM for the fraction associated with small particles, the PAB, and the FAB. These results indicate that the N lost from nylon bags during ruminal incubation of CM contribute to the duodenal N flow. Canola meal probably contains a substantial amount of small particles that leave the bags during ruminal incubation but, contrary to what is now assumed, they are not degraded in the rumen and contribute directly to the MP supply and would contribute to the positive response to CM.