Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/19/2010
Publication Date: 7/1/2010
Citation: Broderick, G.A., Huhtanen, P., Ahvenjarvi, S., Reynal, S.M., Shingfield, K.J. 2010. Quantifying Ruminal Nitrogen Metabolism Using the Omasal Sampling Technique in Cattle--A Meta-Analysis. Journal of Dairy Science. 93:3216-3230. Interpretive Summary: Intervention by the large population of microorganisms living in the rumen (the first compartment of the ruminant stomach) makes protein nutrition of ruminants such as dairy cows much more complex than in non-ruminants such as swine and poultry. Because of extensive transformations in the rumen, composition of the protein digested and absorbed by the cow differs greatly from dietary protein composition fed to the cow. The cow’s requirement for amino acids, the building blocks of protein, is actually met from protein synthesized by the microbes living in the rumen plus from feed protein that escapes microbial breakdown in the rumen. Previously, to be able sample and analyze the digesta (partially digested feed) leaving the rumen, it was necessary to have veterinarians put cannulas (surgical openings) into either the abomasum (the last part of the cow’s stomach) or the duodenum (the first part of the small intestine). Abomasal and duodenal cannulation can be harmful to the animal. Omasal sampling was recently developed as an alternative approach for sampling digesta leaving the rumen. The major advantage of omasal sampling is that it only requires rumen cannulation, where veterinarians make openings directly into the rumen; dairy cows live normal lifetimes after recovering from this surgery. The other major advantage of omasal sampling is that it avoids errors caused by the digestive juices that are normally present in abomasal and intestinal digesta. Absence of these protein-containing secretions means that the protein found in omasal digesta derives only from feeds and rumen microbes, which are the cow’s actual source of protein. Our objective was to determine whether the omasal sampling gave reliable measurements of microbial protein formation and feed protein escape from the rumen. Results showed that it did, meaning that this sampling method can replace other procedures that may be harmful to cows. In addition, the research indicated that microbial protein formation in the rumen increased with feed intake and that, on average, 72% of dietary protein was degraded in the rumen. And we determined that total protein flow estimated by the National Research Council (NRC, used throughout the world to balance rations for dairy cattle) was not different from measured protein flow; however, the NRC predicted too much feed protein escaping the rumen and too little microbial protein produced in the rumen; more research is required to make the NRC system more reliable. Dairy farmers will benefit as application of omasal sampling leads to more answers about how to feed dairy cows more efficiently to improve economic and environmental sustainability of dairying.
Technical Abstract: Mixed model analysis of data from 32 studies (122 diets) was used to validate omasal sampling for quantifying ruminal-N metabolism and to assess the relationships between nonammonia-N flow at the omasal canal and milk protein yield. Data were derived from experiments in cattle fed North American diets (n = 36) based on alfalfa silage, corn silage, and corn grain, and North European diets (n = 86) comprised of grass silage and barley-based concentrates. In all studies, digesta flow was quantified using a triple-marker approach. Linear models accurately predicted microbial-N flow to the omasum from intake of dry matter (DM), organic matter (OM), or TDN. Efficiency of microbial-N synthesis increased with DM intake and there were trends for increased efficiency with elevated dietary concentrations of CP and rumen-degraded protein (RDP) but these effects were small. Regression of intake on omasal flow indicated that an average 72% of dietary CP was degraded in the rumen. The slope from regression of observed omasal flows of rumen-undegraded protein (RUP) on flows predicted by the NRC model indicated that NRC predictions overestimated RUP. Moreover, measured microbial-N flow was on average 26% greater than that predicted by the NRC model. Zero ruminal N-balance (omasal CP flow = CP intake) was obtained at dietary CP and RDP concentrations of 147 and 106 g/kg DM, corresponding to a ruminal ammonia-N and MUN concentration of 7.1 and 8.3 mg/100 ml, respectively. Milk protein yield was positively related to efficiency of microbial-N synthesis and measured RUP concentration. Low variation and high correlation coefficients suggested that the models accurately predicted milk protein yield. Improved efficiency of microbial-N synthesis and reduced ruminal CP degradability were positively associated with efficiency of capture of dietary N in milk N. In conclusion, the results of this study indicate that the omasal sampling technique yields reliable estimates of RDP, RUP, and ruminal microbial protein supply in cattle.