Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2010
Publication Date: 7/1/2010
Citation: Huhtanen, P., Ahvenjarvi, S., Broderick, G.A., Reynal, S.M., Shingfield, K.J. 2010. Quantifying Ruminal Digestion of Organic Matter and Neutral Detergent Fiber Using Omasal Sampling in Cattle--A Meta-Analysis. Journal of Dairy Science. 93:3203-3215. Interpretive Summary: Much of the energy in forages fed to ruminants such as dairy cows is present in fiber and other organic matter which cannot be used by the animal without first being digested by the microorganisms living in the rumen, the first compartment of the cow’s stomach. Rate and extent of energy digestion in the rumen dictates how effectively dairy cows can make milk from their diets. Moreover, because rumen microbes get their energy for growth from organic matter digestion, this also determines the amount of protein produced in the form of microbial cells. Previously, it was necessary to have veterinarians put cannulas (surgical openings) into either the abomasum (the last part of the cow’s stomach) or duodenum (the first part of the small intestine) to be able to measure the extent of digestion in the rumen. However, abomasal and duodenal cannulation can be harmful to the animal. Omasal sampling was recently developed as an alternative approach for sampling digesta (partially digested feed) leaving the rumen. Two advantages of omasal sampling are that: 1) it only requires rumen cannulation (where veterinarians make openings directly into the rumen and dairy cows live normal lifetimes); and 2) it avoids errors caused by the normal secretion of digestive juices into the abomasum and small intestine. Our objective was to determine whether the omasal sampling gave reliable measurements of fiber and organic matter digestion in the rumen and to determine the relationship between digestion in the total gastrointestinal tract and milk yield. Research results indicated that microbial digestion in the rumen accounts for 95% of total fiber digestion occurring in the dairy cow, which means that only about 5% of fiber digestion takes place in the hind-gut (the cecum and large intestine); previous research had suggested that as much as one-third of fiber digestion occurred in the hind-gut. Microbial protein formed in the rumen, and the yield of milk and milk protein, were strongly related to extent of fiber and organic matter digestion in the rumen. Also, although there was a strong relationship between measured rumen digestion and digestion predicted by the National Research Council (NRC, which is used throughout the world to balance rations for dairy cattle)., the NRC model was found to give low estimates of microbial protein supply. This research indicates that the omasal sampling technique yields reliable measurements of dietary energy digestion in the rumen and microbial protein supply and that this method can replace other procedures that are harmful to dairy cows. This research also indicates that the NRC ration formulation system should be revised so it gives more accurate estimates of microbial protein supply. Dairy farmers will benefit because application of omasal sampling will allow more reliable experiments for solving problems on how to meet more efficiently the energy requirements of dairy cows and improve the economic and environmental sustainability of dairying.
Technical Abstract: A data set from 32 studies (122 diets) was used to evaluate the accuracy and precision of the omasal sampling technique by investigating the relationships between ruminal and total digestion of neutral detergent fiber (NDF), between intake and apparent and true ruminal digestion of organic matter (OM), and between omasal NAN flow and milk protein yield. A mixed model regression analysis with random study effect was used to evaluate the relationships. The data were obtained when feeding North American diets (n = 36) based on alfalfa silage, corn silage and corn grain, and North European diets (n = 86) comprised of grass silage supplemented with barley-based concentrates. In all studies, digesta flow was quantified using a triple-marker approach. Standard deviations of ruminal NDF and true OM digestibility were smaller than typically reported in duodenal sampling studies using only chromic oxide as a flow marker. The relationship between total and ruminal NDF digestion was consistent, indicating there was little variation in the proportion of total tract NDF digestion that occurred in the rumen. Furthermore, the slope of this regression indicated that 94.7 (±2.7)% of total NDF digestion occurred in the rumen. The slopes of mixed model regression equations between OM intake and amounts digested indicated that 42 (±2.4) and 74 (±3.1)% of OM was apparently and truly digested in the rumen. The contribution of the rumen to total tract apparent OM digestion was 62 (±2.6)%. The close relationship between omasal flow of nonammonia crude protein and milk protein yield (with adjusted residual mean squared error = 31 g) provided further confidence of the reliability of omasal flow measurements.