Quantifying ruminal digestion of organic matter and neutral detergent fiber using the omasal sampling technique in cattle--a meta-analysis.

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) comprising 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 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 amount digested indicated that 42% (+/-2.4%) and 74% (+/-3.1%) of OM was apparently and truly digested in the rumen, respectively. 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 in the reliability of omasal flow measurements.