Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/7/2006
Publication Date: 4/1/2007
Citation: Ipharraguerre, I.R., Reynal, S.M., Lineiro, M., Broderick, G.A., Clark, J.H. 2007. A comparison of sampling sites, digesta and microbial markers, and microbial references for assessing post-ruminal supply of nutrients in dairy cows. Journal of Dairy Science. 90:1904-1919. Interpretive Summary: All feed consumed by dairy cows and other ruminants is first subjected to an extensive microbial fermentation in the rumen, the first compartment of the ruminant stomach. The nutrients that a dairy cow uses to sustain itself and produce milk are derived from microbial fermentation plus feed that escapes the microbial fermentation in the rumen. Dietary effects on protein flow out of the rumen are important to understand because optimizing protein flow will improve protein efficiency and reduce negative environmental impacts from dairy farming. Sampling digesta that leave the rumen makes it possible to quantify the nutrients flowing to the cow’s intestine. The purpose of this research was to compare sampling at the omasum (the third compartment of the cow’s stomach) or at the duodenum (the start of the small intestine) for measuring digesta flow out of the rumen in lactating dairy cows. Results showed that sampling at the omasum yielded more accurate measurements of nutrient flows out of the rumen. Moreover, sampling at the omasum involves collecting digesta via tubes inserted through rumen cannulas (openings into the rumen made surgically by veterinarians). Rumen cannulation does not affect cow health and longevity. Sampling at the omasum avoids using duodenal cannulas (openings into the small intestine made surgically by veterinarians). Duodenal cannulas are harmful to cows and generally shorten their lives. Because sampling at the omasum is more accurate and easier to accomplish, this research shows that it will be possible to conduct more precise and humane research, and to improve productivity and health of dairy cows, with less environmental pollution.
Technical Abstract: This study evaluated the impact of some methodological factors on the flows of nutrients at the omasal canal and duodenum of dairy cows fed corn-based diets. Three ruminally and duodenally cannulated cows were assigned to an incomplete 4 x 4 Latin square with four 14-day periods and fed diets formulated to contain different amounts and ruminal degradabilities of crude protein. Samples from the omasal canal and duodenum were obtained and processed according to methodologies routinely used in our laboratories and elsewhere. Methodological factors that were evaluated included microbial reference and markers, digesta markers, and sampling sites (techniques). Considerable variation was found for the composition of microbial references and its impact on the intestinal supply of microbial non-ammonia nitrogen. Likewise, it appears that variation in measuring the ruminal outflow of nitrogen fractions of microbial and dietary origin could be reduced by using 15N rather than purines as microbial markers. Sampling from the omasum and duodenum resulted in differences for ruminal outflow and site of digestion, as well as digestibility of some nutrients, particularly nitrogen fractions and starch. A sizable portion of this variation was associated with deviations from the assumed ideal behavior of digesta markers and collection of samples that were unrepresentative of true digesta. Collectively, outcomes from this study indicate that more research will be required to determine the accuracy of nutrient flows and the agreement between measurements at the omasal canal and duodenum when dairy cows are fed a variety of diets under different feeding systems. Therefore, caution is recommended when extrapolating or interpreting the underlying biology of published results as well as the results of their application (e.g., model parameters and predictions).