Determination of ruminal feed digestibility and microbial synthesis based on digesta sampling from the omasal canal

The main objective of the current thesis was to evaluate use of the omasal sampling technique to assess the flow of digesta entering the omasal canal of lactating dairy cows. Digesta flow entering the omasal canal and the proximal duodenum was compared in publication I. The flow of OM entering the omasal canal was lower than in the duodenum that reflects the contribution of endogenous secretions into the abomasum. A higher flow of NDF entering the omasal canal indicated that fibre was digested in the omasum. No differences were detected in total non-ammonia N (NAN) or undegraded dietary N flows between the omasal canal and the duodenum, while microbial N entering the omasal canal tended to be slightly higher. Compared with the omasal canal, measurements at the duodenum indicated a smaller effect of rapeseed meal supplementation on undegraded dietary N flow. Differences in nutrient flow between the omasal canal and the duodenum were primarily due to metabolism and absorption in the omasum and abomasum. In general, the precision of flow measurements was similar for both sampling sites. However, the composition of digesta samples obtained from the omasal canal deviated from true digesta to a greater extent than samples obtained from the duodenum. Supplementation of a grass silage and barley based diet with urea, rapeseed meal or rapeseed cake on ruminal digestibility, microbial protein synthesis and milk yield was studied in publication II. Urea and rapeseed feeds had no effect on microbial N flow entering the omasal canal, despite a predicted deficiency in N availability in the rumen. Both rapeseed meal and cake increased undegraded dietary N flow entering the omasal canal that was reflected as improved milk production. The effect of heterogeneity in particulate matter composition entering the omasal canal on the accuracy of fibre flow measurements was studied in publication III. Digesta particles entering the omasal canal were larger than those in the omasum, duodenum or in faeces, suggesting that samples collected from omasal canal did not provide a reflection of particle size distribution escaping the rumen. Since the concentration of Cr decreased and that of INDF increased with decreasing particle size, NDF flow entering the omasal canal was slightly overestimated based on INDF and slightly underestimated using Cr. However, the results indicated that the omasum may have a greater role in fibre digestion than the intestines. The effects of barley and rapeseed meal supplementation of grass-red clover silage on ruminal digestibility and microbial flow were assessed in publication IV. Barley supplementation increased the flow of liquid associated bacterial and protozoal N, but had no effect on ruminal ammonia concentration or net absorption of N from the rumen. Barley increased daily N retention but decreased ruminal and total tract NDF digestibility. Rapeseed meal supplementation increased non-microbial N flow, net absorption of N from the rumen, urinary N excretion and daily N retention. Barley and rapeseed meal increased milk yield, the effects being additive. Responses attained with barley were explained by increased energy supply to the animal and microbes, whilst the effect of rapeseed meal was mediated through increased supply of ruminally undegradable protein. The contribution of liquid and particle associated bacteria and protozoa to total microbial N flow from the reticulorumen was modified by diet and accounted for proportionately between 0.64-0.71, 0.20-0.30 and 0.04-0.10, respectively. The effect of four indigestible markers (LiCoEDTA, Yb-acetate, Cr-mordanted straw and INDF) and three marker systems on the flow of digesta entering the omasal canal of lactating dairy cows was assessed in publication V. Digesta flow was calculated according to single markers or using the reconstitution system based on combinations of two or three markers. It appears that digesta consists of at least three phases which tend to separate during sampling. As such a triple marker system based on Co and Yb in combination with Cr or INDF is required in order to accurately measure the flow of watersoluble nutrients, OM, N and NDF. Current data demonstrated that precise and accurate measurements of ruminal feed digestibility and microbial synthesis can be attained based on the flow of digesta entering the omasal canal.