Using 15N Dilution Method to Correct for Microbial Contamination When Assessing in situ Protein Degradability of Fresh Ryegrass

This experiment was done to investigate microbial contamination and in situ disappearance rates of dry matter (DM), N and 15N of fresh labeled ryegrass. Perennial ryegrass (Lolium perenne) were labeled with 15N during growth in a glasshouse, harvested at 4th leaves stage and were incubated up to 34 h in situ in the rumen of 3 individually housed sheep. The animals were fed 800 g/d chopped alfalfa and had free access to drinking water. Six bags were placed in the rumen of each sheep simultaneously and removed after 0, 3, 7, 12, 21 and 33 h after incubation. The results were fitted to a model describing the degradation of DM and total N with time. It was found that residues from the washed zero time bags had lower 15N enrichments (7.7% vs. 8.3% enriched) than the original fresh samples. Under-estimation of effective degradability (ED) of protein in fresh forages by about 4% would have potential consequences for predictions of ruminally fermentable and escape protein and thus for dietary protein feeding management. However, because the correction assumes contaminating microbial N is unlabeled, but microbes attached to labeled ryegrass would become labeled to some extent, the true error and effective degradability may still be underestimated. Studies with two markers would help us to better understand the errors associated with the in situ technique.