Near Infrared Spectroscopy to Estimate Microbial Dry Matter and Nitrogen Contamination of In Situ Forage Residues

Introduction The in situ method is used widely to determine ruminal degradability of feed CP and DM. It is an attractive alternative to in vivo methods because it is simple, inexpensive, and rapid. However, several factors influence in situ results, the most important of which is microbial contamination of feed residues after incubation in the rumen. Degradation of forage CP and DM is systematically underestimated because of microbial contamination. Markers have been used to quantify microbial contamination of in situ residues. Microbial DM contamination ranged from 8 13% for forages and 8 25% for concentrates. Crude protein contamination of in situ residues was greater for forages than for concentrates. Microbial CP contamination of in situ forage residues was reported to be 15 50% when S was used as the marker and 46 95% when diaminopimelic acid was used as the marker. Use of microbial markers to measure microbial contamination is time consuming and expensive. It may be possible to quantify microbial DM and N contamination in in situ forage residues using near infrared spectroscopy (NIRS). Our objectives were to: 1) determine the extent of microbial N and DM contamination of in situ residues of ruminally incubated N labeled alfalfa hay, and 2) evaluate the use of NIRS to quantify microbial contamination of in situ forage residues.