Comparison of purine bases and 15N for quantifying microbial nitrogen yield using three marker systems and different sampling sites in zebu cross breed bulls

The present experiment was designed to evaluate the use of microbial markers (MM), sampling sites (SS), and marker systems (MS) to estimate microbial nitrogen (MN) synthesis in bulls and to develop equations to correct MN estimates when only one of the aforementioned techniques was utilized. The MM systems evaluated were (1) purine bases (PB) and (2) 15N labeling. The SS evaluated were: (1) reticulum, (2) omasum, and (3) abomasum, and the single, double and triple MS were evaluated. Eight crossbred (Holstein×Zebu) bulls (353±36.9 kg of BW; 24±1 mol) with ruminal and abomasal cannulas were utilized in this experiment. The following experimental diets were used: (1) 60% corn silage+40% concentrate, (2) 40% corn silage+60% concentrate, (3) 60% fresh sugarcane+40% concentrate, and (4) 40% fresh sugarcane+60% concentrate. Four experimental periods lasting 16 d each were completed with 10 d for adaptation to the experimental diet and 6 d for sampling. Bulls were randomly distributed into two 4×4 Latin squares balanced for residual effects. Data were analyzed in a Latin square design using PROC MIXED. Interactions were observed (P<0.05) in MN, microbial crude protein/total digestible nutrients (MCP/TDN), microbial nitrogen/rumen fermented organic matter (MN/RFOM), microbial nitrogen/rumen truly fermented organic matter (MN/RTFOM), and microbial dry matter/rumen fermented total carbohydrates (MDM/RFTCHO) between SS and MM. For PB, the greatest (P<0.01) values of MN were observed for the digesta sampled in the reticulum and abomasum. In contrast, for 15N, the greatest (P<0.01) values were observed for digesta sampled in the omasum and abomasum. Microbial nitrogen yield was only different (P<0.05) when using reticulum and 15N from those estimated using abomasum and 15N. Thus, the equation developed to correct MN value was: MN (g/d)=27.93±2.46+0.99±0.09×reticulum 15N. The triple MS exhibited the greatest (P<0.01) value of MN compared to the single and double MS. No interactions (P>0.05) were observed between MS and MM or SS; thus, the equation established to correct MN value used only the MS. In conclusion, we have demonstrated that there is no difference using 15N to estimate MN yield if omasum or abomasum are used. Therefore, the omasum can be used as an accurate SS to predict MN. The triple MS presented higher values than the single and double MS. Thus, if single or double MS is used the value must be corrected by the equation obtained using the triple MS.