Evaluation of 15N and purine bases as microbial markers to estimate ruminal bacterial nitrogen outflow in dairy cows

Abstract

The present study aimed to evaluate 15N and purine bases (PB) as markers for ruminal bacterial nitrogen (N) outflow in dairy cows. Additionally, the effects of calcium salts of fatty acids (CSFA) in diets with different levels of rumen degradable protein (RDP) on PB omasal flow, urinary recovery of purine derivatives (PD), and endogenous excretion of PD were evaluated. Eight ruminal cannulated Holstein cows, with 207 ± 23.6 (mean ± SD) days in milk, 618 ± 39.0 kg of body weight, and 33.4 ± 2.45 kg/d of milk yield were used in a replicated 4 × 4 Latin square design. Four treatments were arranged in a 2 × 2 factorial design to evaluate the following factors: CSFA addition [-CS: basal diet without CSFA; and + CS: with 33.2 g/kg dry matter (DM) of CSFA]; and the level of RDP (LRDP: low RDP, with 98 g/kg DM of RDP; and HRDP: high RDP, with 110 g/kg DM of RDP). Particle-associated bacteria (PAB) showed lower (P ≤ 0.008) 15N enrichment and purine N to total bacterial N ratio than liquid associated bacteria (LAB). Increasing RDP concentration reduced (P = 0.007) 15N enrichment and tended (P ≤ 0.095) to reduce PB concentration in bacterial pellets. Purine bases overestimated (P ≤ 0.002) LAB-NAN and total microbial flow but, regarding PAB-NAN flow, there was a trend of interaction (P = 0.067) between marker and fat supplementation effects. In the absence of fat supplementation, PB underestimated (P ≤ 0.05) the flow of PAB-NAN compared to 15N and failed to detect the negative effect of fat supplementation evidenced by 15N labeling. Omasal PB flow tended to increase (P = 0.061) in low protein degradability diets, but PD excretion was constant (322 ± 5,9 mmol/d, mean ± SE) regardless of diet and, as a result, urinary recovery of PB decreased (P = 0.007). Daily excretion of creatinine and endogenous PD was also unaffected by the diet and averaged 0.262 mmol/kg BW and 0.640 mmol/kg BW0.75, respectively. In conclusion, the response of PB and its urinary derivatives to variation in diet composition compared to 15N reveals their weaknesses in the estimation of microbial synthesis.