| Summary: | The purpose of this research was to evaluate whether relationships exist between odd- and branched-chain fatty acids (OBCFAs) originating from milk fat and the corresponding data of ruminal fermentation parameters, microbial populations, and base contents that were used to mark microbial protein in rumen. Nine lactating Holstein dairy cows with similar body weights and parity were selected in this study, and the samples of rumen and milk were collected at the early, middle, and late stages, respectively. The rumen and milk samples were collected over three consecutive days from each cow, and the ruminal and milk OBCFA profiles, ruminal fermentation parameters, bacterial populations, and base contents were measured. The results showed that the concentrations of OBCFAs, with the exception of C11:0 and C15:0, were significantly different between milk and rumen (<i>p</i> < 0.05). The concentrations of <i>anteiso</i>-fatty acids in milk were higher than those in rumen, and the contents of linear odd-chain fatty acids were higher than those of branched-chain fatty acids in both milk and rumen. Significant relationships that existed between the concentrations of C11:0, <i>iso-</i>C15:0, <i>anteiso-</i>C15:0, C15:0, and <i>anteiso-</i>C17:0 in rumen and milk (<i>p</i> < 0.05). The total OBCFA content in milk was positively related to the acetate molar proportion but negatively correlated with isoacid contents (<i>p</i> < 0.05). The populations of<i> </i><i>Ruminococcus</i><i> </i><i>albus</i>, <i>R.</i><i> </i><i>flavefacients</i>, and <i>Eubacterium</i><i> </i><i>ruminantium</i> were significantly related to milk C13:0 contents (<i>p</i> < 0.05). The adenine/N ratio was negatively related to milk OBCFA content (<i>p</i> < 0.05) but positively associated with the <i>iso-</i>C15:0/<i>iso-</i>C17:0 ratio (<i>p</i> < 0.05). Milk OBCFAs were significantly correlated with ruminal fermentation parameters, ruminal bacterial populations, and base contents. Milk OBCFAs had the potential to predict microbial nitrogen flow, and the prediction equations for ruminal microbial nitrogen flow were established for OBCFAs in dairy milk.
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